Re: [PATCH] drm/panel: chipwealth-ch13726a: Fix dark grays rendering green at 120 Hz

[Aaron Kling]

On Tue, Oct 27, 2026 at 9:02 PM Matthew Schwartz
<matthew.schwartz@xxxxxxxxx> wrote:
>
> The init sequence writes 0xf0 0x50 followed by 0xb9 0x00. The panel
> configuration in the Thor's stock Android firmware (decompiled from its
> devicetree overlay, dtbo.img) has two runtime command sequences that
> differ only in the 0xb9 value, both "display off; f0 50; b9 xx; display
> on":
>
>   qcom,mdss-dsi-ram-command:        b9 11
>   qcom,mdss-dsi-bypass-ram-command: b9 00
>
> With 0x00, in the 120 Hz mode, a full-screen gray ramp from black to
> white, shown with identity color management, renders black as green and
> the darkest grays as bright green, both at maximum and at reduced
> brightness, and the tint is more noticeable at lower brightness. The
> 60 Hz mode renders it correctly. With 0x11 the ramp renders correctly in
> both modes and the panel dims without a color shift.
>
> Use 0x11, the value from the vendor's ram-command sequence, instead.
>
> Fixes: 3ee01b8647b5 ("drm/panel: Add panel driver for ChipWealth CH13726A based panels")
> Reported-by: Aaron Kling <webgeek1234@xxxxxxxxx>
> Link: https://lore.kernel.org/all/20260727-ch13726a-washout-v1-1-bf869ddfc5cf@xxxxxxxxx/
> Signed-off-by: Matthew Schwartz <matthew.schwartz@xxxxxxxxx>
> ---
> Aaron, this should be the 120 Hz issue from your washout patch; could
> you give it a try on your Thor? I found the issue here separately and
> saw your patch as I was prepping for submission, and I bet I was seeing
> the same bug you originally saw.

This is something we tried before. And best we can tell, enabling gram
limits frame rate to 60hz on this panel. Framerate tests such as the
ufo test were reporting 60 when set up like this. Can you verify the
refresh rate on your device?

Aaron

>  drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c b/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
> index 562dc573528d..0832e49b77a9 100644
> --- a/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
> +++ b/drivers/gpu/drm/panel/panel-chipwealth-ch13726a.c
> @@ -65,7 +65,8 @@ static int ch13726a_on(struct ch13726a_panel *ctx)
>         ctx->dsi->mode_flags |= MIPI_DSI_MODE_LPM;
>
>         mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xf0, 0x50);
> -       mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xb9, 0x00);
> +       /* 0x00 (the vendor "bypass ram" sequence) renders near-black as green */
> +       mipi_dsi_generic_write_seq_multi(&dsi_ctx, 0xb9, 0x11);
>
>         mipi_dsi_dcs_exit_sleep_mode_multi(&dsi_ctx);
>
>
> base-commit: 8ef66514d7b993c9fb96a5886f001e3556610c1c
> --
> 2.56.0
>