linux-kernel - Re: [PATCH v2] drm/bridge: dw-hdmi: use safe format when first in bridge chain

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Message-ID: <9b98f2be-ed07-c956-adc8-51251b3432ee@baylibre.com>
Date:   Fri, 4 Feb 2022 09:32:30 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     robert.foss@...aro.org
Cc:     Laurent.pinchart@...asonboard.com, jonas@...boo.se,
        jernej.skrabec@...il.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, kieran.bingham@...asonboard.com,
        biju.das.jz@...renesas.com,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
Subject: Re: [PATCH v2] drm/bridge: dw-hdmi: use safe format when first in
 bridge chain

Hi,

On 19/01/2022 13:28, Neil Armstrong wrote:
> When the dw-hdmi bridge is in first place of the bridge chain, this
> means there is no way to select an input format of the dw-hdmi HW
> component.
> 
> Since introduction of display-connector, negotiation was broken since
> the dw-hdmi negotiation 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 negotiation.
> 
> Commit 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts callbacks")
> was introduced to make negotiation work again by making display-connector
> act as a pass-through concerning input & output format negotiation.
> 
> But in the case where the dw-hdmi is single in the bridge chain, for
> example on Renesas SoCs, with the display-connector bridge the dw-hdmi
> is no more single, breaking output format.
> 
> Reported-by: Biju Das <biju.das.jz@...renesas.com>
> Bisected-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> Tested-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> Fixes: 0656d1285b79 ("drm/bridge: display-connector: implement bus fmts callbacks").
> Signed-off-by: Neil Armstrong <narmstrong@...libre.com>
> ---
> Changes since v1:
> - Remove bad fix in dw_hdmi_bridge_atomic_get_input_bus_fmts
> - Fix typos in commit message
> 
>   drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 5 +++--
>   1 file changed, 3 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..97cdc61b57f6 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;
>   

Can a bridge reviewer get an eye on this ? this actually solves an issue reported on a Renesas platform.

Neil