On 8/23/23 12:46, Tomasz Figa wrote:
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Hi Hsia-Jun,
On Tue, Aug 22, 2023 at 8:14 PM Hsia-Jun Li <Randy.Li@xxxxxxxxxxxxx> 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?
If you are asking why we need this:
1. metadata likes dynamic HDR tone data
2. DRM also challenges with this problem, let me quote what sima said:
"another trick that we iirc used for afbc is that sometimes the planes
have a fixed layout
like nv12
and so logically it's multiple planes, but you only need one plane slot
to describe the buffer
since I think afbc had the "we need more than 4 planes" issue too"
Unfortunately, there are vendor pixel formats are not fixed layout.
3. Secure(REE, trusted video piepline) info.
For how to assign such metadata data.
In case with a drm fb_id, it is simple, we just add a drm plane property
for it. The V4L2 interface is not flexible, we could only leave into
CAPTURE request_fd as a control.
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.
I just want to say it can't be allocated at the same place which was for
those DMA bufs(graphics or compressed bitstream).
This also could be answer for your first question, if we place this kind
of buffer in a plane for DMABUF(importing) in V4L2, V4L2 core would try
to prepare it, which could map it into 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 the 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.
I don't want the userspace access it at all. So that won't be a problem.
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
It is the kernel driver that allocate this buffer. For example, v4l2
CAPTURE allocate a buffer for metadata when VIDIOC_REQBUFS.
Or GBM give you a fd which is assigned with a surface.
So we need a kernel interface.
Best,
Tomasz
--
Hsia-Jun(Randy) Li
--
Hsia-Jun(Randy) Li
