From: Vikas Bansal <vikas.bansal@samsung.com>
To: mathias.nyman@intel.com, gregkh@linuxfoundation.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: sumit.batra@samsung.com, Vikas Bansal <vikas.bansal@samsung.com>
Subject: [PATCH] BugFix in XHCI controller driver for scatter gather DMA
Date: Mon, 21 Dec 2015 17:45:41 +0530
Message-id: <1450700141-4798-1-git-send-email-vikas.bansal@samsung.com>
DLP-Filter: Pass
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1818
Lines: 39

From: Sumit Batra <sumit.batra@samsung.com> 

Pre-Condition 
URB with Scatter Gather list is queued to bulk OUT endpoint.
Every buffer in scatter gather list is not a multiple of maximum packet size for that endpoint(short packet).
CHAIN bit is set for all TRBs in a TD so that the DMA happens to all of them at once. 

Issue
DMA operation copies all the CHAINED TRBs at contiguous device memory.
But since the original packet was a short packet, so the actual data is re-aligned after this DMA operation.
At device end this re-aligned data causes data integrity issue with applications like ICMP ping.

Solution
Don't set the CHAINED bit for these TRBs, if their buffers are not a multiple of maximum packet size.
This will reduce the benefit in throughput as required from a scatter gather implementation, but this reduces the CPU utilization.
And solves the data integrity issue on Device End


Signed-off-by: Sumit Batra <sumit.batra@samsung.com>
Signed-off-by: Vikas Bansal <vikas.bansal@samsung.com>
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 7d34cbf..862d7cd 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3110,7 +3110,8 @@ static int queue_bulk_sg_tx(struct xhci_hcd *xhci, gfp_t mem_flags,
 		 * TRB to indicate it's the last TRB in the chain.
 		 */
 		if (num_trbs > 1) {
-			field |= TRB_CHAIN;
+			if(this_sg_len % usb_endpoint_maxp(&urb->ep->desc) == 0)
+				field |= TRB_CHAIN;
 		} else {
 			/* FIXME - add check for ZERO_PACKET flag before this */
 			td->last_trb = ep_ring->enqueue;
--
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