usb: ehci: make HC see up-to-date qh/qtd descriptor ASAP

This patch introduces the helper of ehci_sync_mem to flush
qtd/qh into memory immediately on some ARM, so that HC can
see the up-to-date qtd/qh descriptor asap.

This patch fixs one performance bug on ARM Cortex A9 dual core
platform, which has been reported on quite a few ARM machines
(OMAP4, Tegra 2, snowball...), see details from link of
https://bugs.launchpad.net/bugs/709245.

The patch has been tested ok on OMAP4 panda A1 board, and the
performance of 'dd' over usb mass storage can be increased from
4~5MB/sec to 14~16MB/sec after applying this patch.

SRU Justification:

Impact:
        - without the patch, 'dd' over usb mass storage is about
	4~5MB/sec.

Fix:
        - After applying the patch, 'dd' over usb mass storage is
	about 14~16MB/sec.

BugLink: http://bugs.launchpad.net/bugs/709245

upstream discusstion:
	https://patchwork.kernel.org/patch/1113332/

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

2 files changed, 35 insertions, 0 deletions

diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c
index 2499b3bce36..9dd7c1c44f4 100644
--- a/drivers/usb/host/ehci-q.c
+++ b/drivers/usb/host/ehci-q.c
@@ -995,6 +995,12 @@ static void qh_link_async (struct ehci_hcd *ehci, struct ehci_qh *qh)
         head->qh_next.qh = qh;
         head->hw->hw_next = dma;
 
+        /*
+         * flush qh descriptor into memory immediately,
+         * see comments in qh_append_tds.
+         */
+        ehci_sync_mem();
+
         qh_get(qh);
         qh->xacterrs = 0;
         qh->qh_state = QH_STATE_LINKED;
@@ -1082,6 +1088,18 @@ static struct ehci_qh *qh_append_tds (
                         wmb ();
                         dummy->hw_token = token;
 
+                        /*
+                         * Writing to dma coherent buffer on ARM may
+                         * be delayed to reach memory, so HC may not see
+                         * hw_token of dummy qtd in time, which can cause
+                         * the qtd transaction to be executed very late,
+                         * and degrade performance a lot. ehci_sync_mem
+                         * is added to flush 'token' immediatelly into
+                         * memory, so that ehci can execute the transaction
+                         * ASAP.
+                         */
+                        ehci_sync_mem();
+
                         urb->hcpriv = qh_get (qh);
                 }
         }
diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
index 3ffb27f472c..8281312f187 100644
--- a/drivers/usb/host/ehci.h
+++ b/drivers/usb/host/ehci.h
@@ -753,6 +753,23 @@ static inline unsigned ehci_read_frame_index(struct ehci_hcd *ehci)
 
 #endif
 
+/*
+ * Writing to dma coherent memory on ARM may be delayed via L2
+ * writing buffer, so introduce the helper which can flush L2 writing
+ * buffer into memory immediately, especially used to flush ehci
+ * descriptor to memory.
+ */
+#ifdef  CONFIG_ARM_DMA_MEM_BUFFERABLE
+static inline void ehci_sync_mem()
+{
+        mb();
+}
+#else
+static inline void ehci_sync_mem()
+{
+}
+#endif
+
 /*-------------------------------------------------------------------------*/
 
 #ifndef DEBUG