linux-kernel - Re: USB: DMA: mapping existing buffer is not supported?

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Message-ID: <2023091306-affection-lifter-3d9d@gregkh>
Date:   Wed, 13 Sep 2023 22:32:12 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Randy Li <ayaka@...lik.info>
Cc:     linux-usb@...r.kernel.org,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-doc@...r.kernel.org
Subject: Re: USB: DMA: mapping existing buffer is not supported?

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