Re: [PATCH v16 4/4] drm/bridge: dw-hdmi: fix bus formats negotiation for 8 bit modes

[Neil Armstrong <narmstrong@baylibre.com>]

Hi,

On 26/02/2022 18:13, H. Nikolaus Schaller wrote:
> Commit 7cd70656d1285b ("drm/bridge: display-connector: implement bus fmts callbacks")
> 
> introduced a new mechanism to negotiate bus formats between hdmi connectors
> and bridges which is to be used e.g. for the jz4780 based CI20 board.
> 
> In this case dw-hdmi sets up a list of formats in
> dw_hdmi_bridge_atomic_get_output_bus_fmts().
> 
> This includes e.g. MEDIA_BUS_FMT_UYVY8_1X16 which is chosen for the CI20 but
> only produces a black screen.
> 
> Analysis revealed an omission in
> 
> Commit 6c3c719936dafe ("drm/bridge: synopsys: dw-hdmi: add bus format negociation")
> 
> to check for 8 bit with when adding UYVY8 or YUV8 formats.
> 
> This fix is based on the observation that max_bpc = 0 when running this
> function while info->bpc = 8.

In fact if bpc = 0, it should be considered as 8, so the issue is elsewhere.

> 
> Adding the proposed patch makes the jz4780/CI20 panel work again with default
> MEDIA_BUS_FMT_RGB888_1X24 mode.
> 
> Fixes: 7cd70656d1285b ("drm/bridge: display-connector: implement bus fmts callbacks")
> Fixes: 6c3c719936dafe ("drm/bridge: synopsys: dw-hdmi: add bus format negociation")
> Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
> ---
>   drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> index 43e375da131e8..c08e2cc96584c 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> @@ -2621,11 +2621,13 @@ static u32 *dw_hdmi_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
>   		output_fmts[i++] = MEDIA_BUS_FMT_RGB101010_1X30;
>   	}
>   
> -	if (info->color_formats & DRM_COLOR_FORMAT_YCBCR422)
> -		output_fmts[i++] = MEDIA_BUS_FMT_UYVY8_1X16;
> +	if (max_bpc >= 8 && info->bpc >= 8) {
> +		if (info->color_formats & DRM_COLOR_FORMAT_YCBCR422)
> +			output_fmts[i++] = MEDIA_BUS_FMT_UYVY8_1X16;
>   
> -	if (info->color_formats & DRM_COLOR_FORMAT_YCBCR444)
> -		output_fmts[i++] = MEDIA_BUS_FMT_YUV8_1X24;
> +		if (info->color_formats & DRM_COLOR_FORMAT_YCBCR444)
> +			output_fmts[i++] = MEDIA_BUS_FMT_YUV8_1X24;
> +	}

It should not select YUV here if it's not possible, so something is wrong.

Can you check if https://lore.kernel.org/r/20220119123656.1456355-2-narmstrong@baylibre.com fixes this issue instead ?

Neil

>   
>   	/* Default 8bit RGB fallback */
>   	output_fmts[i++] = MEDIA_BUS_FMT_RGB888_1X24;