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

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Message-ID: <1e783efe-8659-8be1-82b7-15186302e88c@soulik.info>
Date:   Thu, 14 Sep 2023 03:06:58 +0800
From:   Randy Li <ayaka@...lik.info>
To:     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: USB: DMA: mapping existing buffer is not supported?

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