Re: [PATCH] media: uvcvideo: Fix spurious DMA max segment size warnings

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jun 09, 2022 at 12:18:30PM +0300, Laurent Pinchart wrote:
> Hi Takashi,
> 
> (CC'ing Greg and the linux-usb mailing list)
> 
> Thank you for the patch.
> 
> On Thu, Jun 09, 2022 at 10:22:46AM +0200, Takashi Iwai wrote:
> > As default, the DMA max segment size is set to 64k, and uvcvideo may
> > overflow that size easily, resulting in a warning like:
> > 
> >   DMA-API: xhci_hcd 0000:00:14.0: mapping sg segment longer than device claims to support [len=98304] [max=65536]
> > 
> > Explicitly set up the DMA max segment size for avoiding spurious kernel
> > warnings.
> > 
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > ---
> >  drivers/media/usb/uvc/uvc_video.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > index 1b4cc934109e..25aa6e6a6906 100644
> > --- a/drivers/media/usb/uvc/uvc_video.c
> > +++ b/drivers/media/usb/uvc/uvc_video.c
> > @@ -2160,6 +2160,8 @@ int uvc_video_init(struct uvc_streaming *stream)
> >  	for_each_uvc_urb(uvc_urb, stream)
> >  		INIT_WORK(&uvc_urb->work, uvc_video_copy_data_work);
> >  
> > +	dma_set_max_seg_size(uvc_stream_to_dmadev(stream), UINT_MAX);
> > +
> 
> uvc_stream_to_dmadev() returns the pointer to the HCD's struct device,
> which is shared between all drivers on the bus. Is it really fine for a
> USB device driver to change the maximum segment size of the HCD device
> directly ?

Ick, no!  That feels wrong, it should only change things for that one
specific device, not all devices on that bus.

thanks,

greg k-h