linux-kernel - Re: [PATCH 4/4] usb: gadget: uvc: retry vb2_reqbufs() with vb_vmalloc_memops if use

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Message-ID: <cmdnbwc2frixbmpy52g2ywspurtsbcj7xqutv4cptychnl2com@l74jy7kcquvt>
Date: Mon, 12 Jan 2026 18:05:40 +0800
From: Xu Yang <xu.yang_2@....com>
To: Frank Li <Frank.li@....com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Michael Grzeschik <m.grzeschik@...gutronix.de>, jun.li@....com, imx@...ts.linux.dev, linux-usb@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] usb: gadget: uvc: retry vb2_reqbufs() with
 vb_vmalloc_memops if use_sg fail

On Thu, Jan 08, 2026 at 11:16:08AM -0500, Frank Li wrote:
> On Thu, Jan 08, 2026 at 03:43:05PM +0800, Xu Yang wrote:
> > Based on the reality[1][2] that vb2_dma_sg_alloc() can't alloc buffer with
> > device DMA limits, those device will always get below error: "swiotlb
> > buffer is full (sz: 393216 bytes), total 65536 (slots), used 2358 (slots)"
> > and the uvc gadget function can't work at all.
> >
> > The videobuf2-dma-sg.c driver doesn't has a formal improve about this issue
> > till now. To workaround the issue, lets retry vb2_reqbufs() with
> > vb_vmalloc_memops if it fails to allocate buffer with vb2_dma_sg_memops.
> >
> > Link[1]: https://lore.kernel.org/linux-media/20230828075420.2009568-1-anle.pan@nxp.com/
> > Link[2]: https://lore.kernel.org/linux-media/20230914145812.12851-1-hui.fang@nxp.com/
> > Signed-off-by: Xu Yang <xu.yang_2@....com>
> > ---
> >  drivers/usb/gadget/function/uvc_queue.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
> > index 21d80322cb6148ed87eb77f453a1f1644e4923ae..586e5524c171f115d98af5dda43fb800466f46d2 100644
> > --- a/drivers/usb/gadget/function/uvc_queue.c
> > +++ b/drivers/usb/gadget/function/uvc_queue.c
> > @@ -182,7 +182,15 @@ int uvcg_alloc_buffers(struct uvc_video_queue *queue,
> >  {
> >  	int ret;
> >
> > +retry:
> >  	ret = vb2_reqbufs(&queue->queue, rb);
> > +	if (ret < 0 && queue->use_sg) {
> > +		uvc_trace(UVC_TRACE_IOCTL,
> > +			  "failed to alloc buffer with sg enabled, try non-sg mode\n");
> > +		queue->use_sg = 0;
> > +		queue->queue.mem_ops = &vb2_vmalloc_memops;
> 
> How it work if dma_sg_alloc() failure,  vmalloc success, follow dma_map()
> should be failure for vmalloc()

The point is the videobuf2 subsystem doesn't do dma_map() on vmalloc returned big buffer,
however, it do it for dma_sg returned buffer. 

If use vmalloced buffer, UVC gadget already allocate some small buffer for each usb_request
to do dma transfer, so uvc driver will memcopy data from big buffer to small buffer.

If use dma-sg-ed buffer, uvc driver won't memcopy data, instead it will use part of sg
buffer each time.

Then USB system will do usb_gadget_map_request() again before each transfer.

Thanks,
Xu Yang

> 
> Frank
> 
> > +		goto retry;
> > +	}
> >
> >  	return ret ? ret : rb->count;
> >  }
> >
> > --
> > 2.34.1
> >