Skip to content

Commit

Permalink
EHCI: improved logic for isochronous scheduling
Browse files Browse the repository at this point in the history 
This patch (as1608) reworks the logic used by ehci-hcd for scheduling
isochronous transfers.  Now the modular calculations are all based on
a window that starts at the last frame scanned for isochronous
completions.  No transfer descriptors for any earlier frames can
possibly remain on the schedule, so there can be no confusion from
schedule wrap-around.  This removes the need for a "slop" region of
arbitrary size.

There's no need to check for URBs that are longer than the schedule
length.  With the old code they could throw things off by wrapping
around and appearing to end in the near future rather than the distant
future.  Now such confusion isn't possible, and the existing test for
submissions that extend too far into the future will also catch those
that exceed the schedule length.  (But there still has to be an
initial test to handle the case where the schedule already extends as
far into the future as possible.)

Delays caused by IRQ latency won't confuse the algorithm unless they
are ridiculously long (over 250 ms); they will merely reduce how far
into the future new transfers can be scheduled.  A few people have
reported problems caused by delays of 50 ms or so.  Now instead of
failing completely, isochronous transfers will experience a brief
glitch and then continue normally.

(Whether this is truly a good thing is debatable.  A latency as large
as 50 ms generally indicates a bug is present, and complete failure of
audio or video transfers draws people's attention pretty vividly.
Making the transfers more robust also makes it easier for such bugs to
remain undetected.)

Finally, ehci->next_frame is renamed to ehci->last_iso_frame, because
that better describes what it is: the last frame to have been scanned
for isochronous completions.

Signed-off-by: Alan Stern <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
  • Loading branch information
@AlanStern @gregkh
AlanStern authored and gregkh committed Oct 22, 2012
1 parent 6f0c058 commit c3ee9b7
Show file tree
Hide file tree
Showing 2 changed files with 33 additions and 39 deletions.
70 changes: 32 additions & 38 deletions drivers/usb/host/ehci-sched.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1361,7 +1361,7 @@ sitd_slot_ok (
* given EHCI_TUNE_FLS and the slop). Or, write a smarter scheduler!
*/

#define SCHEDULE_SLOP 80 /* microframes */
#define SCHEDULING_DELAY 40 /* microframes */

static int
iso_stream_schedule (
Expand All @@ -1370,7 +1370,7 @@ iso_stream_schedule (
struct ehci_iso_stream *stream
)
{
u32 now, next, start, period, span;
u32 now, base, next, start, period, span;
int status;
unsigned mod = ehci->periodic_size << 3;
struct ehci_iso_sched *sched = urb->hcpriv;
Expand All @@ -1382,12 +1382,6 @@ iso_stream_schedule (
span <<= 3;
}

if (span > mod - SCHEDULE_SLOP) {
ehci_dbg (ehci, "iso request %p too long\n", urb);
status = -EFBIG;
goto fail;
}

now = ehci_read_frame_index(ehci) & (mod - 1);

/* Typical case: reuse current schedule, stream is still active.
Expand All @@ -1396,7 +1390,6 @@ iso_stream_schedule (
* slot in the schedule, implicitly assuming URB_ISO_ASAP.
*/
if (likely (!list_empty (&stream->td_list))) {
u32 excess;

/* For high speed devices, allow scheduling within the
* isochronous scheduling threshold. For full speed devices
Expand All @@ -1408,36 +1401,41 @@ iso_stream_schedule (
else
next = now;

/* Fell behind (by up to twice the slop amount)?
* We decide based on the time of the last currently-scheduled
* slot, not the time of the next available slot.
/*
* Use ehci->last_iso_frame as the base. There can't be any
* TDs scheduled for earlier than that.
*/
excess = (stream->next_uframe - period - next) & (mod - 1);
if (excess >= mod - 2 * SCHEDULE_SLOP)
start = next + excess - mod + period *
DIV_ROUND_UP(mod - excess, period);
else
start = next + excess + period;
if (start - now >= mod) {
ehci_dbg(ehci, "request %p would overflow (%d+%d >= %d)\n",
urb, start - now - period, period,
mod);
status = -EFBIG;
base = ehci->last_iso_frame << 3;
next = (next - base) & (mod - 1);
start = (stream->next_uframe - base) & (mod - 1);

/* Is the schedule already full? */
if (unlikely(start < period)) {
ehci_dbg(ehci, "iso sched full %p (%u-%u < %u mod %u)\n",
urb, stream->next_uframe, base,
period, mod);
status = -ENOSPC;
goto fail;
}

/* Behind the scheduling threshold? Assume URB_ISO_ASAP. */
if (unlikely(start < next))
start += period * DIV_ROUND_UP(next - start, period);

start += base;
}

/* need to schedule; when's the next (u)frame we could start?
* this is bigger than ehci->i_thresh allows; scheduling itself
* isn't free, the slop should handle reasonably slow cpus. it
* isn't free, the delay should handle reasonably slow cpus. it
* can also help high bandwidth if the dma and irq loads don't
* jump until after the queue is primed.
*/
else {
int done = 0;
start = SCHEDULE_SLOP + (now & ~0x07);

/* NOTE: assumes URB_ISO_ASAP, to limit complexity/bugs */
base = now & ~0x07;
start = base + SCHEDULING_DELAY;

/* find a uframe slot with enough bandwidth.
* Early uframes are more precious because full-speed
Expand All @@ -1464,19 +1462,16 @@ iso_stream_schedule (

/* no room in the schedule */
if (!done) {
ehci_dbg(ehci, "iso resched full %p (now %d max %d)\n",
urb, now, now + mod);
ehci_dbg(ehci, "iso sched full %p", urb);
status = -ENOSPC;
goto fail;
}
}

/* Tried to schedule too far into the future? */
if (unlikely(start - now + span - period
>= mod - 2 * SCHEDULE_SLOP)) {
ehci_dbg(ehci, "request %p would overflow (%d+%d >= %d)\n",
urb, start - now, span - period,
mod - 2 * SCHEDULE_SLOP);
if (unlikely(start - base + span - period >= mod)) {
ehci_dbg(ehci, "request %p would overflow (%u+%u >= %u)\n",
urb, start - base, span - period, mod);
status = -EFBIG;
goto fail;
}
Expand All @@ -1490,7 +1485,7 @@ iso_stream_schedule (

/* Make sure scan_isoc() sees these */
if (ehci->isoc_count == 0)
ehci->next_frame = now >> 3;
ehci->last_iso_frame = now >> 3;
return 0;

fail:
Expand Down Expand Up @@ -2220,16 +2215,16 @@ static void scan_isoc(struct ehci_hcd *ehci)
now_frame = (uf >> 3) & fmask;
live = true;
} else {
now_frame = (ehci->next_frame - 1) & fmask;
now_frame = (ehci->last_iso_frame - 1) & fmask;
live = false;
}
ehci->now_frame = now_frame;

frame = ehci->next_frame;
for (;;) {
union ehci_shadow q, *q_p;
__hc32 type, *hw_p;

frame = ehci->last_iso_frame;
restart:
/* scan each element in frame's queue for completions */
q_p = &ehci->pshadow [frame];
Expand Down Expand Up @@ -2334,7 +2329,6 @@ static void scan_isoc(struct ehci_hcd *ehci)
/* Stop when we have reached the current frame */
if (frame == now_frame)
break;
frame = (frame + 1) & fmask;
ehci->last_iso_frame = (frame + 1) & fmask;
}
ehci->next_frame = now_frame;
}
2 changes: 1 addition & 1 deletion drivers/usb/host/ehci.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -143,7 +143,7 @@ struct ehci_hcd { /* one per controller */
struct ehci_qh *intr_unlink_last;
unsigned intr_unlink_cycle;
unsigned now_frame; /* frame from HC hardware */
unsigned next_frame; /* scan periodic, start here */
unsigned last_iso_frame; /* last frame scanned for iso */
unsigned intr_count; /* intr activity count */
unsigned isoc_count; /* isoc activity count */
unsigned periodic_count; /* periodic activity count */
Expand Down

0 comments on commit c3ee9b7

Please sign in to comment.