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 17:18:15 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Pekka Paalanen <ppaalanen@...il.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

Hi,

On 25/03/2020 14:49, Pekka Paalanen wrote:
> 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?

mmappable, yes in our WIP V4L2 driver in non-secure path, meaningful, absolutely never.

So yeah, these should not be mmappable since not meaningful.

> 
> 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.

Actually, the Amlogic platforms offers secure video path using these exact
modifiers, AFAIK it doesn't support the NV12 dual-write output in secure.

AFAIK last submission is from AMD, and it doesn't talk at all about mmapability
of the secure BOs.

Neil

> 
> 
> Thanks,
> pq
> 




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