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Goals
The purpose of this page is to help educate about display output on upstream Linux (not to be confused with the 'downstream' vendor kernel provided in Yocto).
Direct Rendering Manager (DRM)
DRM is the driver subsystem in the kernel that is able to communicate with video drivers, such as the Freescale I.MX6 processors. Through ioctl() calls, multiple userspace programs can draw to a display at any given time as DRM will manage these requests. DRM is also able to interface with the graphics processor (GPU) in order to hardware accelerate these requests. This is separate from framebuffer in that it only allowed raw writes to a display. DRM exposes some information via sysfs in the /sys/class/drm directory.
For example, to retrieve info about the DRM devices found on your system (Gateworks GW540x Linux 4.18):
root@bionic-armhf:~# ls /sys/class/drm
card0 card1 card1-HDMI-A-1 card1-LVDS-1 renderD128 version
# show
- card0 is from etnaviv the GPU driver (if enabled)
- card1 is from the IPU display driver
Kernel Mode Settings (KMS)
KMS is part of DRM, but solely focuses on configuring display modes (i.e. screen resolution, refresh rate etc).
When booting your kernel, mode settings are read by what appears in the kernel command line, else defaults are chosen. DRM exports the available modes for a given display in the DRM 'modes' file:
root@bionic-armhf:~# for i in $(ls -1 /sys/class/drm/*/modes); do echo "$i:"; cat $i; done /sys/class/drm/card1-HDMI-A-1/modes: 1920x1080 1280x1024 1440x900 1280x800 1152x864 1024x768 1024x768 800x600 800x600 640x480 640x480 720x400 /sys/class/drm/card1-LVDS-1/modes:
- the above shows that the HDMI is connected to a 1080p capable monitor with several resolutions available and that the board also has an LVDS connector but it is disabled
Mode settings can be passed to the kernel via the video= parameter from within U-Boot. The format of this is as follows: video=<conn>:<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]
Note the following variables (see also modedb.txt):
<conn>: Connector<xres> x <yres>: ResolutionM: Display timings will be calculated using VESA(TM) Coordinated Video Timings instead of a lookup table (Please see modedb.txt for more details)R: Do "Reduce Blanking" calculation for digital displays-<bpp>: color depthi: Calculate timings for an interlaced Modem: Add margins to the calculation (1.8% of xres rounded down to 8 pixels and 1.8% of yres)e: Output Forced OnD: Output Forced On and use Digital Outputsd: Output Forced Off
Examples:
- Enable LVDS for a 1280x800 display and disable HDMI:
setenv video 'video=LVDS-1:1280x800@60M video=HDMI-A-1:d' - Enable HDMI for a 800x600 display with 60Hz refresh rate and disable LVDS:
setenv video 'video=LVDS-1:d video=HDMI-A-1:800x600@60' - Disable both the LVDS and HDMI outputs on a Gateworks GW540x:
setenv video 'video=LVDS-1:d video=HDMI-A-1:d'
Limitations
Using the upstream kernel currently has some limitations on the imx platform. For example, the driver is limited to the number of display outputs it can do. That means if both LVDS and HDMI were connected, both resolutions would be scaled to the display with the lower resolution. Both outputs are also mirror's of each other and cannot be run independently. However, using the 'downstream' vendor kernel that we provide on our Yocto BSPs do have features that allow for independently controlled displays, even allowing for an 'overlay' buffer for the first two displays.
To see more on the downstream vendor kernel provided in Yocto, please see here.
