linux-kernel - Re: [RFC PATCH] drm/ssd130x: Allocate buffer in the CRTC's .atomic

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Date:   Fri, 01 Sep 2023 11:19:30 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Maxime Ripard <mripard@...nel.org>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH] drm/ssd130x: Allocate buffer in the CRTC's
 .atomic_check() callback

Maxime Ripard <mripard@...nel.org> writes:

> On Fri, Sep 01, 2023 at 09:48:09AM +0200, Javier Martinez Canillas wrote:
>> Thomas Zimmermann <tzimmermann@...e.de> writes:
>> 
>> > Hi Javier,
>> >
>> > another idea about this patch: why not just keep the allocation in the 
>> > plane's atomic check, but store the temporary buffers in a plane struct. 
>> > You'd only grow the arrays length in atomic_check and later fetch the 
>> > pointers in atomic_update. It needs some locking, but nothing complicated.
>> >
>> 
>> Yes, that would work too. Another option is to just move the buffers to
>> struct ssd130x_device as it was before commit 45b58669e532 ("drm/ssd130x:
>> Allocate buffer in the plane's .atomic_check() callback") but just make
>> them fixed arrays with the size of the biggest format.
>> 
>> That will be some memory wasted but will prevent the problem of trying to
>> allocate buffers after drm_atomic_helper_swap_state() has been called.
>
> If we want to go that road, we don't even need an extra allocation, it
> can be part of the state or object structure itself.
>

Yes, I meant to have it as fixed-length arrays. But still the best option
seems to be to allocate them but in the CRTC's .atomic_check() and store
them in a CRTC private state as Thomas suggested.

Geert will post a v2 of his R1 support patches, I'll wait for those and
after that try to implement Thomas' suggestion.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat