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

[Neil Armstrong]

Hi,

On 25/03/2020 10:04, Simon Ser wrote:
> On Wednesday, March 25, 2020 9:50 AM, Neil Armstrong <narmstrong at baylibre.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