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 11:24:15 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Simon Ser <contact@...rsion.fr>
Cc:     "daniel@...ll.ch" <daniel@...ll.ch>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "mjourdan@...libre.com" <mjourdan@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.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

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.

We need a solution for those if we want embedded and secure products to be supported
upstream, otherwise they will stay in an obscure off-tree linux tree and for example
AOSP support (which will support these secure video buffers) will use these vendor
specific hacks.

Neil