Re: [PATCH v3 4/6] drm/tidss: Set bus_format correctly from bridge/connector

From: Tomi Valkeinen <tomi.valkeinen@ti.com>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Nikhil Devshatwar <nikhil.nd@ti.com>
Cc: Sekhar Nori <nsekhar@ti.com>, Yuti Amonkar <yamonkar@cadence.com>,
	dri-devel@lists.freedesktop.org,
	Swapnil Jakhade <sjakhade@cadence.com>
Subject: Re: [PATCH v3 4/6] drm/tidss: Set bus_format correctly from bridge/connector
Date: Tue, 1 Dec 2020 17:53:18 +0200	[thread overview]
Message-ID: <9785cafb-4b04-6132-a636-3d43c1f89d3b@ti.com> (raw)
In-Reply-To: <20201130094617.GE4141@pendragon.ideasonboard.com>

On 30/11/2020 11:46, Laurent Pinchart wrote:
> Hi Nikhil,
> 
> On Mon, Nov 30, 2020 at 12:05:03PM +0530, Nikhil Devshatwar wrote:
>> On 14:51-20201125, Tomi Valkeinen wrote:
>>> On 19/11/2020 18:01, Nikhil Devshatwar wrote:
>>>> Remove the old code to iterate over the bridge chain, as this is
>>>> already done by the framework.
>>>> The bridge state should have the negotiated bus format and flags.
>>>> Use these from the bridge's state.
>>>> If the bridge does not support format negotiation, error out
>>>> and fail.
>>>>
>>>> Signed-off-by: Nikhil Devshatwar <nikhil.nd@ti.com>
>>>> Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
>>>> ---
>>>>
>>>> Notes:
>>>>     changes from v2:
>>>>     * Remove the old code and use the flags from the bridge state
>>>>
>>>>  drivers/gpu/drm/tidss/tidss_encoder.c | 36 +++++++++++----------------
>>>>  1 file changed, 14 insertions(+), 22 deletions(-)
>>>
>>> If a first bridge (after the crtc) supports two bus formats as input, how does tidss choose between
>>> those? This patch just picks bstate->input_bus_cfg.format, and it's not clear to me which one that
>>> is (the first one?). Also, we don't check if tidss actually supports the bus format.
>>
>> The selection is done by the framework in
>> select_bus_fmt_recursive at drivers/gpu/drm/drm_bridge.c:810
>>
>> My understanding is that currently, the format negotiation logic does
>> not negotiate all the way till encoder, it stops only at the
>> first_bridge.
> 
> Should we then implement a bridge in the tidss driver to model the
> internal encoder, in order to support format negotiation all the way to
> the tidss ?

I don't know, but it feels perhaps a bit odd. Then we would have crtc + encoder + bridge, which are
actually all about the same HW block. And this would have to be done for all DRM drivers.

 Tomi

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