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

[matt mooney]

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.

-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@xxxxxxxxxx>
> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> ---
>  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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