USB: ehci: qh/qtd cleanup comments

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>

1 files changed, 40 insertions, 10 deletions

diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c
index c0e752cffc68..64942ec584c2 100644
--- a/drivers/usb/host/ehci-q.c
+++ b/drivers/usb/host/ehci-q.c
@@ -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 @@ halt:
                 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 @@ halt:
                         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)))