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

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Message-ID: <305382fd-2312-59d9-e2d3-25a17e0a2158@linaro.org>
Date:   Tue, 16 May 2023 09:21:06 +0200
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Aradhya Bhatia <a-bhatia1@...com>,
        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>,
        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

On 15/05/2023 17:59, Aradhya Bhatia wrote:
> 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.

This is because the bridge negociation logic will test with all possible
output formats from the chain, and won't stop at first working test.

If your bridge only supports a single input format, it should return the
same format whatever output_fmt is tried.

So indeed remove this test on mhdp aswell, or filter out invalid output
formats.

The MEDIA_BUS_FMT_FIXED is when there's no output format to test, so this
should be always supported.

Neil

> 
> 
> Regards
> Aradhya