On Embedded Linux systems, there are multiple platform plugins that you can use: EGLFS, VkKhrDisplay, LinuxFB, or Wayland. The availability of these plugins depends on how Qt is configured. Of these, Wayland requires the presence of a compositor, and provides a full windowing system supporting multiple windows, similarly to X11 or Windows. The others work without any windowing system, meaning the Qt application is in full control of the rendering and output. They typically support one fullscreen Qt "window" per screen.
EGLFS 是许多主板上的默认插件。若不适合，使用
QT_QPA_PLATFORM
环境变量去请求另一插件。另外，为快速测试，使用
-platform
命令行自变量采用相同句法。
注意： 从 5.0 起，Qt 不再有自己的窗口系统 QWS 实现。对于单流程用例， Qt Platform Abstraction 是优秀解决方案；多流程用例的支持是透过 Wayland .
见 配置嵌入式 Linux 设备 for an overview of configuring Qt for cross-compilation using an Embedded Linux toolchain.
EGL is an interface between OpenGL and the native windowing system. Qt can use EGL for context and surface management, however the API contains no platform-specifics. Creating a 本机窗口 , which won't necessarily be an actual window on the screen, must still be done by platform-specific means. This is why we need the board or GPU-specific adaptation code. Typically, these adaptations are provided as:
EGLFS is a platform plugin for running Qt applications on top of EGL and OpenGL ES 2.0, without an actual windowing system like X11 or Wayland. It is the recommended plugin for modern Embedded Linux devices that include a GPU.
In addition to Qt Quick and native OpenGL applications, EGLFS supports software-rendered windows, like QWidget , too. For QWidget , the widgets' contents are rendered using the CPU into images, which are then uploaded into textures and composited by the plugin.
EGLFS forces the first top-level window - either a QWidget 或 QQuickView - to become fullscreen. This window is also chosen to be the root widget window into which all other top-level widgets are composited. For example, dialogs, popup menus, or combo boxes. This behavior is necessary because with EGLFS there is always exactly one native window and one EGL window surface; these belong to the widget or window that is created first. This approach works well when there is a main window that exists for the application's lifetime and all other widgets are either not top-levels or are created afterward, once the main window is shown.
There are further restrictions for OpenGL-based windows. EGLFS supports a single fullscreen GL window (as of Qt 5.3), like OpenGL-based QWindow ， QQuickView ，或 QOpenGLWidget . Opening additional OpenGL windows or mixing such windows with QWidget -based content isn't supported; Qt terminates the application with an error message.
Furthermore, APIs designed for desktop platforms or environments with a windowing system, such as 拖放 , are not supported on EGLFS.
若有必要，
eglfs
can be configured using the following environment variables:
| 环境变量 | 描述 |
|---|---|
QT_QPA_EGLFS_INTEGRATION
|
In addition to the compiled-in
hooks
, it is also possible to use dynamically loaded plugins to provide device or vendor-specific adaptation. This environment variable enforces a specific plugin. For example, setting it to
eglfs_kms
uses the KMS/DRM backend. This is only an option when no static or compiled-in hooks were specified in the device makespecs. In practice, the traditional compiled-in hooks are rarely used, almost all backends are now migrated to plugins. The device makespecs still contain a relevant, though optional,
EGLFS_DEVICE_INTEGRATION
entry: the name of the preferred backend for that particular device. Avoid setting this environment variable if there is more than one plugin present on the target system. In a desktop environment, the KMS or X11 backends are prioritized, depending on the presence of the
DISPLAY
环境变量。
注意：
On some boards a special value of
|
QT_QPA_EGLFS_PHYSICAL_WIDTH
and
QT_QPA_EGLFS_PHYSICAL_HEIGHT
|
Specifies the physical screen's width and height in millimeters. Note that since Qt 6 the physical screen size is no longer used to determine logical dpi. |
QT_QPA_EGLFS_ROTATION
|
Specifies the rotation applied to software-rendered content in
QWidget
-based applications. Supported values are 180, 90, and -90. This variable does not apply to OpenGL-based windows, including Qt Quick. Qt Quick applications can apply transformations in their QML scene instead. The standard
eglfs
mouse cursor always takes the value into account, with an appropriately positioned and rotated pointer image, regardless of the application type. However, special cursor implementations, such as the KMS/DRM backend's hardware cursor, may not support rotation.
|
QT_QPA_EGLFS_FORCEVSYNC
|
When set,
eglfs
requests
FBIO_WAITFORVSYNC
on the framebuffer device after each call to eglSwapBuffers(). This variable is only relevant for backends relying on the legacy Linux
fbdev
subsystem. Normally, with a default swap interval of 1, Qt assumes that calling eglSwapBuffers() takes care of vsync; if it doesn't (for example, due to driver bugs), try setting
QT_QPA_EGLFS_FORCEVSYNC
to a non-zero value.
|
QT_QPA_EGLFS_FORCE888
|
When set, the red, green, and blue color channel sizes are ignored when
eglfs
creates a new context, window or off-screen surface. Instead, the plugin requests a configuration with 8 bits per channel. This can be helpful on devices where configurations with less than 32 or 24 bits per pixel (for example, 5-6-5 or 4-4-4) are chosen by default despite knowing they are not ideal, for example, due to banding effects. Instead of changing application code, this variable provides a shortcut to force 24 or 32 bpp configurations.
|
Additionally, the following less commonly used variables are available:
| 环境变量 | 描述 |
|---|---|
QT_QPA_EGLFS_FB
|
Overrides the framebuffer device. The default is
/dev/fb0
. On most embedded platforms this variable isn't very relevant because the framebuffer is used only to query settings like the display dimensions. However, on certain devices, this variable provides the ability to specify which display to use in multiple display setups, similar to the
fb
parameter in LinuxFB.
|
QT_QPA_EGLFS_WIDTH
and
QT_QPA_EGLFS_HEIGHT
|
Contains the screen's width and height in pixels. While
eglfs
tries to determine the dimensions from the framebuffer device
/dev/fb0
, this doesn't always work. It may be necessary to manually specify the sizes.
|
QT_QPA_EGLFS_DEPTH
|
Overrides the color depth for the screen. On platforms where the framebuffer device
/dev/fb0
is not available or the query is not successful, a default of
32
is used. Use this variable to override any such defaults.
注意： This variable only affects the color depth value reported by QScreen . It has no connection to EGL configurations and the color depth used for OpenGL rendering. |
QT_QPA_EGLFS_SWAPINTERVAL
|
By default, a swap interval of
1
is requested. This variable enables synchronizing to the display's vertical refresh. Use this variable to override the swap interval's value. For instance, passing 0 disables blocking on swap, resulting in running as fast as possible without any synchronization.
|
QT_QPA_EGLFS_DEBUG
|
When set, some debugging information is printed on the debug output. For example, the input
QSurfaceFormat
and the properties of the chosen EGL configuration are printed while creating a new context. When used together with Qt Quick's
QSG_INFO
variable, you can get useful information for troubleshooting issues related to the EGL configuration.
|
除了
QT_QPA_EGLFS_DEBUG
,
eglfs
also supports Qt's modern categorized logging system. The following logging categories are available:
qt.qpa.egldeviceintegration
– Enables logging for dynamically loaded backends. Use this category to check what backend is in use.
qt.qpa.input
– Enables debug output both from the
evdev
and
libinput
input handlers. Use this category to check if a given input device was recognized and opened.
qt.qpa.eglfs.kms
– Enables verbose logging in the KMS/DRM backend.
After running
configure
, make sure to inspect its output. This is the easiest, quickest way to identify whether you have the necessary EGLFS backend, libudev, or libinput enabled. In short, if there's an undesired "no" in your
configure
output, run:
./configure -v
to turn on the verbose output, so that you can see the compiler and linker invocations for each configure test.
注意： If you encounter errors about missing headers, libraries, or seemingly cryptic linker failures, often, they are a sign of an incomplete or broken sysroot and isn't related to Qt.
As an example, when targeting the Raspberry Pi with the Broadcom proprietary graphics drivers, the output should contain something like the following:
QPA backends: EGLFS ................................ yes EGLFS details: EGLFS i.Mx6 ........................ no EGLFS i.Mx6 Wayland ................ no EGLFS EGLDevice .................... no EGLFS GBM .......................... no EGLFS Mali ......................... no EGLFS Raspberry Pi ................. yes EGL on X11 ......................... no
If this is not the case, it's not advisable to proceed further with the build since accelerated graphics won't be functional without the Raspberry Pi-specific backend, even if the rest of Qt compiles successfully.
While EGLFS only supports OpenGL (ES), VkKhrDisplay is an experimental platform plugin that supports rendering with the Vulkan API. To enumerate displays and set up rendering, it relies on the VK_KHR_display family of extensions. Note that it is not given that a Vulkan implementation within a graphics stack supports this functionality. Currently this platform plugin has been verified and tested with Mesa and V3DV running on a Raspberry Pi 4.
This platform plugin does not support OpenGL or any software rendering. Attempting to show a
QWidget
-based user interface will therefore fail. The only supported
QWindow
surface type is
QSurface::VulkanSurface
. For Qt Quick applications this implies that Vulkan-based rendering must be enforced either by setting
QSG_RHI_BACKEND=vulkan
in the environment, or by calling
QQuickWindow::setGraphicsApi
(
QSGRendererInterface::Vulkan
); early on before creating a
QQuickWindow
or
QQuickView
.
To use this platform plugin, run the application with
-platform vkkhrdisplay
or set
QT_QPA_PLATFORM
to
vkkhrdisplay
. The plugin is built only when Qt is configured with Vulkan support.
Advanced EGLFS-style configuration (e.g. the JSON configuration file) or outputting to multiple screens from the same application are not currently implemented. Applications can however choose the screen to use via environment variables.
To determine the index values, check the logs printed on the debug output by the plugin. Currently these logs are uncategorized (printed via qDebug ) since inspecting them is essential in most cases, in order to ensure the plugin picks the appropriate display and mode.
QT_VK_DISPLAY_INDEX
- When set, the display with the given index is used.
QT_VK_MODE_INDEX
- When set, the mode with the given index is used.
QT_VK_PHYSICAL_DEVICE_INDEX
- When set, the Vulkan physical device with the given index is used. This will not be relevant in most cases on embedded. Note that this variable is also used by the rest of the Qt graphics stack.
Input (keyboard, mouse, touch) handling is similar to EGLFS, supporting
evdev
,
libinput
，和
tslib
. There is no mouse cursor rendering implemented, however. This is because there is no concept of a hardware cursor in this environment, and rendering a cursor with Vulkan within the platform plugin, similarly to what EGLFS does with OpenGL, is problematic for multiple reasons. Therefore this platform plugin is not well suited for mouse-based input at the moment.
The related environment variables are:
QT_QPA_DISABLE_INPUT
- Disables keyboard/mouse/touch input.
QT_QPA_NO_LIBINPUT
- Prefers the evdev-based input handlers even when
libinput
可用。
QT_QPA_TSLIB
- Requests using the legacy
tslib
库。
This plugin writes directly to the framebuffer via Linux's fbdev subsystem. Only software-rendered content is supported. Note that on some setups the display performance is expected to be limited. To use Qt Quick applications with this platform plugin, the
software
scenegraph backend must be used, either by setting
QT_QUICK_BACKEND=software
in the environment, or by calling
setGraphicsApi
() 采用
QSGRendererInterface::Software
.
QWidget
applications, or
QWindow
with a surface type of
QSurface::RasterSurface
, are supported, but this does not include special widgets such as
QOpenGLWidget
.
As fbdev is being deprecated in the Linux kernel, DRM dumb buffer support is also available. To use it, set the
QT_QPA_FB_DRM
environment variable to a non-zero value. When set, provided that dumb buffers are supported by your system, legacy framebuffer devices like
/dev/fb0
won't be accessed. Instead, the rendering is set up via the DRM APIs, similar to the
eglfs_kms
backend in EGLFS. The output is double-buffered and page flipped, providing proper vsync for software-rendered content as well.
注意： When dumb buffers are in use, none of the options described below are applicable since properties like physical and logical screen sizes are all queried automatically.
The
linuxfb
plugin allows you to specify additional settings via the
QT_QPA_PLATFORM
environment variable or
-platform
command-line option. For example,
QT_QPA_PLATFORM=linuxfb:fb=/dev/fb1
specifies that the framebuffer device
/dev/fb1
must be used instead of the default
fb0
. To specify multiple settings, separate the mwith a colon (:).
| 设置 | 描述 |
|---|---|
fb=/dev/fbN
|
Specifies the framebuffer devices. On multiple display setups, this setting allows you to run the application on different displays. Currently, there's no way to use multiple framebuffers from one Qt application. |
size=
<width>
x
<height>
|
Specifies the screen size in pixels. The plugin tries to query the display dimensions, both physical and logical, from the framebuffer device. However, this query may not always lead to proper results; it may be necessary to specify the values explicitly. |
mmsize=
<width>
x
<height>
|
Specifies the physical width and height in millimeters. |
offset=
<width>
x
<height>
|
Specifies the top-left corner of the screen offset in pixels. The default position is at
(0, 0)
.
|
nographicsmodeswitch
|
Specifies not to switch the virtual terminal to graphics mode (
KD_GRAPHICS
). Typically,
enabling
graphics mode disables the blinking cursor and screen blanking. However, when this parameter is set, those two features are also skipped.
|
tty=/dev/ttyN
|
Overrides the virtual console. Only used when
nographicsmodeswitch
isn't set.
|
As of Qt 5.9, the behavior of EGLFS and LinuxFB has been synchronized, with regards to the window sizing policy: the first top-level window is forced to cover the entire screen, with both platform plugins. If this is not desired, set the
QT_QPA_FB_FORCE_FULLSCREEN
环境变量到
0
to restore the behavior from earlier Qt versions.
The level of support to target one or more displays from one single Qt application varies between the platform plugins. Support often depends on the device and its graphics stack.
When the KMS/DRM backend is in use, EGLFS reports all available screens in QGuiApplication::screens (). Applications can target different screens with different windows via QWindow::setScreen ().
注意： The restriction of one single fullscreen window per screen still applies. Changing screens after making the QWindow visible isn't supported either. Therefore, it's essential that embedded applications make all the necessary QWindow::setScreen () calls before calling QWindow::show ().
When you start developing on a given embedded device, often it's necessary to verify the behavior of the device and drivers, and that the connected displays are working as they should. One easy way is to use the
hellowindow
example. Launching it with the
-platform eglfs --multiscreen --timeout
arguments shows a rotating Qt logo on each connected screen for a few seconds.
The KMS/DRM backend also supports custom configurations via a JSON file. To enable this, set the
QT_QPA_EGLFS_KMS_CONFIG
environment variable to the name of the file. You can also embed this file into the application via the Qt resource system.
Most of these configuration options apply to all KMS/DRM-based backends, regardless of the buffer management technology (GBM or EGLStreams).
Here's an example configuration:
{
"device": "/dev/dri/card1",
"hwcursor": false,
"pbuffers": true,
"outputs": [
{
"name": "VGA1",
"mode": "off"
},
{
"name": "HDMI1",
"mode": "1024x768"
}
]
}
Here we configure the specified device so that:
Additionally, such a configuration also disables looking for a device via
libudev
; instead the specified device is used.
当
mode
is not defined, the system's preferred mode is chosen. The accepted values for
mode
是：
off
,
current
,
preferred
,
skip
, width
x
height, width
x
height
@
vrefresh, or a modeline string.
Specifying
current
chooses a mode with a resolution that matches the current one. Because mode-setting is done only when the desired mode is actually different from the active one (unless forced via the
QT_QPA_EGLFS_ALWAYS_SET_MODE
environment variable), this value is useful to preserve the current mode and any content in the planes not touched by Qt.
skip
causes the connector for the output to be ignored as if it were disconnected.
off
is similar, but it changes the mode and turns off the display.
By default, all screens reported by the DRM layer are treated as one big virtual desktop. The mouse cursor implementation takes this into account and moves across the screens as expected. Although not recommended, you can disable the virtual desktop by setting
separateScreens
to
false
in the configuration.
By default, the virtual desktop is formed left to right, based on the order of connectors as reported by the system. To change this, set
virtualIndex
to a value starting from 0.
For example, the following configuration uses the preferred resolution but ensures that the left side of the virtual desktop is the screen connected to the HDMI port; while the right side is the screen connected to the DisplayPort:
{
"device": "drm-nvdc",
"outputs": [
{
"name": "HDMI1",
"virtualIndex": 0
},
{
"name": "DP1",
"virtualIndex": 1
}
]
}
The order of elements in the array is not relevant. Outputs with unspecified virtual indices are placed after the others, with the original order in the DRM connector list preserved.
To create a vertical desktop space (that is, to stack top to bottom instead of left to right), add a
virtualDesktopLayout
property after
device
with the value of
vertical
.
警告： It's recommended that all screens in the virtual desktop use the same resolution, otherwise elements like the mouse cursor may behave in unexpected ways when entering areas that only exist on one given screen.
当
virtualIndex
is not sufficient, the
virtualPos
property can be used to explicitly specify the top-left position of the screen in question. Taking the previous example and assuming a resolution of 1080p for HDMI1, the following code snippet places a second HDMI-based screen below the first one:
{
...
"outputs": [
...
{
"name": "HDMI2",
"virtualPos": "0, 1080"
}
]
}
注意： Avoid such configurations when mouse support is desired. The mouse cursor's behavior may be unexpected with non-linear layouts. Touch should present no issues.
In some cases, the automatic querying of the physical screen size via DRM may fail. Normally the
QT_QPA_EGLFS_PHYSICAL_WIDTH
and
QT_QPA_EGLFS_PHYSICAL_HEIGHT
environment variable would be used to provide the missing values. This is not suitable anymore when multiple screens are present. Instead, use the
physicalWidth
and
physicalHeight
properties in the
outputs
list to specify the sizes in millimeters.
注意： Different physical sizes and thus differing logical DPIs are discouraged because it may lead to unexpected issues due to some graphics stack components not knowing about multiple screens and relying solely on the first screen's values.
Each active output from the
outputs
array corresponds to one
QScreen
instance reported from
QGuiApplication::screens
(). By default, the primary screen that
QGuiApplication::primaryScreen
() reports is the screen that is registered first. If you're not using
virtualIndex
, this means the decision is based on the DRM connector order. To override this, set the
primary
特性到
true
on the desired entry in the
outputs
列表。
For example, to ensure the screen corresponding to the VGA output is the primary even when the system happens to report the HDMI one first, do the following:
{
"device": "/dev/dri/card0",
"outputs": [
{ "name": "HDMI1" },
{ "name": "VGA1", "mode": "1280x720", "primary": true },
{ "name": "LVDS1", "mode": "off" }
]
}
For troubleshooting, it might be useful to enable debug logs from the KMS/DRM backend. To do this, enable the
qt.qpa.eglfs.kms
categorized logging rule.
注意： In an embedded environment, virtual desktops are more limited compared to a full windowing system. Windows overlapping multiple screens, non-fullscreen windows, and moving windows between screens, should be avoided and may not function as expected.
The most common and best supported use case for a multi-screen setup is to open a dedicated
QQuickWindow
or
QQuickView
for each screen. With the default
threaded
render loop of the Qt Quick scenegraph, each of these windows will get its own dedicated render thread. This is good because the threads can be throttled independently based on vsync, and will not interfere with each other. With the
basic
loop, this can get problematic, causing animations to degrade.
For example, discovering all connected screens and creating a QQuickView for each of them can be done like this:
int main(int argc, char **argv)
{
QGuiApplication app(argc, argv);
QVector<QQuickView *> views;
for (QScreen *screen : app.screens()) {
QQuickView *view = new QQuickView;
view->setScreen(screen);
view->setResizeMode(QQuickView::SizeRootObjectToView);
view->setSource(QUrl("qrc:/main.qml"));
QObject::connect(view->engine(), &QQmlEngine::quit, qGuiApp, &QCoreApplication::quit);
views.append(view);
view->showFullScreen();
}
int result = app.exec();
qDeleteAll(views);
return result;
}
Screen cloning (mirroring) is supported. This is enabled via the
clones
特性：
{
"device": "/dev/dri/card0",
"outputs": [
{ "name": "HDMI1", "mode": "1920x1080" },
{ "name": "DP1", "mode": "1920x1080", "clones": "HDMI1" }
]
}
In this case, the content on the display connected via DisplayPort will be the same as on the HDMI one. This is ensured by scanning out the same buffer on both.
However, this feature can only work if the resolutions are the same, there are no incompatibilities when it comes to accepted buffer formats, and the application doesn't have any output on the QScreen associated with a clone destination. In practice, the latter means that no QWindow associated with the QScreen in question - DP1 in the example - must ever perform a QOpenGLContext::swapBuffers () operation. It's up to the configuration and the application to ensure these.
Headless mode via DRM render nodes is supported. This allows performing GPU compute (OpenGL compute shaders, OpenCL) or off-screen OpenGL rendering without needing DRM master privileges. In this mode, applications can function even when there is already another process outputting to the screen.
Just switching
device
from
/dev/dri/card0
to
/dev/dri/renderD128
is futile on its own since there are a number of operations that cannot be performed in headless mode. Therefore, this must be combined with a
headless
特性，例如：
{
"device": "/dev/dri/renderD128",
"headless": "1024x768"
}
Keep in mind that windows are still sized to match the - now virtual - screen size, hence the need for specifying a size in the
headless
property. There is also a lack of vsync-based throttling.
Once enabled, applications have two typical choices to perform off-screen rendering in headless mode:
Use an ordinary window, such as a
QOpenGLWindow
subclass, targeting the window's default framebuffer, meaning a
gbm_surface
in practice:
MyOpenGLWindow w;
w.show(); // will not actually show up on screen
w.grabFramebuffer().save("output.png");
Or the typical offscreen approach with an extra FBO:
QOffscreenSurface s;
s.setFormat(ctx.format());
s.create();
ctx.makeCurrent(&s);
QOpenGLFramebufferObject fbo(1024, 768);
fbo.bind();
ctx.functions()->glClearColor(1, 0, 0, 1);
ctx.functions()->glClear(GL_COLOR_BUFFER_BIT);
fbo.toImage().save("output.png");
ctx.doneCurrent();
KMS/DRM can be used with two different DRM APIs which are legacy and atomic . The main benefit of DRM atomic API is to allow several DRM plane updates within the same renderloop, whereas legacy API would require one plane update per vsync.
Atomic API is useful when your application needs to blend content into overlays keeping all the updates within the same vsync. Still not all devices support this API and it could be unavailable on some older devices. KMS backend will by default use the legacy API, but you can enable the DRM atomic API with
QT_QPA_EGLFS_KMS_ATOMIC
environment variable set to 1.
Using a smaller framebuffer than screen resolution can also be useful. This is possible with DRM atomic using the
size
parameter in the JSON file. The example below uses a 1280x720 framebuffer on a 3840x2160 videomode :
{
"device": "/dev/dri/card0",
"outputs": [
{ "name": "HDMI1", "mode": "3840x2160", "size": "1280x720", "format": "argb8888" }
]
}
This backend, typically used on Tegra devices, is similar to the KMS/DRM backend mentioned above, except that it relies on the EGLDevice and EGLStream extensions instead of GBM.
For technical details about this approach, check out this presentation .
As of Qt 5.7 this backend shares many of its internal implementation with the GBM-based backend. This means that multiple screens and the advanced configuration via
QT_QPA_EGLFS_KMS_CONFIG
are supported. Some settings, such as
hwcursor
and
pbuffers
are not applicable however.
By default, the backend will automatically choose the correct EGL layer for the default plane of each output. When necessary, this can be overridden by setting the
QT_QPA_EGLFS_LAYER_INDEX
environment variable to the index of the desired layer. This approach does not currently support multiple outputs, so its usage should be limited to systems with a single screen. To see which layers are available, and to debug potential startup issues, enable the logging category
qt.qpa.eglfs.kms
.
In some cases, it may be necessary to perform a video mode set on application startup even when the screen reports that the desired resolution is already set. This is normally optimized away, but if the screen stays powered down, try setting the environment variable
QT_QPA_EGLFS_ALWAYS_SET_MODE
to a non-zero value and relaunch the application.
To configure the behavior of the EGLStream object used by the backend, use the
QT_QPA_EGLFS_STREAM_FIFO_LENGTH
environment variable. This assumes that
KHR_stream_fifo
is supported by the target system. By default, the stream operates in mailbox mode. To switch to FIFO mode, set a value of 1 or greater. The value specifies the maximum number of frames the stream can hold.
On some systems it may become necessary to target a specific overlay plane through a pre-defined connector. Just forcing a layer index via
QT_QPA_EGLFS_LAYER_INDEX
does not perform plane configuration and is therefore not suitable in itself. Instead, in such special scenarios use the
QT_QPA_EGLFS_KMS_CONNECTOR_INDEX
and
QT_QPA_EGLFS_KMS_PLANE_INDEX
environment variables. When these are set, only the specified connector and plane will be in use, all other outputs will get ignored. The backend will take care of picking the EGL layer that corresponds to the desired plane, and configuring the plane.
Touchscreens require additional considerations in multi-display systems because touch events have to be routed to the correct virtual screen, and this requires a correct mapping between touchscreens and display outputs.
The mapping is done via the JSON configuration file specified in
QT_QPA_EGLFS_KMS_CONFIG
and described in the previous sections. When a
touchDevice
property is present in an element of the
outputs
array, the value is treated as a device node and the touch device is associated with the display output in question.
For example, assuming our touchscreen has a device node of /dev/input/event5 and is a touchscreen integrated into the monitor connected via HDMI as the secondary screen, the following configuration ensures correct touch (and synthesized mouse) event translation:
{
"device": "drm-nvdc",
"outputs": [
{
"name": "HDMI1",
"touchDevice": "/dev/input/event5",
"virtualIndex": 1
},
{
"name": "DP1",
"virtualIndex": 0
}
]
}
注意：
When in doubt, enable logging from both the graphics and input subsystems by setting the environment variable
QT_LOGGING_RULES=qt.qpa.*=true
before launching the application. This will help identify the correct input device nodes and may uncover output configuration issues that can be difficult to debug otherwise.
注意：
As of Qt 5.14, the above is only supported for the evdevtouch and libinput backends. Other variants will continue to route events to the primary screen. To force the usage of evdevtouch on systems where multiple input backends are available, set the environment variable
QT_QPA_EGLFS_NO_LIBINPUT
to
1
.
Other backends, which are typically based on targeting the framebuffer or a composition API directly via the vendor's EGL implementation, usually provide limited or no support for multiple displays. On i.MX6-based boards with Vivante GPUs the
QT_QPA_EGLFS_FB
environment variable can be used to specify the framebuffer to target, similarly to linuxfb. On the Raspberry Pi the
QT_QPA_EGLFS_DISPMANX_ID
environment variable can be used to specify the screen to output to. The value corresponds to one of the
DISPMANX_ID_
constants, refer to the Dispmanx documentation. Note that these approaches, unlike KMS/DRM, will not typically allow output to multiple screens from the same application. Alternatively, driver-specific environment variables or kernel parameters may also be available as well to control the used framebuffer. Refer to the embedded board's documentation.
Systems with a fixed amount of dedicated video memory may need extra care before running Qt applications based on Qt Quick or classes like QOpenGLWidget . The default setting may be insufficient for such applications, especially when they are displayed on a high resolution (for example, full HD) screen. In this case, they may start failing in unexpected ways. It is recommended to ensure that there is at least 128 MB of GPU memory available. For systems that do not have a fixed amount of memory reserved for the GPU, this is not an issue.
使用
fb
plugin parameter to specify the framebuffer device to use.
The console-oriented platform plugins like eglfs and linuxfb install signal handlers by default to capture interrupt (
SIGINT
), suspend and continue (
SIGTSTP
,
SIGCONT
) and termination (
SIGTERM
). This way the keyboard, terminal cursor, and possibly other graphics state can be restored when the application terminates or gets suspended due to
kill
，或
Ctrl+C
or
Ctrl+Z
. (although terminating or suspending via the keyboard is only possible when
QT_QPA_ENABLE_TERMINAL_KEYBOARD
is set, as outlined above in the Input section). However, in some cases capturing
SIGINT
can be undesirable as it may conflict with remote debugging for instance. Therefore, the environment variable
QT_QPA_NO_SIGNAL_HANDLER
is provided to opt-out from all built-in signal handling.
Qt normally uses
fontconfig
to provide access to system fonts. If
fontconfig
is not available, Qt will fall back to using
QBasicFontDatabase
. In this case, Qt applications will look for fonts in Qt's
lib/fonts
directory. Qt will automatically detect pre-rendered fonts and TrueType fonts. This directory can be overridden by setting the
QT_QPA_FONTDIR
环境变量。
For more information on the supported formats, see Qt for Embedded Linux Fonts .
注意：
Qt no longer ships any fonts in the
lib/fonts
directory. This means that it is up to the platform (the system image) to provide the necessary fonts.
This is the X11 plugin used on regular desktop Linux platforms. In some embedded environments, that provide X and the necessary development files for xcb , this plugin functions just like it does on a regular PC desktop.
注意： On some devices there is no EGL and OpenGL support available under X because the EGL implementation is not compatible with Xlib. In this case the XCB plugin is built without EGL support, meaning that Qt Quick 2 or other OpenGL-based applications do not work with this platform plugin. It can still be used, however, to run software-rendered applications (based on QWidget for example).
As a general rule, the usage of XCB on embedded devices is not advisable. Plugins like eglfs are likely to provide better performance and hardware acceleration.
Wayland is a light-weight windowing system; or more precisely, it is a protocol for clients to talk to a display server.
Qt Wayland provides a
wayland
platform plugin that allows Qt applications to connect to a Wayland compositor.
更多细节，见 Wayland and Qt .
When performance is critical to your application, avoid the use of Qt modules that rely on software rendering, such as Qt Charts . Prefer modules that rely on hardware rendering instead, where possible.
遵循 最佳 QML 和 Qt Quick 实践 , especially with regards to including the QML CMake API , so that qmllint , QML 脚本编译器 (qmlsc), and the QML 类型编译器 (qmltc) are available. In addition, it's preferable to write declarative QML and minimize Javascript. For more information on how using excessive JavaScript may impact performance see QML 性能注意事项和建议 .
For drawing custom UI elements, use images/textures and 着色器效果 . Don't use the QML Canvas type. Shaders require hardware acceleration (GPU).
With Qt Quick, it's possible to use either hardware-accelerated or software-rendered backends. For complex Uls, using Qt Widgets on embedded targets is not recommended, as it will always use a software backend.
There are trade-offs here:
You need to be cautious with higher resolutions. Resolutions of 720p and higher may reduce performance.
Use a Window as your application's root element with the application's background color .
The reasoning for this is that a Window component has a color property that has the effect of being a buffer clear. Rendering the background using a full-screen Rectangle as an application's root Item would cause an additional draw call. For some RHI backends this may be the same thing, but there is a difference between a glClear call and drawing a quad. In most cases, a single, opaque image might not have a large performance impact but if you were to use an alpha value in that item's color, you might see a significant performance impact.