DRM/KMS framework on STM32MP157f without the ltdc peripheral

[KChar.1]

Hey there,

I am working with a custom board based on an OSD32MP157F SoM. The pinout is very similar with the DK2 board with some minor differences, mainly the DSI_RESET pin. I have done several tests on STM32MP157f-dk2 boards with an eDP display that connects through a MIPI/eDP bridge to the DSI port of the DK. This works fine for a while now.

In the custom board I do not have included any ltdc peripheral as I thought that it is not needed for driving the DSI bridge. I made a new image with the corresponding pinout and the bridge i2c driver initializes normally as seen in the bootlog.

[ 1.735336] lt8911exb 0-0029: Success request pwr-gpio
[ 1.740362] lt8911exb 0-0029: Success request rst-gpio
[ 1.747446] lt8911exb 0-0029: LT8911EXB i2c test success chipid: 0xe0517
[ 1.785822] lt8911exb 0-0029: LT8911 tx pll locked
[ 1.833083] lt8911exb 0-0029: video check: Vtotal = 0
[ 1.838486] lt8911exb 0-0029: video check: Hact(word counter) = 0
[ 1.844158] lt8911exb 0-0029: video check: Vact = 0
[ 2.143622] lt8911exb 0-0029: LT8911EXB setup finish.

But when I try "modtest -M stm" I get a

failed to open device 'stm': No such file or directory

I investigated the entries in /dev and there is no drm entry either.

During troubleshooting I found this post from a few years ago that shows great similarities with my current issue. Looking at the LTDC wiki I can see that the LTDC binds to the DRM/KMS framework. My main hypothesis at the moment is that the DRM framework never initializes since there is no LTDC entry or peripheral. I was wondering if this hypothesis is correct and if yes if there are any ways of using the DRM framework only for DSI.

I attach some parts of the device tree here in case they are helpful. Any ideas or suggestions are highly appreciated

[PatrickF]

Hi @KChar.1​ ,

DSI get their pixel internally from LTDC, so LTDC is mandatory (without any pins if used with DSI).

I'n not DT expert, but you might have a look to https://wiki.st.com/stm32mpu/wiki/LTDC_device_tree_configuration with examples used for DK2 boards

Note that on our boards, we usually define the LTDC pins as we have the options to use // display (e.g. for RGB->HDMI bridge)

Regards

[KChar.1]

Hey @PatrickF​ , thank you so much for this! Indeed I was confusing the hdmi_transmitter with the LTDC. I declared the ltdc without any pins on my DT and I can see that modetest -M works but it can not find any panels:

Encoders:
id crtc type possible crtcs possible clones
 
Connectors:
id encoder status name size (mm) modes encoders
 
CRTCs:
id fb pos size
35 0 (0,0) (0x0)
 #0 nan 0 0 0 0 0 0 0 0 0 flags: ; type: 
 props:
 24 VRR_ENABLED:
 flags: range
 values: 0 1
 value: 0
 28 GAMMA_LUT:
 flags: blob
 blobs:
 
 value:
 29 GAMMA_LUT_SIZE:
 flags: immutable range
 values: 0 4294967295
 value: 256
 
Planes:
id crtc fb CRTC x,y x,y gamma size possible crtcs
31 0 0 0,0 0,0 0 0x00000001
 formats: AR24 XR24 RG24 RG16 AR15 XR15 AR12 XR12 C8 
 props:
 8 type:
 flags: immutable enum
 enums: Overlay=0 Primary=1 Cursor=2
 value: 1
 30 IN_FORMATS:
 flags: immutable blob
 blobs:
 
 value:
 01000000000000000900000018000000
 01000000400000004152323458523234
 52473234524731364152313558523135
 41523132585231324338202000000000
 ff010000000000000000000000000000
 0000000000000000
 in_formats blob decoded:
 AR24: LINEAR
 XR24: LINEAR
 RG24: LINEAR
 RG16: LINEAR
 AR15: LINEAR
 XR15: LINEAR
 AR12: LINEAR
 XR12: LINEAR
 C8 : LINEAR
 33 alpha:
 flags: range
 values: 0 65535
 value: 65535
 34 zpos:
 flags: immutable range
 values: 0 0
 value: 0
36 0 0 0,0 0,0 0 0x00000001
 formats: AR24 RG24 RG16 AR15 AR12 C8 
 props:
 8 type:
 flags: immutable enum
 enums: Overlay=0 Primary=1 Cursor=2
 value: 0
 30 IN_FORMATS:
 flags: immutable blob
 blobs:
 
 value:
 01000000000000000600000018000000
 01000000300000004152323452473234
 52473136415231354152313243382020
 3f000000000000000000000000000000
 0000000000000000
 in_formats blob decoded:
 AR24: LINEAR
 RG24: LINEAR
 RG16: LINEAR
 AR15: LINEAR
 AR12: LINEAR
 C8 : LINEAR
 38 alpha:
 flags: range
 values: 0 65535
 value: 65535
 39 zpos:
 flags: immutable range
 values: 1 1
 value: 1
 
Frame buffers:
id size pitch

When I try to load the panel manually with

# modprobe panel-orisetech-otm8009a

I get no errors.

I can see that there are no connectors on the modetest which makes me think that the dsi is not recognised. I leave my dsi dt entry below:

&dsi {
	 status = "okay";
 
	 ports {
		 port@0 {
			 reg = <0>;
			 dsi_in: endpoint {
				remote-endpoint = <&ltdc_ep1_out>;
				//remote-endpoint = <&dsi_in>;
			 };
		 };
 
		 port@1 {
			 reg = <1>;
			 dsi_out: endpoint {
				 remote-endpoint = <&panel_in>;
			 };
		 };
	 };
 
 
 
	 panel_otm8009a: panel-otm8009a@0 {
		 compatible = "orisetech,otm8009a";
		 reg = <0>;
		 reset-gpios = <&gpioe 9 GPIO_ACTIVE_LOW>;
		 power-supply = <&v3v3>;
		 status = "okay";
		 
		 port {
			 panel_in: endpoint {
				 remote-endpoint = <&dsi_out>;
			 };
			 
		 };
	 };
 };

Any ideas on how to continue troubleshooting the issue?

[KChar.1]

I looked a bit closer to this and I realized that after changing the ldtc entry I forgot to change the DSI_RST entry to the correct config. Works fine now! Thank you so much @PatrickF​