linux-kernel - Re: [PATCH 2/2] USB: hcd: only unmap the actual completed DMA buffer

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Message-ID: <51A39441.9000609@cogentembedded.com>
Date:	Mon, 27 May 2013 21:13:37 +0400
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Ming Lei <ming.lei@...onical.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 2/2] USB: hcd: only unmap the actual completed DMA buffer

Hello.

On 27-05-2013 20:13, Ming Lei wrote:

> This patch only unmap the actual completed DMA buffer instead of
> the whole transfer buffer.

     Who will unmap the rest of the buffer?

> It is common to see only part of DMA transfer is completed, especially
> in case of DMA_FROM_DEVICE because the length of incoming traffic often
> is unknown before submitting URB, so this patch may improve USB
> DMA unmapping which runs in hard irq context.

> The patch has been tested on ARMv7(Pandaboard), and it is observed that
> at average ~25us is saved about ehci interrupt handling on below usbnet
> test case:

> 	- Pandaboard: IP address is IP_A
> 	- on one x86 box, run below command:
> 		#ping -f -s 1472 IP_A
> 	- compute ehci interrupt handling time on Pandaboard during ping
> 	  test

    This seems just crazy to me. What has been mapped, should be unmapped.

> Cc: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Ming Lei <ming.lei@...onical.com>
> ---
>   drivers/usb/core/hcd.c |    4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

WBR, Sergei


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