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

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Date:   Thu, 20 Jan 2022 14:25:43 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     robert.foss@...aro.org, jernej.skrabec@...il.com, jonas@...boo.se,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        kieran.bingham@...asonboard.com,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        laurent.pinchart@...asonboard.com, biju.das.jz@...renesas.com
Subject: Re: [PATCH v2] drm/bridge: dw-hdmi: use safe format when first in
 bridge chain

Hi,

On 20/01/2022 12:14, Jagan Teki wrote:
> On Wed, Jan 19, 2022 at 5:58 PM Neil Armstrong <narmstrong@...libre.com> 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)) {
> 
> What if we use bridge helper, drm_bridge_chain_get_first_bridge in
> order to find the first bridge in chain?

drm_bridge_chain_get_first_bridge() would be similar, since we already access bridge_chain
in the previous check, it's simpler like that.

Neil

> 
> Thanks,
> Jagan.
>