Hi,
I have been working on supporting a composite builtin driver with UVC
using a dwc3 USB controller. At userspace I am using userptr in order
to push the buffers to the driver. Currently we are working with
scatter gather support from the videobuf2-dma-sg API, this in order to
avoid unnecessary copies that are normally used in the memcpy of
uvc_video_encode_data function inside uvc_video.c, and using
uvc_video_encode_isoc_sg instead. Everything works fine when we are
using memory coming from a malloc at userspace, but when we use the
CMEM allocated memory pointers we are getting the following error when
queueing the buffers:
Cannot allocate memory (12).
I managed to track down the issue up until the vb2_dma_sg_get_userptr
function from the videobuf2-dma-sg API, and I see that we are failing
at creating the vector of pages thus failing to create the scatterlist
for the buffer. I tried to workaround this by creating a manual
scatterlist with a mapped memory vaddr taking inspiration on how the
videobuf2-vmalloc API works (it is important to mention this works for
the CMEM pointers, but doing the memcpy), by doing this inside the
vb2_dma_sg_get_userptr function:
unsigned long *nums = frame_vector_pfns(vec);
buf->vaddr = (__force void *)
ioremap(__pfn_to_phys(nums[0]), size + offset);
buf->num_pages = 1;
sg_alloc_table(buf->dma_sgt, buf->num_pages, GFP_KERNEL);
sgt = buf->dma_sgt;
for_each_sgtable_sg(sgt, sg, i) {
sg_set_buf(sg, (void *)(buf->vaddr), size+offset);
}
The problem is, the buffers we are receiving at the host are
corrupted, which lets me think that we are probably reading from
another memory that has no relation with the CMEM pointer memory.
In summary, this is the current situation we have and the CMEM_alloc +
videobuf2-dma-sg is the desired one that is not working.
allocation \ mem ops videobuf2-vmalloc videobuf2-dma-sg
----------------------- --------------------
-------------------
malloc working working
CMEM_alloc working not working
My question here is: Is this approach correct? Or are we trying to do
something that doesn't have support? Is there another correct way that
we should be trying in order to send the CMEM buffers without doing
any memory copy at all? As of right now, the UVC functions only
support doing the memcpy or using the scatterlist, which is why we are
using the second one. Any thoughts or insight will be very
appreciated.
Thank you in advance,
--
Roberto Gutierrez Sánchez
Computer Engineer
Embedded Software Developer
RidgeRun Embedded Solutions
