usb: gadget: uvc: reduce the request size to increase the throughput

One misconception of queueing request to the usb gadget controller is,
that the amount of data that one usb_request is representing is the same
as the hardware is able to transfer in one interval.

This exact idea applies to the uvc_video gadget that assumes that the
request needs to have the size as the gadgets bandwidth parameters are
configured with. (maxpacket, multiplier and burst)

Although it is actually no problem for the hardware to queue a big
request, in the case of isochronous transfers it is impractical to queue
big amount of data per request to the hardware. Especially if the
necessary bandwidth was increased on purpose to maintain high amounts of
data.

E.g. in the dwc3-controller it has the negative impact that the endpoint
FIFO will not be filled fast enough by the internal hardware engine.
Since each transfer buffer (TRB) represents one big request, the
hardware will need a long time to prefill the hardware FIFO. This can be
avoided by queueing more smaller requests which will then lead to
smaller TRBs which the hardware engine can keep up with.

This patch is simply dropping the maxburst as an multiplier for the size
calculation and therefor decreases the overall maximum request size.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
---
This patch is created as an result from the discussion in the following thread:

https://lore.kernel.org/linux-usb/20231031231841.dbhtrgqounu2rvgn@synopsys.com/

 drivers/usb/gadget/function/uvc_queue.c | 1 -
 drivers/usb/gadget/function/uvc_video.c | 1 -
 2 files changed, 2 deletions(-)

Hi,

On Fri, Nov 10, 2023, Michael Grzeschik wrote:
> One misconception of queueing request to the usb gadget controller is,
> that the amount of data that one usb_request is representing is the same
> as the hardware is able to transfer in one interval.
> 
> This exact idea applies to the uvc_video gadget that assumes that the
> request needs to have the size as the gadgets bandwidth parameters are
> configured with. (maxpacket, multiplier and burst)
> 
> Although it is actually no problem for the hardware to queue a big
> request, in the case of isochronous transfers it is impractical to queue
> big amount of data per request to the hardware. Especially if the
> necessary bandwidth was increased on purpose to maintain high amounts of
> data.
> 
> E.g. in the dwc3-controller it has the negative impact that the endpoint
> FIFO will not be filled fast enough by the internal hardware engine.
> Since each transfer buffer (TRB) represents one big request, the
> hardware will need a long time to prefill the hardware FIFO. This can be
> avoided by queueing more smaller requests which will then lead to
> smaller TRBs which the hardware engine can keep up with.

Just want to clarify here to avoid any confusion, the hardware TX FIFO
size is relatively small, usually can be smaller than the TRB. It should
be fine whether the TRB is larger or smaller than the FIFO size. The
hardware does not "prefill" the FIFO. It just fills whichever TRB it's
currently processing (I think you may be mixing up with TRB cache).

The performance impact from this change is to reduce the USB bandwidth
usage. The current calculation in uvc function can use 48KB/uframe for
each request in SuperSpeed, the max size for isoc in SuperSpeed. I know
many hosts can't handle this kind of transfer rate from their hardware.
(e.g. It gets worse when scheduling transfers for multiple endpoints and
under multiple tier hubs).

The bandwidth can be impacted by multiple factors and not just from the
gadget side as noted in the discussion before.

> 
> This patch is simply dropping the maxburst as an multiplier for the size
> calculation and therefor decreases the overall maximum request size.
> 
> Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
> ---
> This patch is created as an result from the discussion in the following thread:
> 
> https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20231031231841.dbhtrgqounu2rvgn@synopsys.com/__;!!A4F2R9G_pg!fTaIo4tDljSbEvUY5SZLkNvKWcz0YeN0Ekzs0CPWyD73RGRmErRC2frODFgnMB1M4Nse0oKKgwxC65gePhGAtauKJq1Vnzlj$ 
> 
>  drivers/usb/gadget/function/uvc_queue.c | 1 -
>  drivers/usb/gadget/function/uvc_video.c | 1 -
>  2 files changed, 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
> index 0aa3d7e1f3cc32..1d3c3c09ff97cb 100644
> --- a/drivers/usb/gadget/function/uvc_queue.c
> +++ b/drivers/usb/gadget/function/uvc_queue.c
> @@ -55,7 +55,6 @@ static int uvc_queue_setup(struct vb2_queue *vq,
>  	sizes[0] = video->imagesize;
>  
>  	req_size = video->ep->maxpacket
> -		 * max_t(unsigned int, video->ep->maxburst, 1)

I think you're reducing a bit too much here? Also, take advantage of
burst. So, perhaps keep request size to max(16K, MPS * maxburst)?

Can be more or less depending on how much video data is needed to
transfer over a video frame.

BR,
Thinh

>  		 * (video->ep->mult);
>  
>  	/* We divide by two, to increase the chance to run
> diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
> index 91af3b1ef0d412..c6b61843bad3d7 100644
> --- a/drivers/usb/gadget/function/uvc_video.c
> +++ b/drivers/usb/gadget/function/uvc_video.c
> @@ -329,7 +329,6 @@ uvc_video_alloc_requests(struct uvc_video *video)
>  	BUG_ON(video->req_size);
>  
>  	req_size = video->ep->maxpacket
> -		 * max_t(unsigned int, video->ep->maxburst, 1)
>  		 * (video->ep->mult);
>  
>  	video->ureq = kcalloc(video->uvc_num_requests, sizeof(struct uvc_request), GFP_KERNEL);
> -- 
> 2.39.2
> 
>

On Fri, Nov 10, 2023, Thinh Nguyen wrote:
> Hi,
> 
> On Fri, Nov 10, 2023, Michael Grzeschik wrote:
> > One misconception of queueing request to the usb gadget controller is,
> > that the amount of data that one usb_request is representing is the same
> > as the hardware is able to transfer in one interval.
> > 
> > This exact idea applies to the uvc_video gadget that assumes that the
> > request needs to have the size as the gadgets bandwidth parameters are
> > configured with. (maxpacket, multiplier and burst)
> > 
> > Although it is actually no problem for the hardware to queue a big
> > request, in the case of isochronous transfers it is impractical to queue
> > big amount of data per request to the hardware. Especially if the
> > necessary bandwidth was increased on purpose to maintain high amounts of
> > data.
> > 
> > E.g. in the dwc3-controller it has the negative impact that the endpoint
> > FIFO will not be filled fast enough by the internal hardware engine.
> > Since each transfer buffer (TRB) represents one big request, the
> > hardware will need a long time to prefill the hardware FIFO. This can be
> > avoided by queueing more smaller requests which will then lead to
> > smaller TRBs which the hardware engine can keep up with.
> 
> Just want to clarify here to avoid any confusion, the hardware TX FIFO
> size is relatively small, usually can be smaller than the TRB. It should
> be fine whether the TRB is larger or smaller than the FIFO size. The
> hardware does not "prefill" the FIFO. It just fills whichever TRB it's
> currently processing (I think you may be mixing up with TRB cache).
> 
> The performance impact from this change is to reduce the USB bandwidth
> usage. The current calculation in uvc function can use 48KB/uframe for
> each request in SuperSpeed, the max size for isoc in SuperSpeed. I know
> many hosts can't handle this kind of transfer rate from their hardware.
> (e.g. It gets worse when scheduling transfers for multiple endpoints and
> under multiple tier hubs).
> 
> The bandwidth can be impacted by multiple factors and not just from the
> gadget side as noted in the discussion before.
> 
> > 
> > This patch is simply dropping the maxburst as an multiplier for the size
> > calculation and therefor decreases the overall maximum request size.
> > 
> > Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
> > ---
> > This patch is created as an result from the discussion in the following thread:
> > 
> > https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20231031231841.dbhtrgqounu2rvgn@synopsys.com/__;!!A4F2R9G_pg!fTaIo4tDljSbEvUY5SZLkNvKWcz0YeN0Ekzs0CPWyD73RGRmErRC2frODFgnMB1M4Nse0oKKgwxC65gePhGAtauKJq1Vnzlj$ 
> > 
> >  drivers/usb/gadget/function/uvc_queue.c | 1 -
> >  drivers/usb/gadget/function/uvc_video.c | 1 -
> >  2 files changed, 2 deletions(-)
> > 
> > diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
> > index 0aa3d7e1f3cc32..1d3c3c09ff97cb 100644
> > --- a/drivers/usb/gadget/function/uvc_queue.c
> > +++ b/drivers/usb/gadget/function/uvc_queue.c
> > @@ -55,7 +55,6 @@ static int uvc_queue_setup(struct vb2_queue *vq,
> >  	sizes[0] = video->imagesize;
> >  
> >  	req_size = video->ep->maxpacket
> > -		 * max_t(unsigned int, video->ep->maxburst, 1)
> 
> I think you're reducing a bit too much here? Also, take advantage of
> burst. So, perhaps keep request size to max(16K, MPS * maxburst)?

I mean to potentially cap at 16K, it didn't translate correctly to
code.... I meant something like this:

max_size = mult * maxburst * MPS;
clamp(max_size, mult * MPS, 16K);

> 
> Can be more or less depending on how much video data is needed to
> transfer over a video frame.

Can the uvc gadget figure out the max video data over a video frame? Or
is it not available at this layer? I figure different setup requires
more video data payload.

If we have this info, then you can properly cap the request size.

Thanks,
Thinh