USB: EHCI: Allow users to override 80% max periodic bandwidth

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.

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% set in stone by USB 2.0 specification seems to be
chosen pretty arbitrary to me, just to serve as a reasonable default.

NOTE 1
~~~~~~

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.  Alan Stern suggested that bulk then
would be problematic (less than 300*8 bittimes left per microframe), but
I think that is still enough for control traffic.

NOTE 2
~~~~~~

Sarah Sharp expressed concern that maxing out periodic bandwidth
could lead to vendor-specific hardware bugs on host controllers, because

> It's entirely possible that you'll run into
> vendor-specific bugs if you try to pack the schedule with isochronous
> transfers.  I don't think any hardware designer would seriously test or
> validate their hardware with a schedule that is basically a violation of
> the USB bus spec (more than 80% for periodic transfers).

So far I've only tested this patch on my HP Mini 5103 with N10 chipset

    kirr@mini:~$ lspci
    00:00.0 Host bridge: Intel Corporation N10 Family DMI Bridge
    00:02.0 VGA compatible controller: Intel Corporation N10 Family Integrated Graphics Controller
    00:02.1 Display controller: Intel Corporation N10 Family Integrated Graphics Controller
    00:1b.0 Audio device: Intel Corporation N10/ICH 7 Family High Definition Audio Controller (rev 02)
    00:1c.0 PCI bridge: Intel Corporation N10/ICH 7 Family PCI Express Port 1 (rev 02)
    00:1c.3 PCI bridge: Intel Corporation N10/ICH 7 Family PCI Express Port 4 (rev 02)
    00:1d.0 USB Controller: Intel Corporation N10/ICH 7 Family USB UHCI Controller #1 (rev 02)
    00:1d.1 USB Controller: Intel Corporation N10/ICH 7 Family USB UHCI Controller #2 (rev 02)
    00:1d.2 USB Controller: Intel Corporation N10/ICH 7 Family USB UHCI Controller #3 (rev 02)
    00:1d.3 USB Controller: Intel Corporation N10/ICH 7 Family USB UHCI Controller #4 (rev 02)
    00:1d.7 USB Controller: Intel Corporation N10/ICH 7 Family USB2 EHCI Controller (rev 02)
    00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev e2)
    00:1f.0 ISA bridge: Intel Corporation NM10 Family LPC Controller (rev 02)
    00:1f.2 SATA controller: Intel Corporation N10/ICH7 Family SATA AHCI Controller (rev 02)
    01:00.0 Network controller: Broadcom Corporation BCM4313 802.11b/g/n Wireless LAN Controller (rev 01)
    02:00.0 Ethernet controller: Marvell Technology Group Ltd. 88E8059 PCI-E Gigabit Ethernet Controller (rev 11)

and the system works stable with 110us/uframe (~88%) isoc bandwith allocated for
above-mentioned isochronous transfers.

NOTE 3
~~~~~~

This feature is off by default. I mean max periodic bandwidth is set to
100us/uframe by default exactly as it was before the patch. So only those of us
who need the extreme settings are taking the risk - normal users who do not
alter uframe_periodic_max sysfs attribute should not see any change at all.

NOTE 4
~~~~~~

I've tried to update documentation in Documentation/ABI/ thoroughly, but
only "TBD" was put into Documentation/usb/ehci.txt -- the text there seems
to be outdated and much needing refreshing, before it could be amended.

Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 7 insertions, 10 deletions

diff --git a/drivers/usb/host/ehci-sched.c b/drivers/usb/host/ehci-sched.c
index 6c9fbe352f73..2abf8543f083 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 > ehci->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 = ehci->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))
+				> (ehci->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 = ehci->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 = ehci->uframe_periodic_max - stream->c_usecs;
 			do {
 				tmp = 1 << uf;
 				tmp <<= 8;