linux-kernel - Re: [PATCH] drm/vkms: set plane modifiers

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Message-ID: <20211216170532.GA16349@elementary>
Date:   Thu, 16 Dec 2021 18:05:32 +0100
From:   José Expósito <jose.exposito89@...il.com>
To:     Simon Ser <contact@...rsion.fr>
Cc:     rodrigosiqueiramelo@...il.com, hamohammed.sa@...il.com,
        airlied@...ux.ie, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, melissa.srw@...il.com,
        cphealy@...il.com
Subject: Re: [PATCH] drm/vkms: set plane modifiers

On Wed, Dec 15, 2021 at 05:23:10PM +0000, Simon Ser wrote:
> You'll need to set the format_mod_supported hook as well, otherwise the kernel
> will expose a bogus IN_FORMATS prop with one modifier and zero formats.

Hi Simon,

Thank you very much for reviewing this patch, I just started learning
about DRM and this kind of reviews are really helpful :)

I was a bit confused by the docs[1]: (*stars* added in the relevant parts)

> format_mod_supported:
>
> This *optional* hook is used for the DRM to determine if the given
> format/modifier combination is valid for the plane. This allows the
> DRM to generate the correct format bitmask (which formats apply to
> which modifier), and to valdiate modifiers at atomic_check time.
>
> *If not present*, then any modifier in the plane’s modifier list is
> allowed with any of the plane’s formats.

How I read it: "format_mod_supported" can be ignored where no filtering
is intended.

Looking at "create_in_format_blob" this does not look like a bug... But
I am not sure. You probably know the answer.
Whether the anwser is to chage the docs or "create_in_format_blob" I can
send a patch.

For what is worth, after extrating the relevant bits from Weston's
"drm_plane_populate_formats" function to my test program [2], formats
are not listed if "format_mod_supported" is not implemented. The same
applies to drm_info.
So, for the moment I emailed v2 implementing "format_mod_supported" [3].

By the way, this could be related to [4], see commit
b36a6bb8a151c056e1046e9d5b1192d90d9941c9. If we decided that the docs
are wrong I can send a patch as well.

Thanks again for your input,
Jose

[1] https://www.kernel.org/doc/html/latest/gpu/drm-kms.html?highlight=in_formats#c.drm_plane_funcs
[2] https://github.com/JoseExposito/drm-sandbox/blob/main/in_formats.c
[3] https://lore.kernel.org/dri-devel/20211216170140.15803-1-jose.exposito89@gmail.com/T/
[4] https://lists.freedesktop.org/archives/wayland-devel/2021-December/042072.html