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.
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.
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.
Sincerely
Randy
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
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
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?
> 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?
> 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?
thanks,
greg k-h