linux-kernel - Re: [PATCH 4/8] drm/ssd130x: Add support for DRM_FORMAT

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Message-ID: <87351qabe6.fsf@minerva.mail-host-address-is-not-set>
Date:   Fri, 14 Jul 2023 15:08:33 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] drm/ssd130x: Add support for DRM_FORMAT_R1

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

Hello Geert,

> Hi Javier,

[...]

>> >
>> > The display should not be updated after .remove(), so I think plain
>> > devm_kcalloc() should be fine.
>>
>> That was precisely my point, that there could be atomic commits even after
>> the driver has been removed (e.g: if using DRM fbdev emulation, user-space
>> can keep the /dev/fb0 opened and continue updating the framebuffer. That's
>> not released until the fd is closed and struct fb_ops .fb_destroy called.
>>
>> But that's a general rule in DRM, any user-visible resource must not be
>> allocated using device managed resources and instead use the drm_device
>> managed resources helpers. To make sure that are not released until the
>> last call to drm_dev_put():
>
> These buffers are not user-visible, so they should not be accessed
> after .remove().  When these are accessed, the next step would be
> to write the buffer data to the device, which would also fail miserably,
> as the regmap, GPIO, and regulator are hardware resources managed
> through devm_*().
>

Right, given that we do the shadow buffer -> native buffer copy, I thought
of it as if as a user-visible (well, user-accessible I guess) but you are
correct that's not and trying to write to the device will fail anyways.

So devm_* should be enough indeed and if there are problems with that, it
would be a different bug in the driver to fix.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat