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USB: ehci: qh/qtd cleanup comments

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Provide better comments about qh_completions() and QTD handling.
That code can be *VERY* confusing, since it's evolved over a few
years to cope with both hardware races and silicon quirks.

Remove two unlikely() annotations that match the GCC defaults
(and are thus pointless); add an "else" to highlight code flow.

This patch doesn't change driver behavior.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
David Brownell 2008-04-10 14:21:06 -07:00 committed by Greg Kroah-Hartman
parent 6427f79953
commit a082b5c788
1 changed files with 40 additions and 10 deletions
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50
drivers/usb/host/ehci-q.c
View file

@ -336,11 +336,20 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
/* always clean up qtds the hc de-activated */
if ((token & QTD_STS_ACTIVE) == 0) {
/* on STALL, error, and short reads this urb must
* complete and all its qtds must be recycled.
*/
if ((token & QTD_STS_HALT) != 0) {
stopped = 1;
/* magic dummy for some short reads; qh won't advance.
* that silicon quirk can kick in with this dummy too.
*
* other short reads won't stop the queue, including
* control transfers (status stage handles that) or
* most other single-qtd reads ... the queue stops if
* URB_SHORT_NOT_OK was set so the driver submitting
* the urbs could clean it up.
*/
} else if (IS_SHORT_READ (token)
&& !(qtd->hw_alt_next
@ -354,18 +363,18 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
&& HC_IS_RUNNING (ehci_to_hcd(ehci)->state))) {
break;
/* scan the whole queue for unlinks whenever it stops */
} else {
stopped = 1;
if (unlikely (!HC_IS_RUNNING (ehci_to_hcd(ehci)->state)))
/* cancel everything if we halt, suspend, etc */
if (!HC_IS_RUNNING(ehci_to_hcd(ehci)->state))
last_status = -ESHUTDOWN;
/* ignore active urbs unless some previous qtd
* for the urb faulted (including short read) or
* its urb was canceled. we may patch qh or qtds.
/* this qtd is active; skip it unless a previous qtd
* for its urb faulted, or its urb was canceled.
*/
if (likely(last_status == -EINPROGRESS &&
!urb->unlinked))
else if (last_status == -EINPROGRESS && !urb->unlinked)
continue;
/* issue status after short control reads */
@ -375,7 +384,7 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
continue;
}
/* token in overlay may be most current */
/* qh unlinked; token in overlay may be most current */
if (state == QH_STATE_IDLE
&& cpu_to_hc32(ehci, qtd->qtd_dma)
== qh->hw_current)
@ -402,11 +411,16 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
if (likely(last_status == -EINPROGRESS))
last_status = qtd_status;
/* if we're removing something not at the queue head,
* patch the hardware queue pointer.
*/
if (stopped && qtd->qtd_list.prev != &qh->qtd_list) {
last = list_entry (qtd->qtd_list.prev,
struct ehci_qtd, qtd_list);
last->hw_next = qtd->hw_next;
}
/* remove qtd; it's recycled after possible urb completion */
list_del (&qtd->qtd_list);
last = qtd;
}
@ -431,7 +445,15 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
qh_refresh(ehci, qh);
break;
case QH_STATE_LINKED:
/* should be rare for periodic transfers,
/* We won't refresh a QH that's linked (after the HC
* stopped the queue). That avoids a race:
* - HC reads first part of QH;
* - CPU updates that first part and the token;
* - HC reads rest of that QH, including token
* Result: HC gets an inconsistent image, and then
* DMAs to/from the wrong memory (corrupting it).
*
* That should be rare for interrupt transfers,
* except maybe high bandwidth ...
*/
if ((cpu_to_hc32(ehci, QH_SMASK)
@ -549,6 +571,12 @@ qh_urb_transaction (
this_qtd_len = qtd_fill(ehci, qtd, buf, len, token, maxpacket);
len -= this_qtd_len;
buf += this_qtd_len;
/*
* short reads advance to a "magic" dummy instead of the next
* qtd ... that forces the queue to stop, for manual cleanup.
* (this will usually be overridden later.)
*/
if (is_input)
qtd->hw_alt_next = ehci->async->hw_alt_next;
@ -568,8 +596,10 @@ qh_urb_transaction (
list_add_tail (&qtd->qtd_list, head);
}
/* unless the bulk/interrupt caller wants a chance to clean
* up after short reads, hc should advance qh past this urb
/*
* unless the caller requires manual cleanup after short reads,
* have the alt_next mechanism keep the queue running after the
* last data qtd (the only one, for control and most other cases).
*/
if (likely ((urb->transfer_flags & URB_SHORT_NOT_OK) == 0
|| usb_pipecontrol (urb->pipe)))