RE: [PATCH] drm/bridge: dw-hdmi: use safe format when first in bridge chain

[Biju Das]

Hi Neil,

Thanks for the patch

> Subject: [PATCH] drm/bridge: dw-hdmi: use safe format when first in bridge
> chain
> 
> When the dw-hdmi bridge is in first place of the bridge chain, this means
> there is now way to select an input format of the dw-hdmi HW component.
> 
> Since introduction of display-connector, negociation was broken since the
> dw-hdmi negociation code only worked when the dw-hdmi bridge was in last
> position of the bridge chain or behind another bridge also supporting
> input & output format negociation.
> 
> Commit 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts
> callbacks") was introduced to make negociation work again by making
> display-connector act as a pass-through concerning input & output format
> negociation.
> 
> But in the case were the dw-hdmi was single in the bridge chain, for
> example on Renesas SoCs, with the disply-connector bridge the dw-hdmi is
> no more single, breaking output format.
> 
> Reported-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
> Bisected-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> Tested-by: Kieran Bingham <kieran.bingham+renesas@xxxxxxxxxxxxxxxx>
> Fixes: 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts
> callbacks").
> Signed-off-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> index 54d8fdad395f..9f2e1cac0ae2 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> @@ -2551,8 +2551,9 @@ static u32
> *dw_hdmi_bridge_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
>  	if (!output_fmts)
>  		return NULL;
> 
> -	/* If dw-hdmi is the only bridge, avoid negociating with ourselves
> */
> -	if (list_is_singular(&bridge->encoder->bridge_chain)) {
> +	/* If dw-hdmi is the first or only bridge, avoid negociating with
> ourselves */
> +	if (list_is_singular(&bridge->encoder->bridge_chain) ||
> +	    list_is_first(&bridge->chain_node,
> +&bridge->encoder->bridge_chain)) {
>  		*num_output_fmts = 1;
>  		output_fmts[0] = MEDIA_BUS_FMT_FIXED;
> 
> @@ -2673,6 +2674,10 @@ static u32
> *dw_hdmi_bridge_atomic_get_input_bus_fmts(struct drm_bridge *bridge,
>  	if (!input_fmts)
>  		return NULL;
> 
> +	/* If dw-hdmi is the first bridge fall-back to safe output format */
> +	if (list_is_first(&bridge->chain_node, &bridge->encoder-
> >bridge_chain))
> +		output_fmt = MEDIA_BUS_FMT_FIXED;
> +

Based on my debugging, this looks redundant, as get_output_bus_fmts already sets output_fmt = MEDIA_BUS_FMT_FIXED,
And *num_output_fmts = 1, so the function parameter output_fmt in input_bus_fmts will have MEDIA_BUS_FMT_FIXED
And no need to override output_fmt value.  I may be wrong here.

Regards,
Biju

>  	switch (output_fmt) {
>  	/* If MEDIA_BUS_FMT_FIXED is tested, return default bus format */
>  	case MEDIA_BUS_FMT_FIXED:
> --
> 2.25.1