linux-kernel - Re: [PATCH] BugFix in XHCI controller driver for scatter gather DMA

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Date:	Mon, 21 Dec 2015 21:23:35 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Vikas Bansal <vikas.bansal@...sung.com>, mathias.nyman@...el.com,
	gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc:	sumit.batra@...sung.com
Subject: Re: [PATCH] BugFix in XHCI controller driver for scatter gather DMA

Hello.

On 12/21/2015 03:46 PM, Vikas Bansal wrote:

> From: Sumit Batra <sumit.batra@...sung.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@...sung.com>
> Signed-off-by: Vikas Bansal <vikas.bansal@...sung.com>
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 7d34cbf..7363dee 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -3110,7 +3110,9 @@ 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)

    Indent with 2 extra tabs ISO 1 please -- it'll be easier on the eyes 
(blends with the next statement otherwise).

> +				field |= TRB_CHAIN;
>   		} else {
>   			/* FIXME - add check for ZERO_PACKET flag before this */
>   			td->last_trb = ep_ring->enqueue;

MBR, Sergei

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