Driving a 137,930 KHz 1080x1920 portrait HDMI screen from an IMX8M

Question

I need to make a fixed-frequency screen (with a new custom mode line):

{ DRM_MODE("1080x1920", DRM_MODE_TYPE_DRIVER, 137930,
            1080, 1140, 1150, 1185, 0,
            1920, 1928, 1932, 1940, 0,
            DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC),
            .vrefresh = 60, .picture_aspect_ratio = HDMI_PICTURE_ASPECT_16_9, },

(in edid_cea_modes[] in imx-hdp.c) work with the iMX8M.

My problem is that there is no 137,930 KHz mode in

/* Table 8. HDMI TX clock control settings (pixel clock is output) */
const u32 t28hpc_hdmitx_clock_control_table_pixel_out[T28HPC_HDMITX_CLOCK_CONTROL_TABLE_ROWS_PIXEL_OUT][T28HPC_HDMITX_CLOCK_CONTROL_TABLE_COLS_PIXEL_OUT] = {
{ 27000, 1000, 270000, 0x03, 0x1, 0x1, 240, 0x0BC, 0x030, 80, 0x026, 0x026, 2160000, 0, 2, 2, 2, 4, 135000, 0x3, 27000, 1},
{ 27000, 1250, 337500, 0x03, 0x1, 0x1, 300, 0x0EC, 0x03C, 100, 0x030, 0x030, 2700000, 0, 2, 2, 2, 4, 168750, 0x3, 33750, 1},
{ 27000, 1500, 405000, 0x03, 0x1, 0x1, 360, 0x11C, 0x048, 120, 0x03A, 0x03A, 3240000, 0, 2, 2, 2, 4, 202500, 0x3, 40500, 1},
{ 27000, 2000, 540000, 0x03, 0x1, 0x1, 240, 0x0BC, 0x030, 80, 0x026, 0x026, 2160000, 0, 2, 2, 2, 4, 270000, 0x2, 54000, 1},
{ 54000, 1000, 540000, 0x03, 0x1, 0x1, 480, 0x17C, 0x060, 80, 0x026, 0x026, 4320000, 1, 2, 2, 2, 4, 270000, 0x3, 54000, 1},
{ 54000, 1250, 675000, 0x04, 0x1, 0x1, 400, 0x13C, 0x050, 50, 0x017, 0x017, 2700000, 0, 1, 1, 2, 4, 337500, 0x2, 67500, 1},
{ 54000, 1500, 810000, 0x04, 0x1, 0x1, 480, 0x17C, 0x060, 60, 0x01C, 0x01C, 3240000, 0, 2, 2, 2, 2, 405000, 0x2, 81000, 1},
{ 54000, 2000, 1080000, 0x03, 0x1, 0x1, 240, 0x0BC, 0x030, 40, 0x012, 0x012, 2160000, 0, 2, 2, 2, 1, 540000, 0x1, 108000, 1},
{ 74250, 1000, 742500, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 80, 0x026, 0x026, 5940000, 1, 2, 2, 2, 4, 371250, 0x3, 74250, 1},
{ 74250, 1250, 928125, 0x04, 0x1, 0x1, 550, 0x1B4, 0x06E, 50, 0x017, 0x017, 3712500, 1, 1, 1, 2, 4, 464062, 0x2, 92812, 1},
{ 74250, 1500, 1113750, 0x04, 0x1, 0x1, 660, 0x20C, 0x084, 60, 0x01C, 0x01C, 4455000, 1, 2, 2, 2, 2, 556875, 0x2, 111375, 1},
{ 74250, 2000, 1485000, 0x03, 0x1, 0x1, 330, 0x104, 0x042, 40, 0x012, 0x012, 2970000, 0, 2, 2, 2, 1, 742500, 0x1, 148500, 1},
{ 99000, 1000, 990000, 0x03, 0x1, 0x1, 440, 0x15C, 0x058, 40, 0x012, 0x012, 3960000, 1, 2, 2, 2, 2, 495000, 0x2, 99000, 1},
{ 99000, 1250, 1237500, 0x03, 0x1, 0x1, 275, 0x0D8, 0x037, 25, 0x00B, 0x00A, 2475000, 0, 1, 1, 2, 2, 618750, 0x1, 123750, 1},
{ 99000, 1500, 1485000, 0x03, 0x1, 0x1, 330, 0x104, 0x042, 30, 0x00D, 0x00D, 2970000, 0, 2, 2, 2, 1, 742500, 0x1, 148500, 1},
{ 99000, 2000, 1980000, 0x03, 0x1, 0x1, 440, 0x15C, 0x058, 40, 0x012, 0x012, 3960000, 1, 2, 2, 2, 1, 990000, 0x1, 198000, 1},
{148500, 1000, 1485000, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 40, 0x012, 0x012, 5940000, 1, 2, 2, 2, 2, 742500, 0x2, 148500, 1},
{148500, 1250, 1856250, 0x04, 0x1, 0x1, 550, 0x1B4, 0x06E, 25, 0x00B, 0x00A, 3712500, 1, 1, 1, 2, 2, 928125, 0x1, 185625, 1},
{148500, 1500, 2227500, 0x03, 0x1, 0x1, 495, 0x188, 0x063, 30, 0x00D, 0x00D, 4455000, 1, 1, 1, 2, 2, 1113750, 0x1, 222750, 1},
{148500, 2000, 2970000, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 40, 0x012, 0x012, 5940000, 1, 2, 2, 2, 1, 1485000, 0x1, 297000, 1},
{198000, 1000, 1980000, 0x03, 0x1, 0x1, 220, 0x0AC, 0x02C, 10, 0x003, 0x003, 1980000, 0, 1, 1, 2, 1, 990000, 0x0, 198000, 1},
{198000, 1250, 2475000, 0x03, 0x1, 0x1, 550, 0x1B4, 0x06E, 25, 0x00B, 0x00A, 4950000, 1, 1, 1, 2, 2, 1237500, 0x1, 247500, 1},
{198000, 1500, 2970000, 0x03, 0x1, 0x1, 330, 0x104, 0x042, 15, 0x006, 0x005, 2970000, 0, 1, 1, 2, 1, 1485000, 0x0, 297000, 1},
{198000, 2000, 3960000, 0x03, 0x1, 0x1, 440, 0x15C, 0x058, 20, 0x008, 0x008, 3960000, 1, 1, 1, 2, 1, 1980000, 0x0, 396000, 1},
{297000, 1000, 2970000, 0x03, 0x1, 0x1, 330, 0x104, 0x042, 10, 0x003, 0x003, 2970000, 0, 1, 1, 2, 1, 1485000, 0x0, 297000, 1},
{297000, 1500, 4455000, 0x03, 0x1, 0x1, 495, 0x188, 0x063, 15, 0x006, 0x005, 4455000, 1, 1, 1, 2, 1, 2227500, 0x0, 445500, 1},
{297000, 2000, 5940000, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 20, 0x008, 0x008, 5940000, 1, 1, 1, 2, 1, 2970000, 0x0, 594000, 1},
{594000, 1000, 5940000, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 10, 0x003, 0x003, 5940000, 1, 1, 1, 2, 1, 2970000, 0x0, 594000, 1},
{594000, 750, 4455000, 0x03, 0x1, 0x1, 495, 0x188, 0x063, 10, 0x003, 0x003, 4455000, 1, 1, 1, 2, 1, 2227500, 0x0, 445500, 0},
{594000, 625, 3712500, 0x04, 0x1, 0x1, 550, 0x1B4, 0x06E, 10, 0x003, 0x003, 3712500, 1, 1, 1, 2, 1, 1856250, 0x0, 371250, 0},
{594000, 500, 2970000, 0x03, 0x1, 0x1, 660, 0x20C, 0x084, 10, 0x003, 0x003, 5940000, 1, 1, 1, 2, 2, 1485000, 0x1, 297000, 1},

in t28hpc_hdmitx_table.c.

I need to learn:

Is the table the full extent of what the GPU/DCSS can do? (In which case it can't drive my 1080x1920 portrait screen.)
If 137,930 KHz modes are physically possible in the DCSS, where do I get the other numbers from? (I assume that they're register settings.)

Start with the datasheet for your display. That will tell you what the range is for pixel clock, horizontal frequency, and vertical frequency. The other numbers may be given, or you may have to experiment a little based on other working values. The good news is that the video drivers are quite flexible, so you should be able to add your mode. — stark, Jan 20 '21 at 14:57
@stark The EDID from the display was the basis for my DRM_MODE mode line, so those values were already known. The problem was iMX8M GPU/DCSS-specific. — fadedbee, Jan 20 '21 at 16:01

fadedbee · Accepted Answer · 2021-01-20T16:04:46.317

I've made this work.

The first step was to add a new row to t28hpc_hdmitx_clock_control_table_pixel_out[][] in drivers/gpu/drm/imx/hdp/t28hpc_hdmitx_table.c:

{138375, 1000, 1383750, 0x03, 0x1, 0x1, 615, 0x1E8, 0x07B, 40, 0x012, 0x012, 5535000, 1, 2, 2, 2, 2, 691875, 0x2, 138375, 1},

I generated these values with a day of staring at this table and finding patterns. Hint: one relation between fields involves a + 4 modification.

This still failed, but now with the error:

[    2.147134] [drm] API_AFE_t28hpc_hdmitx.c:406 pixel_freq_khz: 138375, feedback_factor: 1000
[    2.155556] [drm] API_AFE_t28hpc_hdmitx.c:409 Pixel clock frequency (138375 KHz) is supported in this color depth (8-bit). Settings found in row 16
[    2.168785] [drm] VCO frequency is 5535000
[    2.172900] [drm] API_AFE_t28hpc_hdmitx.c:499 VCO frequency (5535000 KHz) not supported
[    2.180925] [drm:hdmi_phy_init_t28hpc] *ERROR* failed to set phy pclock
[    2.187557] [drm:imx_hdp_imx_bind] *ERROR* Failed to initialise HDP PHY

I fixed this by the modifying a row in t28hpc_hdmitx_pll_tuning_table_pixel_out[][] from:

{11, 4950000, 4950000, 0x6, 0x3, 0x1, 0x00, 0x07,  550, 0x42, 213, 7, 258},

to

{11, 5535000, 5535000, 0x6, 0x3, 0x1, 0x00, 0x07,  615, 0x42, 213, 7, 258},

All that was required was to change instances of 4950000 to 5535000 and 550 to 615 to account for the frequency differences.

I've ended up with a screen that runs 0.3% too fast, due to there being no integer solutions which yield exactly 137,930 KHz, but that's better than a black screen.

With a few hour's additional work, I can probably tweak the porches to get 60Hz at a 138,375 KHz pixel clock.

Driving a 137,930 KHz 1080x1920 portrait HDMI screen from an IMX8M

1 Answers1