On 2023/9/14 03:19, Greg KH wrote:
On Thu, Sep 14, 2023 at 03:06:58AM +0800, Randy Li wrote:
Hello
I was trying to understand why USB webcams (UVC) have to copy video data
through the CPU (uvc_video_complete() schedules uvc_video_copy_data_work()
for this purpose). During my investigation, I noticed that functions
like|usb_sg_*() and |usb_buffer_*() are not available since kernel version
2.6.12.
What do you mean by "not available"? I see them in the tree today, why
do you think they are not present?
usb_buffer_dmasync_sg(), usb_buffer_map(), usb_buffer_dmasync() and usb_buffer_unmap() are all disabled
by #if 0 in include/usb/usb.h
usb_buffer_map_sg() and usb_buffer_unmap_sg() are just declaration without definition.
I can comprehend that USB host controllers such as xHCI, commonly found in
embedded implementations like dwc3, do not support IOMMU. However, this
should not prevent them from operating on a contiguous buffer.
Are you sure that the protocol for these devices allow this?
I thought that is what hcd_buffer_alloc() does.
If the USB subsystem can no longer work with existing buffers, I propose
that we consider removing the remaining documentation in the "Working with
existing buffers" section of Documentation/driver-api/usb/dma.rst.
I don't understand, what is wrong with the information there exactly?
Have you tried following the suggestions there?
Besides my answer to first question, I found no code use them today.
thanks,
greg k-h
