Thoughts dereferenced from the scratchpad noise. | Diving deep into Linux DRM bridge chaining

Introduction

Here are very popular schematics:

old-new-linux-graphics-stack

It shows in a simple and clear way the Linux Kernel Graphics Stack structure before and after September 2009 when the DRM system was introduced. From then, many drivers were rewritten to match the new environment and use the full potential of newly added functionalities.

But plenty of drivers still need to be ported for DRM system or have some key functionalities missing. This post describes a story of implementing a feature called bridge chaining.

DRM bridges chaining implementation process

One of the key features of the DRM system is the possibility to integrate several bridges into one video chain to convert from one video format to another. This feature is described in Linux Kernel documentation as still in-flux and not really fully sorted out yet, so, problems with implementation were expected. But nobody said it is going to be easy!

Structure

Hardware used:

i.mx8mmini SoC;
sn65dsi84 MIPI DSI to LVDS bridge;
it6263 LVDS to HDMI bridge.
HDMI panel.

The plan was to generate DSI video signal from SoC, convert it to LVDS, then convert it to HDMI, and, finally, feed the HDMI panel. The entire operation was being controlled by the system via the I2C interface to which all bridges were connected. So, the hardware structure is following:

bridge-chaining-hardware

Software used:

GNU/Linux v5.15;
Weston (the reference implementation of a Wayland server) v10.0.1.

Responsibilities for generating graphics among software were split into two parts; both use DRM system:

kernel space: responsible for generating graphics during boot (logo, console, etc.), is directly driven by Linux Kernel;
userspace: responsible for generating graphics after boot, is driven by several userspace clients and graphic server (Weston in this case).

The switch is performed when Weston is being loaded by systemd.

Implementation process

Hardware setup and configuration

Hardware setup was rather easy:

Connect all the needed wires;
Do not miss any data and clock I2C wires, otherwise it could be difficult to debug later.

So as devicetree configuration:

Enable needed subsystems (LCDIF, MIPI DSI, etc.);

Register bridges under chosen I2C buses and configure them, here is for example it6263 and sn65dsi84 configurations (for details, refer to it6263.c devicetree documentation and ti-sn65dsi83.c devicetree documentation):

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&i2c3 {
    lvds-to-hdmi-bridge@4c {
        compatible = "ite,it6263";
        reg = <0x4c>;
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_it6263_en>;
        reset-gpios = <&gpio5 2 GPIO_ACTIVE_LOW>;
        status = "okay";

        port {
            it6263_in: endpoint {
                remote-endpoint = <&lvds_out>;
            };
        };
    };

mipi_to_lvds: sn65dsi84@2c {
    compatible = "ti,sn65dsi83";
    reg = <0x2c>;
    enable-gpios = <&gpio1 6 GPIO_ACTIVE_HIGH>;
    interrupts-extended = <&gpio1 5 GPIO_ACTIVE_HIGH>;
    clocks = <&mipi_dsi 0>, <&clk IMX8MM_CLK_LCDIF_PIXEL>;
    clock-names = "mipi_clk", "pixel_clock";
    display = <&display_subsystem>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_i2c1_sn65dsi84>;
    sync-delay = <512>;
    dsi-lanes = <4>;
    status = "okay";

    lvds_ports: ports {
        #address-cells = <1>;
        #size-cells = <0>;

        port@0 {
            reg = <0>;
            lvds_in: endpoint {
                remote-endpoint = <&mipi_out>;
                data-lanes = <1 2 3 4>;
            };
        };

        port@2 {
            reg = <2>;
            lvds_out: endpoint {
                remote-endpoint = <&it6263_in>;
                attach-bridge;
        };
    };
};

Configure power controllers (for details, refer to the appropriate devicetree documentation):

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&regulators {
    reg_lvds_pwr: lvds_pwr {
        compatible = "regulator-fixed";
        pinctrl-names = "default";
        regulator-name = "lvds_pwr_en";
        pinctrl-0 = <&pinctrl_lcd_3v3_enable>;
        gpio = <&gpio4 29 GPIO_ACTIVE_HIGH>;
        enable-active-high;
        regulator-boot-on;
        regulator-always-on;
    };
    reg_5v_bl: 5v_bl {
        compatible = "regulator-fixed";
        pinctrl-names = "default";
        regulator-name = "5v_bl_en";
        pinctrl-0 = <&pinctrl_lcd_5v_enable>;
        gpio = <&gpio5 5 GPIO_ACTIVE_HIGH>;
        enable-active-high;
        regulator-boot-on;
        regulator-always-on;
    };
};

Add i.mx input-output multiplexers (for details, refer to the appropriate devicetree documentation):

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&iomuxc {
    pinctrl_lcd_3v3_enable: lcd_3v3_en {
        fsl,pins = <
            MX8MM_IOMUXC_SAI3_RXC_GPIO4_IO29 0x19
        >;
    };
    pinctrl_lcd_5v_enable: lcd_5v_en {
        fsl,pins = <
            MX8MM_IOMUXC_SPDIF_EXT_CLK_GPIO5_IO5 0x19
        >;
    };
    pinctrl_it6263_en: it6263_en {
        fsl,pins = <
            MX8MM_IOMUXC_SAI3_MCLK_GPIO5_IO2 0x19
        >;
    };
    pinctrl_i2c1_sn65dsi84: i2c1-sn65dsi83grp {
        fsl,pins = <
            MX8MM_IOMUXC_GPIO1_IO05_GPIO1_IO5 0x04
            MX8MM_IOMUXC_GPIO1_IO06_GPIO1_IO6 0x106
        >;
    };
};

After above changes both chips were detected on I2C buses, here is sn65dsi84 on i2c-0 under address 2c:

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# i2cdetect -y 0
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         UU -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- 2c -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Drivers

Then, it was high time to add appropriate drivers to the image. In theory, after adding drivers, the entire graphic chain presented in Structure chapter above should work, with no need for additional kernel or Weston configuration. Let’s see then.

Following drivers were chosen to drive this chain:

for sn65dsi84: driver from varigit/linux-imx repository
for it6263: driver from nxp-imx/linux-imx repository

Checking whether the system recognized the drivers may be done via sysfs:

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# ls /sys/bus/i2c/drivers/ | grep -E '(it6263|sn65dsi83)'
it6263
sn65dsi83

Bridge chaining

The first boot result was not very pleasurable: several errors in dmesg from sn65dsi84 driver and no messages from it6263 driver. No video output was noticed as well:

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# dmesg | grep it6263
# dmesg | grep sn65dsi84
[    3.575954] sn65dsi84 0-002c: even_odd_swap property not found, using default
[    3.583172] sn65dsi84 0-002c: lvds-channels property not found, using default
[    3.590351] /soc@0/bus@30800000/i2c@30a20000/sn65dsi84@2c: could not find display-timings node
[    3.598985] /soc@0/bus@30800000/i2c@30a20000/sn65dsi84@2c: no timings specified
(...)

Ok then, all sn65dsi84 errors were fixed, it was a matter of devicetree configuration. But still no video on the panel.

Note: the above errors and the way they were fixed are not explained because the sn65dsi84 driver from varigit/linxu-imx repository, the source of these messages, will be replaced by another. This driver is presented here only to demonstrate that some drivers are not entirely compatible with the DRM system.

Several checks were done to check every part of the system separately:

Checking sn65dsi84 with LVDS panel proved correct driver and hardware configuration by showing some video output;
Checking communication via I2C with it6263 proved correct connection (some register write/read operations were done);
Checking the HDMI panel proved correct connection and functionality by showing video output connected to another platform.

Still, there are no clues as to where the problem is.

While checking the code of the sn65dsi84 driver from the varigit/linux-imx repository the absence of a call to drm_bridge_attach() function in the attach function was noticed. So, it6263 simply was not attached to the chain by sn65dsi84 as it should be. After several experiments decision to use sn65dsi84 driver from nxp-imx/linux-imx repository was made, here is a call to drm_bridge_attach() from the repository:

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static int sn65dsi83_attach(struct drm_bridge *bridge,
                enum drm_bridge_attach_flags flags)
{
    struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);

    (...)

    return drm_bridge_attach(bridge->encoder, ctx->panel_bridge,
                &ctx->bridge, flags);
    (...)
}

So, after performing all operations needed to attach sn65dsi84 to the previous bridge or encoder in the chain this call executes the next bridge’s attach function.

After switching to another sn65dsi84 driver, both bridges were successfully probed and recognized by the DRM system:

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# cat /sys/bus/i2c/devices/*c/name
sn65dsi84
(...)
it6263
# dmesg | grep it6263
[    3.130625] it6263_probe has been called
[    3.131740] it6263_reset has been called
(...)
[    3.694544] it6263_bridge_attach has been called
(...)
# dmesg | grep sn65dsi84
[    3.658664] sn65dsi83_probe has been called
[    3.661588] sn65dsi83_parse_dt has been called
(...)
[    3.689746] sn65dsi83_attach has been called
(...)

Note: dmesg logs from above are provided by adding printk() to the drivers source code.

After further analysis, the following problems were pointed out:

No video output on the panel during boot as well as in userspace;

Weston argues that one drm_connector does not have CRTC assigned:

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[11:44:08.403] weston 10.0.1
(...)
[11:44:09.436] Output 'DSI-1' using color profile: built-in default sRGB SDR profile
[11:44:09.444] Output DSI-1 (crtc 33) video modes:
               1280x720@59.9 16:9, current, 74.2 MHz
[11:44:09.444] Output 'DSI-1' enabled with head(s) DSI-1
[11:44:09.444] Module '/usr/lib/libgbm.so' already loaded
[11:44:09.444] Output 'HDMI-A-1' using color profile: built-in default sRGB SDR profile
[11:44:09.444] Output 'HDMI-A-1': No available CRTCs.
[11:44:09.444] Enabling output "HDMI-A-1" failed.
[11:44:09.444] Error: cannot enable output 'HDMI-A-1' without heads.

Weston issue

According to Linux Kernel DRM/KMS system documentation the KMS structure should be following:

kms-structure

The red-marked drm_connector structure is surplus and is causing the Weston issues.

According to comments in drm_bridge_connector.c and to comments in drm_bridge.c file, to create drm_connector for a chain of bridges, DRM drivers should match the following requirements:

Encoder or display controller drivers should:
- Create drm_connector structure and initialize it by using drm_bridge_connector_init() function from drm_bridge_connector.c;
Bridge drivers should:
- Export following functions via drm_bridge_funcs (some of them are optional): drm_bridge_funcs.detect(), drm_bridge_funcs.get_modes(), drm_bridge_funcs.get_edid(), drm_bridge_funcs.get_edid(), drm_bridge_funcs.hpd_enable() and drm_bridge_funcs.hpd_disable().;
- Export its type and functionalities via flags drm_bridge.typeand drm_bridge.ops
- Implement conditional connector creation or not create connectors by themselves.

To implement the above requirements the following files were modified:

sec-dsim.c (driver for MIPI DSI transmitter) - to implement the creation of drm_connector structure for the chain of bridges;
ti-sn65dsi84.c - to export appropriate functions via drm_bridge_funcs and to export type and functionalities via drm_bridge.typeand drm_bridge.ops;
it6263.c - to export appropriate functions via drm_bridge_funcs, implement conditional creation of drm_connector structure and to export type and functionalities via drm_bridge.typeand drm_bridge.ops.

Weston has not argued anymore about connectors:

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(...)
[13:52:54.907] weston 10.0.1
(...)
[13:38:50.377] Output 'HDMI-A-1' using color profile: built-in default sRGB SDR profile
[13:38:50.386] Output HDMI-A-1 (crtc 33) video modes:
               1280x720@60.0, preferred, current, 64.0 MHz
               1280x720@60.0 16:9, 74.2 MHz
               1280x720@50.0 16:9, 74.2 MHz
               832x624@74.6, 57.3 MHz
               800x600@75.0, 49.5 MHz
               640x480@75.0, 31.5 MHz
               640x480@72.8, 31.5 MHz
               640x480@66.7, 30.2 MHz
               720x400@70.1, 28.3 MHz
[13:38:50.386] Output 'HDMI-A-1' enabled with head(s) HDMI-A-1
[13:38:50.386] Compositor capabilities:
               arbitrary surface rotation: yes
               screen capture uses y-flip: yes
               cursor planes: yes
               arbitrary resolutions: no
               view mask clipping: yes
               explicit sync: yes
               color operations: no
               presentation clock: CLOCK_MONOTONIC, id 1
               presentation clock resolution: 0.000000001 s
[13:38:50.397] Loading module '/usr/lib/weston/desktop-shell.so'
[13:38:50.413] launching '/usr/libexec/weston-keyboard'
[13:38:50.417] Warning: support for deprecated wl_shell interface is enabled. Please migrate legacy clients to xdg-shell.
[13:38:50.420] Loading module '/usr/lib/weston/screen-share.so'
[13:38:50.426] Loading module '/usr/lib/weston/systemd-notify.so'
[13:38:50.427] info: add 1 socket(s) provided by systemd
[13:38:50.427] launching '/usr/libexec/weston-desktop-shell'
(...)

But still no video output on the panel.

No video output issue

For now, the KMS infrastructure looks like this:

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# modeset
(...)
Encoders:
id      crtc    type    possible crtcs  possible clones
34      33      DSI     0x00000001      0x00000001

Connectors:
id      encoder status          name            size (mm)       modes   encoders
35      34      connected       HDMI-A-1        340x190         9       34
  modes:
        index name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot
  #0 1280x720 60.00 1280 1328 1360 1440 720 727 735 741 64020 flags: phsync, pvsync; type: preferred, driver
  #1 1280x720 60.00 1280 1390 1430 1650 720 725 730 750 74250 flags: phsync, pvsync; type: driver
  #2 1280x720 50.00 1280 1720 1760 1980 720 725 730 750 74250 flags: phsync, pvsync; type: driver
  #3 832x624 74.55 832 864 928 1152 624 625 628 667 57284 flags: nhsync, nvsync; type: driver
  #4 800x600 75.00 800 816 896 1056 600 601 604 625 49500 flags: phsync, pvsync; type: driver
  #5 640x480 75.00 640 656 720 840 480 481 484 500 31500 flags: nhsync, nvsync; type: driver
  #6 640x480 72.81 640 664 704 832 480 489 492 520 31500 flags: nhsync, nvsync; type: driver
  #7 640x480 66.67 640 704 768 864 480 483 486 525 30240 flags: nhsync, nvsync; type: driver
  #8 720x400 70.08 720 738 846 900 400 412 414 449 28320 flags: nhsync, pvsync; type: driver
  props:
        1 EDID:
                flags: immutable blob
                blobs:

                value:
                        00ffffffffffff002074560100000000
                        141f010380221378fe6435a5544f9e27
                        125054bfea0061400101010101010101
                        010101010101021900a050d015203020
                        (...)

CRTCs:
id      fb      pos     size
33      40      (0,0)   (1280x720)
  #0 1280x720 60.00 1280 1328 1360 1440 720 727 735 741 64020 flags: phsync, pvsync; type: preferred, driver
(...)

Planes:
id      crtc    fb      CRTC x,y        x,y     gamma size      possible crtcs
31      33      40      0,0             0,0     0               0x00000001
  formats: XR24 AR24 RG16 XB24 AB24 RX24 RA24 AR15 XR15 AB15 XB15 BG16
(...)

Quick summary:

System correctly recognizes the chain;
EDID is being read and interpreted;
Connector got nine modes from EDID with one set as preferred;
Preferred mode is set as current;
All elements of the chain are connected to each other and have expected information attached to it;
No video on the HDMI panel during boot as well as in userspace.

After several days in complete darkness, a light in the tunnel was found. During one of many experiments, penguins (Linux Kernel logo) showed up during boot. It happened after forcing the system to use a mode with a specific pixel clock frequency. All other modes were checked to prove that the issue was hidden in modes. To force the kernel, video command line parameter was used; on the other hand, to force Weston, weston.ini was used.

Additionally following changes were added:

Commit c35a6e6c8c59e43b5080a6c77c2866ed9e01a77b from nxp-ixm/linux-imx repository;

Changed sn65dsi84 configuration from burst video mode to burst with sync pulses video mode:

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diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index d0a5c5328eae..62058ee91b0a 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -274,7 +274,7 @@ static int sn65dsi83_attach(struct drm_bridge *bridge,

        dsi->lanes = ctx->dsi_lanes;
        dsi->format = MIPI_DSI_FMT_RGB888;
-       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST;
+       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_SYNC_PULSE;

        ret = mipi_dsi_attach(dsi);
        if (ret < 0) {

And… That it, the panel shown perfect picture during boot as well as in userspace! It seems that connector had not only suitable modes for used hardware but nonsuitable as well.

DRM system has tools to filter out inappropriate modes. Every bridge driver should specify limitations to modes in drm_bridge_funcs.mode_valid() function. After doing so, the list of modes in the connector was limited to only two modes, so the system had only two choices:

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# modetest
(...)
Connectors:
id      encoder status          name            size (mm)       modes   encoders
35      34      connected       HDMI-A-1        340x190         2       34
  modes:
        index name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot
  #0 1280x720 60.00 1280 1390 1430 1650 720 725 730 750 74250 flags: phsync, pvsync; type: driver
  #1 1280x720 50.00 1280 1720 1760 1980 720 725 730 750 74250 flags: phsync, pvsync; type: driver
(...)

In such cases the Linux Kernel sets as current the mode with the bigger resolution and frame rate. As a result, the mode with a resolution 1280x720 and a frequency of 60 Hz was chosen:

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# modetest
(...)
CRTCs:
id      fb      pos     size
33      40      (0,0)   (1280x720)
  #0 1280x720 60.00 1280 1390 1430 1650 720 725 730 750 74250 flags: phsync, pvsync; type: driver
(...)

Summary

The DRM system is a great step forward compared to big all-in-one graphic servers. The idea of its abstractions presents that even such complex systems can be built and described in a straightforward way even for newbies, it still requires some attention and care from developers, though.

For those who want to dive deeper into the DRM system, I recommend the following resources:

Daniil Klimuk

Embedded Systems Trainee. Likes to dig in some tech.

Diving deep into Linux DRM bridge chaining

Introduction

DRM bridges chaining implementation process

Structure

Implementation process

Hardware setup and configuration

Drivers

Bridge chaining

Weston issue

No video output issue

Summary

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