Re: [RFC, PATCH] USB: EHCI: Allow users to override 80% max periodic bandwidth

[matt mooney <mfm@muteddisk.com>]

On 10:30 Wed 22 Jun     , matt mooney wrote:
> On 20:02 Wed 22 Jun     , Kirill Smelkov wrote:
> > There are cases, when 80% max isochronous bandwidth is too limiting.
> > 
> > For example I have two USB video capture cards which stream uncompressed
> > video, and to stream full NTSC + PAL videos we'd need
> > 
> >     NTSC 640x480 YUV422 @30fps      ~17.6 MB/s
> >     PAL  720x576 YUV422 @25fps      ~19.7 MB/s
> > 
> > isoc bandwidth.
> > 
> > Now, due to limited alt settings in capture devices NTSC one ends up
> > streaming with max_pkt_size=2688  and  PAL with max_pkt_size=2892, both
> > with interval=1. In terms of microframe time allocation this gives
> > 
> >     NTSC    ~53us
> >     PAL     ~57us
> > 
> > and together
> > 
> >     ~110us  >  100us == 80% of 125us uframe time.
> > 
> > So those two devices can't work together simultaneously because the'd
> > over allocate isochronous bandwidth.
> > 
> > 80% seemed a bit arbitrary to me, and I've tried to raise it to 90% and
> > both devices started to work together, so I though sometimes it would be
> > a good idea for users to override hardcoded default of max 80% isoc
> > bandwidth.
> 
> There is nothing arbitrary about 80%. The USB 2.0 Specification constrains
> periodic transfers for high-speed endpoints to 80% of the microframe. See
> section 5.6.4.
> 

Looking at the patch, I see that you probably already knew that.

So I digress and defer to the USB experts ;)

-mfm

> > After all, isn't it a user who should decide how to load the bus? If I
> > can live with 10% or even 5% bulk bandwidth that should be ok. I'm a USB
> > newcomer, but that 80% seems to be chosen pretty arbitrary to me, just
> > to serve as a reasonable default.
> > 
> > NOTE: for two streams with max_pkt_size=3072 (worst case) both time
> > allocation would be 60us+60us=120us which is 96% periodic bandwidth
> > leaving 4% for bulk and control. I think this should work too.
> > 
> > Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > ---
> >  drivers/usb/host/ehci-hcd.c   |   16 ++++++++++++++++
> >  drivers/usb/host/ehci-sched.c |   17 +++++++----------
> >  2 files changed, 23 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
> > index c606b02..1d36e72 100644
> > --- a/drivers/usb/host/ehci-hcd.c
> > +++ b/drivers/usb/host/ehci-hcd.c
> > @@ -112,6 +112,14 @@ static unsigned int hird;
> >  module_param(hird, int, S_IRUGO);
> >  MODULE_PARM_DESC(hird, "host initiated resume duration, +1 for each 75us\n");
> >  
> > +/*
> > + * max periodic time per microframe
> > + * (be careful, USB 2.0 requires it to be 100us = 80% of 125us)
> > + */
> > +static unsigned int uframe_periodic_max = 100;
> > +module_param(uframe_periodic_max, uint, S_IRUGO);
> > +MODULE_PARM_DESC(uframe_periodic_max, "maximum allowed periodic part of a microframe, us");
> > +
> >  #define	INTR_MASK (STS_IAA | STS_FATAL | STS_PCD | STS_ERR | STS_INT)
> >  
> >  /*-------------------------------------------------------------------------*/
> > @@ -571,6 +579,14 @@ static int ehci_init(struct usb_hcd *hcd)
> >  	hcc_params = ehci_readl(ehci, &ehci->caps->hcc_params);
> >  
> >  	/*
> > +	 * tell user, if using non-standard (80% == 100 usec/uframe) bandwidth
> > +	 */
> > +	if (uframe_periodic_max != 100)
> > +		ehci_info(ehci, "using non-standard max periodic bandwith "
> > +				"(%u%% == %u usec/uframe)",
> > +				100*uframe_periodic_max/125, uframe_periodic_max);
> > +
> > +	/*
> >  	 * hw default: 1K periodic list heads, one per frame.
> >  	 * periodic_size can shrink by USBCMD update if hcc_params allows.
> >  	 */
> > diff --git a/drivers/usb/host/ehci-sched.c b/drivers/usb/host/ehci-sched.c
> > index d12426f..fb374f2 100644
> > --- a/drivers/usb/host/ehci-sched.c
> > +++ b/drivers/usb/host/ehci-sched.c
> > @@ -172,7 +172,7 @@ periodic_usecs (struct ehci_hcd *ehci, unsigned frame, unsigned uframe)
> >  		}
> >  	}
> >  #ifdef	DEBUG
> > -	if (usecs > 100)
> > +	if (usecs > uframe_periodic_max)
> >  		ehci_err (ehci, "uframe %d sched overrun: %d usecs\n",
> >  			frame * 8 + uframe, usecs);
> >  #endif
> > @@ -709,11 +709,8 @@ static int check_period (
> >  	if (uframe >= 8)
> >  		return 0;
> >  
> > -	/*
> > -	 * 80% periodic == 100 usec/uframe available
> > -	 * convert "usecs we need" to "max already claimed"
> > -	 */
> > -	usecs = 100 - usecs;
> > +	/* convert "usecs we need" to "max already claimed" */
> > +	usecs = uframe_periodic_max - usecs;
> >  
> >  	/* we "know" 2 and 4 uframe intervals were rejected; so
> >  	 * for period 0, check _every_ microframe in the schedule.
> > @@ -1286,9 +1283,9 @@ itd_slot_ok (
> >  {
> >  	uframe %= period;
> >  	do {
> > -		/* can't commit more than 80% periodic == 100 usec */
> > +		/* can't commit more than uframe_periodic_max usec */
> >  		if (periodic_usecs (ehci, uframe >> 3, uframe & 0x7)
> > -				> (100 - usecs))
> > +				> (uframe_periodic_max - usecs))
> >  			return 0;
> >  
> >  		/* we know urb->interval is 2^N uframes */
> > @@ -1345,7 +1342,7 @@ sitd_slot_ok (
> >  #endif
> >  
> >  		/* check starts (OUT uses more than one) */
> > -		max_used = 100 - stream->usecs;
> > +		max_used = uframe_periodic_max - stream->usecs;
> >  		for (tmp = stream->raw_mask & 0xff; tmp; tmp >>= 1, uf++) {
> >  			if (periodic_usecs (ehci, frame, uf) > max_used)
> >  				return 0;
> > @@ -1354,7 +1351,7 @@ sitd_slot_ok (
> >  		/* for IN, check CSPLIT */
> >  		if (stream->c_usecs) {
> >  			uf = uframe & 7;
> > -			max_used = 100 - stream->c_usecs;
> > +			max_used = uframe_periodic_max - stream->c_usecs;
> >  			do {
> >  				tmp = 1 << uf;
> >  				tmp <<= 8;
> > -- 
> > 1.7.6.rc1
> > 
> > --
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