On Thu, Sep 14, 2023 at 04:09:06AM +0800, Randy Li wrote: > > 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. Sorry, I was looking at the usb_sg_* calls, those are there if you want to use them. But again, why not just use the normal sg field in the urb itself for the scatter-gather pointer? Will that not work for you? > > > 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. The old-style usb-storage driver uses the usb_sg_* calls, and the uas.c driver uses the sg fields and provides line-speed transfers (the speed limit is in the device, not the kernel). thanks, greg k-h
