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

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Message-ID: <c0f75cbb-4d5d-0954-4bb6-20a82cfe5e2f@soulik.info>
Date:   Thu, 14 Sep 2023 04:09:06 +0800
From:   Randy Li <ayaka@...lik.info>
To:     Greg KH <gregkh@...uxfoundation.org>
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 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