linux-kernel - [PATCH 3/3] add dma_coherent_write

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Date:	Wed, 31 Aug 2011 17:30:14 -0400
From:	Mark Salter <msalter@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux-arm-kernel@...ts.infradead.org, ming.lei@...onical.com,
	stern@...land.harvard.edu, Mark Salter <msalter@...hat.com>
Subject: [PATCH 3/3] add dma_coherent_write_sync calls to USB EHCI driver

The EHCI driver polls DMA coherent memory for control data written by the
driver. On some architectures, such as ARMv7, the writes from the driver
may get delayed in a write buffer even though it is written to DMA coherent
memory. This delay led to serious performance issues on an ARMv7 based
platform using a USB disk drive. Before using this patch, 'hdparm -t' showed
a read speed of 5.7MB/s. After applying this patch, hdparm showed 23.5MB/s.

Signed-off-by: Mark Salter <msalter@...hat.com>
---
 drivers/usb/host/ehci-q.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/drivers/usb/host/ehci-q.c b/drivers/usb/host/ehci-q.c
index 0917e3a..75d9838 100644
--- a/drivers/usb/host/ehci-q.c
+++ b/drivers/usb/host/ehci-q.c
@@ -114,6 +114,7 @@ qh_update (struct ehci_hcd *ehci, struct ehci_qh *qh, struct ehci_qtd *qtd)
 	/* HC must see latest qtd and qh data before we clear ACTIVE+HALT */
 	wmb ();
 	hw->hw_token &= cpu_to_hc32(ehci, QTD_TOGGLE | QTD_STS_PING);
+	dma_coherent_write_sync();
 }
 
 /* if it weren't for a common silicon quirk (writing the dummy into the qh
@@ -404,6 +405,7 @@ qh_completions (struct ehci_hcd *ehci, struct ehci_qh *qh)
 					wmb();
 					hw->hw_token = cpu_to_hc32(ehci,
 							token);
+					dma_coherent_write_sync();
 					goto retry_xacterr;
 				}
 				stopped = 1;
@@ -753,8 +755,10 @@ qh_urb_transaction (
 	}
 
 	/* by default, enable interrupt on urb completion */
-	if (likely (!(urb->transfer_flags & URB_NO_INTERRUPT)))
+	if (likely(!(urb->transfer_flags & URB_NO_INTERRUPT))) {
 		qtd->hw_token |= cpu_to_hc32(ehci, QTD_IOC);
+		dma_coherent_write_sync();
+	}
 	return head;
 
 cleanup:
@@ -1081,6 +1085,7 @@ static struct ehci_qh *qh_append_tds (
 			/* let the hc process these next qtds */
 			wmb ();
 			dummy->hw_token = token;
+			dma_coherent_write_sync();
 
 			urb->hcpriv = qh_get (qh);
 		}
-- 
1.7.6

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