• Backup Information
  • DRM/KMS
  • FBDEV
• Step-to-step Procedure from Scratch
  • 1. Get Linux Kernel Source
  • 2. Get and Merge the TMDS Driver
    • 2.1. (If you haven’t done it) Include the Digilent Dynamic Clock Driver
  • 3. Setup the Petalinux Project
  • 4. Configure the Device Tree
  • 5. Build and Run
• Debugging Tools
  • modetest
• Possible Problems

Backup Information#

DRM/KMS#

A brief description of Linux DRM/KMS by ST:

The DRM/KMS framework is dedicated to the management of the display, graphic and composition subsystems. With the help of other Linux multimedia frameworks and applications, the DRM/KMS framework is typically used:

• to compose animated contents taking advantages of the hardware acceleration.
• to control both display interfaces and external displays including their settings (resolution, frequency, multi-screen…).
• to display this animated contents on display panels or HDMI outputs.

There’re some slides that give a deeper insight. To me it is just a pipeline consisting:

• Framebuffer: something manipulates with the GPU/VRAM hardware
• Planes: like display layers
• CRTC: that controls the timing and outputs composited planes
• Encoder: that encodes the raw data by certain protocol
• Bridge: functional blocks inserted in the pipeline (often refers to PHY hardwares?)
• Connector: to the display device
• Panel: an abstraction of the display device

Notice that the DRM/KMS pipeline is somehow abstract, e.g. an encoder block contains not only the driver, but also hardware resources that the actual video data is flowing through.

In the example of Zynq, Xilinx provides a module pl_disp that provides framebuffer, CRTC and planes. I think it corresponds to the AXI_VTC and VDMA/frmbuf_rd IP cores. The hardware that fits the idea of encoder and connector already exists, but cannot be recognized by the Linux system. So what we gonna do is to implement a driver for them.

FBDEV#

Short for Framebuffer Device. The Framebuffer structure is the predecessor of the DRM/KMS, it’s not actively maintained nowadays, or somehow obsolete. The X Windows System we will use later, however, is based on fbdev drivers. It confuses me a lot that the X system seems to run well even through I have only built a DRM driver.

I’ve found this in the Linux kernel doc:

Drivers implementing fbdev emulation with the helpers can call drm_fb_helper_hotplug_changed from this hook to inform the fbdev helper of output changes.

From that I would guess that the DRM subsystem is able to feign itself as a fbdev for the old programs.

Step-to-step Procedure from Scratch#

Environment reference:

Petalinux : v2023.1
linux-xlnx : v5.4 (xlnx_rebase_v5.4)
OS : Ubuntu 20.04 LTS
Hardware Platform : x86_64

1. Get Linux Kernel Source#

Clone the repository of Linux kernel for Xilinx SoCs.

git clone https://github.com/Xilinx/linux-xlnx.git

Checkout to desired kernel version, v5.4 in this example.

git checkout xlnx_rebase_v5.4

2. Get and Merge the TMDS Driver#

Clone the repo of my DRM-TMDS driver:

git clone https://github.com/tsaoo/drm-tmds-pl-drv.git
cd drm-tmds-pl-drv

Checkout to MATCH the kernel version selected before.

# Check the git page for all available version
git checkout v5.4

Copy the source files to the Linux kernel. Replace the ${KERNEL} below with the absolute location of the linux-xlnx kernel root directory.

cp drivers/gpu/drm/xlnx/* ${KERNEL}/drivers/gpu/drm/xlnx/

Edit the Kconfig and the Makefile so that the driver can be included when the kernel is compiled.

nano ${KERNEL}/drivers/gpu/drm/xlnx/Kconfig

# Add the following lines somewhere:
config DRM_TMDS_PL_DRV
	tristate "DRM TMDS on PL"
	depends on DRM_XLNX
	help
	  DRM driver for TMDS pipeline on PL

nano ${KERNEL}/drivers/gpu/drm/xlnx/Makefile

# Append the following line to the end of the makefile:
obj-$(CONFIG_DRM_TMDS_PL_DRV) += drm-tmds-pl-drv.o

2.1. (If you haven’t done it) Include the Digilent Dynamic Clock Driver#

Check the Digilent official repo for their dynamic clock IP core:

cp drivers/clk/clk-dglnt-dynclk.c ${KERNEL}/drivers/clk/

nano ${KERNEL}/drivers/clk/Kconfig

# Insert the following lines somewhere:
config CLK_DGLNT_DYNCLK
	tristate on ARCH_ZYNQ || MICROBLAZE
	help
		---help---
		Support for the Digilent AXI Dynamic Clock core for Xilinx FPGAs.

nano ${KERNEL}/drivers/clk/Makefile

# Add the this line to the end:
obj-$(CONFIG_CLK_DGLNT_DYNCLK) += clk-dglnt-dynclk.o

3. Setup the Petalinux Project#

Configure Petalinux project to use the modified local Linux kernel.

petalinux-config

Navigate and configure:

• Linux Components Selection --->
  • linux-kernel --->
    • (X) ext-local-src
  • External linux-kernel local source settings --->
    • External linux-kernel local source path = /…/linux-xlnx_rebase_v5.4
    • External linux-kernel license checksum = file:///…/linux-xlnx_rebase_v5.4/LICENSES/preferred/MIT;md5=xxxx

It is (might be) optional to designate the license file and its checksum, but I always do so. The MD5 checksum can be obtained by: md5sum MIT.

NOTE

Don’t forget the “file://” at the head of the license file path

Every Petalinux version has a corresponding linux-xlnx kernel version, and there’s a version-checking procedure to prevent mismatch. But you CAN build different kernel though, simply disable the checking step.

## In early Petalinux version the file or directory may not exist, create them anyway
nano ${PROJECT}/project-spec/meta-user/recipes-kernel/linux/linux-xlnx_%.bbappend

## Append this line:
KERNEL_VERSION_SANITY_SKIP="1"

Then configure the kernel building procedure to include the TMDS driver.

petalinux-config -c kernel

Navitage and configure:

• Devices Drivers --->
  • <*> Digilent axi_dynclk Driver
  • Graphics support --->
    • <*> Xilinx DRM KMS Driver
      • <*> DRM Driver for TMDS on
      • <*> Xilinx DRM KMS bridge
      • <*> Xilinx DRM PL display driver
    • <*> Xilinx DRM VTC Driver

It is not harmful to turn on everything under the Xilinx DRM KMS subsection, and enable framebuffer device support under the Graphics support. But I’m not sure whether they are necessary. If you are using kernel version 4.x or before, it is likely that there’s a Xilinx DRM option in addition to Xilinx DRM KMS Driver. It would suggest you disable the former.

4. Configure the Device Tree#

Edit the device tree referring to dts_example.dtsi in the repo.

nano ${PROJECT}/project-spec/meta-user/recipes-bsp/device-tree/files/system-user.dtsi

5. Build and Run#

Use petalinux-build and petalinux-package as normal.

Debugging Tools#

modetest#

Provided by libdrm. It is used to poll the sectors in the DRM/KMS subsystem. libdrm is optional in the Petalinux rootfs.

petalinx-config -c rootfs

Navigate and configure:

• Filesystem Packages --->
  • X11 --->
    • base --->
      • libdrm --->
        • [*] libdrm
        • [*] libdrm-tests

I also enabled other stuff in libdrm subsection, just in case.

Check a blog in Chinese for more info.

Some frequent uses of mine:

# To poll every part in the DRM subsystem:
modetest -M xlnx

# To test the connector id 30 and crtc id 28 in 1024x768 size and RG24 format:
modetest -M xlnx -s 30@28:1024x768@RG24

Possible Problems#

TODO

I remember Petalinux would throw an error when build the kernel of v4.14. Some sorf of xxx_shipped.l file in the kernel must be edited, but I can’t tell which by now. Record it if come across again.