linux-kernel - Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

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Message-ID: <hG3hgN80Bt03njzCaW7h3xaog7ppTTBzmsShC0L5LdCbr5dFkHMJHHxizeYa_IYP7uCwMG-vPJOWMhueq2LirNKFhulkkni2KFf3XA24bb8=@emersion.fr>
Date:   Thu, 11 Jul 2019 08:28:25 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        Brian Starkey <brian.starkey@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

On Thursday, July 11, 2019 11:21 AM, Daniel Vetter <daniel@...ll.ch> wrote:

> On Tue, Jun 25, 2019 at 10:38:31PM -0300, Rodrigo Siqueira wrote:
>
> > The compute_crc() function is responsible for calculating the
> > framebuffer CRC value; due to the XRGB format, this function has to
> > ignore the alpha channel during the CRC computation. Therefore,
> > compute_crc() set zero to the alpha channel directly in the input
> > framebuffer, which is not a problem since this function receives a copy
> > of the original buffer. However, if we want to use this function in a
> > context without a buffer copy, it will change the initial value. This
> > patch makes compute_crc() calculate the CRC value without modifying the
> > input framebuffer.
>
> Uh why? For writeback we're writing the output too, so we can write
> whatever we want to into the alpha channel. For writeback we should never
> accept a pixel format where alpha actually matters, that doesn't make
> sense. You can't see through a real screen either, they are all opaque :-)

I'm not sure about that. See e.g.
https://en.wikipedia.org/wiki/See-through_display

Many drivers already accept FBs with alpha channels for the primary
plane.
https://drmdb.emersion.fr/formats?plane=1

Just making sure we aren't painting ourselves into a corner. :P