linux-kernel - Re: [PATCH v6 3/8] drm/bridge: mhdp8546: Add minimal format negotiation

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Date:   Mon, 15 May 2023 21:29:20 +0530
From:   Aradhya Bhatia <a-bhatia1@...com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        Jyri Sarha <jyri.sarha@....fi>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Rahul T R <r-ravikumar@...com>,
        Swapnil Jakhade <sjakhade@...ence.com>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Francesco Dolcini <francesco@...cini.it>
CC:     DRI Development List <dri-devel@...ts.freedesktop.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Devarsh Thakkar <devarsht@...com>,
        Jayesh Choudhary <j-choudhary@...com>
Subject: Re: [PATCH v6 3/8] drm/bridge: mhdp8546: Add minimal format
 negotiation

Hi Tomi,

On 12-May-23 14:45, Tomi Valkeinen wrote:
> On 09/05/2023 12:30, Aradhya Bhatia wrote:
>> From: Nikhil Devshatwar <nikhil.nd@...com>
>>
>> With new connector model, mhdp bridge will not create the connector and
>> SoC driver will rely on format negotiation to setup the encoder format.
>>
>> Support minimal format negotiations hooks in the drm_bridge_funcs.
>> Complete format negotiation can be added based on EDID data.
>> This patch adds the minimal required support to avoid failure
>> after moving to new connector model.
>>
>> Signed-off-by: Nikhil Devshatwar <nikhil.nd@...com>
>> Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...com>
> 
> You need to add your SoB to this and the other patches.

Okay!

> 
>> ---
>>
>> Notes:
>>
>>      changes from v1:
>>      * cosmetic fixes, commit message update.
>>
>>      changes from v5:
>>      * dropped the default_bus_format variable and directly assigned
>>        MEDIA_BUS_FMT_RGB121212_1X36 to input_fmts.
>>
>>   .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 25 +++++++++++++++++++
>>   1 file changed, 25 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> index f6822dfa3805..623e4235c94f 100644
>> --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
>> @@ -2146,6 +2146,30 @@ cdns_mhdp_bridge_atomic_reset(struct drm_bridge
>> *bridge)
>>       return &cdns_mhdp_state->base;
>>   }
>>   +static u32 *cdns_mhdp_get_input_bus_fmts(struct drm_bridge *bridge,
>> +                     struct drm_bridge_state *bridge_state,
>> +                     struct drm_crtc_state *crtc_state,
>> +                     struct drm_connector_state *conn_state,
>> +                     u32 output_fmt,
>> +                     unsigned int *num_input_fmts)
>> +{
>> +    u32 *input_fmts;
>> +
>> +    *num_input_fmts = 0;
>> +
>> +    if (output_fmt != MEDIA_BUS_FMT_FIXED)
>> +        return NULL;
> 
> The tfp410 and sii902x drivers don't have the above check. Why does mhdp
> need it? Or the other way, why don't tfp410 and sii902x need it?

I had removed this condition in order to follow status quo, from the
ITE-66121 HDMI bridge driver.

The idea would have been to drop this for MHDP as well, but I guess I
overlooked this one.

However...

> I guess at the moment we always do get MEDIA_BUS_FMT_FIXED as the out
> fmt (in all three bridge drivers), don't we?

... I tested again to ensure that the above is indeed the case. And
ended up catching some odd behavior.

It turns out that for all the HDMI bridges (TFP410, SII902X, ITE-66121),
the format negotiation doesn't stop at output_fmt = MEDIA_BUS_FMT_FIXED.
The {bridge}_get_input_format API gets called again with the output_fmt
= MEDIA_BUS_FMT_RGB24_1X24.

This doesn't happen with the MHDP driver. Format negotiation with MHDP
bridge stops after one round, at output_fmt = MEDIA_BUS_FMT_FIXED.


Regards
Aradhya