linux-kernel - Re: [PATCH v4] drm/ssd130x: Allocate buffers in the plane's .atomic

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Message-ID: <g6bu3b26evk464x4cn77xnzjiuotsq7pfvyakfvwnir5e3ihnk@2jh4dd56fsza>
Date:   Wed, 26 Jul 2023 15:54:34 +0200
From:   Maxime Ripard <mripard@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4] drm/ssd130x: Allocate buffers in the plane's
 .atomic_check callback

On Wed, Jul 26, 2023 at 02:33:06PM +0200, Geert Uytterhoeven wrote:
> > >> Also, Javier pointed me to a discussion you had on IRC about using devm
> > >> allocation here. We can't really do that. KMS devices are only freed
> > >> once the last userspace application closes its fd to the device file, so
> > >> you have an unbounded window during which the driver is still callable
> > >> by userspace (and thus can still trigger an atomic commit) but the
> > >> buffer would have been freed for a while.
> > >
> > > It should still be safe for (at least) the data_array buffer. That
> > > buffer is only used to store pixels in hardware format, and immediately
> > > send them to the hardware.  If this can be called that late, it will
> > > fail horribly, as you can no longer talk to the hardware at that point
> > > (as ssd130x is an i2c driver, it might actually work; but a DRM driver
> > >  that calls devm_platform_ioremap_resource() will crash when writing
> > >  to its MMIO registers)?!?
> >
> > At the very least the SPI driver will fail since the GPIO that is used to
> > toggle the D/C pin is allocated with devm_gpiod_get_optional(), but also
> > the regulator, backlight device, etc.
> >
> > But in any case, as mentioned it is only relevant if the data_array buffer
> > is allocated at probe time, and from Maxime's explanation is more correct
> > to do it in the .atomic_check handler.
> 
> You need (at least) data_array for clear_screen, too, which is called
> from .atomic_disable().

I'm not sure I get what your concern is?

Even if we entirely disable the plane, the state will not have been
destroyed yet so you still have at least access to the data_array from
the old state.

Maxime

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