linux-kernel - Re: [PATCH v4 7/8] drm/fourcc: amlogic: Add modifier definitions for the Scatter layout

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Date:   Wed, 25 Mar 2020 15:49:21 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Simon Ser <contact@...rsion.fr>,
        "mjourdan@...libre.com" <mjourdan@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-amlogic@...ts.infradead.org" 
        <linux-amlogic@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 7/8] drm/fourcc: amlogic: Add modifier definitions
 for the Scatter layout

On Wed, 25 Mar 2020 11:24:15 +0100
Neil Armstrong <narmstrong@...libre.com> wrote:

> Hi,
> 
> On 25/03/2020 10:04, Simon Ser wrote:
> > On Wednesday, March 25, 2020 9:50 AM, Neil Armstrong <narmstrong@...libre.com> wrote:
> >   
> >> Amlogic uses a proprietary lossless image compression protocol and format
> >> for their hardware video codec accelerators, either video decoders or
> >> video input encoders.
> >>
> >> This introduces the Scatter Memory layout, means the header contains IOMMU
> >> references to the compressed frames content to optimize memory access
> >> and layout.
> >>
> >> In this mode, only the header memory address is needed, thus the content
> >> memory organization is tied to the current producer execution and cannot
> >> be saved/dumped neither transferrable between Amlogic SoCs supporting this
> >> modifier.  
> > 
> > I don't think this is suitable for modifiers. User-space relies on
> > being able to copy a buffer from one machine to another over the
> > network. It would be pretty annoying for user-space to have a blacklist
> > of modifiers that don't work this way.
> > 
> > Example of such user-space:
> > https://gitlab.freedesktop.org/mstoeckl/waypipe/
> >   
> 
> I really understand your point, but this is one of the use-cases we need solve.
> This is why I split the fourcc patch and added an explicit comment.
> 
> Please point me a way to display such buffer, the HW exists, works like that and
> it's a fact and can't change.
> 
> It will be the same for secure zero-copy buffers we can't map from userspace, but
> only the HW decoder can read/write and HW display can read.

The comparison to secure buffers is a good one.

Are buffers with the DRM_FORMAT_MOD_AMLOGIC_FBC_LAYOUT_SCATTER modifier
meaningfully mmappable to CPU always / sometimes / never /
varies-and-cannot-know?

Maybe this type should be handled similar to secure buffers, with the
exception that they are not actually secured but only mostly
inaccessible. Then again, I haven't looked at any of the secure buffer
proposals.


Thanks,
pq

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