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SubjectRe: [PATCH v4 7/8] drm/fourcc: amlogic: Add modifier definitions for the Scatter layout

On 25/03/2020 14:49, Pekka Paalanen wrote:
> On Wed, 25 Mar 2020 11:24:15 +0100
> Neil Armstrong <> wrote:
>> Hi,
>> On 25/03/2020 10:04, Simon Ser wrote:
>>> On Wednesday, March 25, 2020 9:50 AM, Neil Armstrong <> 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:
>> 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.


> Thanks,
> pq

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