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

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Message-ID: <2023091323-trombone-storeroom-cbd3@gregkh>
Date:   Wed, 13 Sep 2023 21:19:01 +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 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