[RFC]: shmem fd for non-DMA buffer sharing cross drivers

[Tomasz Figa]

Hi Hsia-Jun,

On Tue, Aug 22, 2023 at 8:14 PM Hsia-Jun Li <Randy.Li at synaptics.com> wrote:
>
> Hello
>
> I would like to introduce a usage of SHMEM slimier to DMA-buf, the major
> purpose of that is sharing metadata or just a pure container for cross
> drivers.
>
> We need to exchange some sort of metadata between drivers, likes dynamic
> HDR data between video4linux2 and DRM.

If the metadata isn't too big, would it be enough to just have the
kernel copy_from_user() to a kernel buffer in the ioctl code?

> Or the graphics frame buffer is
> too complex to be described with plain plane's DMA-buf fd.
> An issue between DRM and V4L2 is that DRM could only support 4 planes
> while it is 8 for V4L2. It would be pretty hard for DRM to expend its
> interface to support that 4 more planes which would lead to revision of
> many standard likes Vulkan, EGL.

Could you explain how a shmem buffer could be used to support frame
buffers with more than 4 planes?

>
> Also, there is no reason to consume a device's memory for the content
> that device can't read it, or wasting an entry of IOMMU for such data.

That's right, but DMA-buf doesn't really imply any of those. DMA-buf
is just a kernel object with some backing memory. It's up to the
allocator to decide how the backing memory is allocated and up to the
importer on whether it would be mapped into an IOMMU.

> Usually, such a metadata would be the value should be written to a
> hardware's registers, a 4KiB page would be 1024 items of 32 bits registers.
>
> Still, I have some problems with SHMEM:
> 1. I don't want thhe userspace modify the context of the SHMEM allocated
> by the kernel, is there a way to do so?

This is generally impossible without doing any of the two:
1) copying the contents to an internal buffer not accessible to the
userspace, OR
2) modifying any of the buffer mappings to read-only

2) can actually be more costly than 1) (depending on the architecture,
data size, etc.), so we shouldn't just discard the option of a simple
copy_from_user() in the ioctl.

> 2. Should I create a helper function for installing the SHMEM file as a fd?

We already have the udmabuf device [1] to turn a memfd into a DMA-buf,
so maybe that would be enough?

[1] https://elixir.bootlin.com/linux/v6.5-rc7/source/drivers/dma-buf/udmabuf.c

Best,
Tomasz

>
> --
> Hsia-Jun(Randy) Li