netdev - Re: [PATCH v2 1/3] net/usb/r815x: replace USB buffer from stack to DMA-able

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Date:	Mon, 29 Jul 2013 00:46:02 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	oneukum@...e.de
Cc:	hayeswang@...ltek.com, gregkh@...uxfoundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH v2 1/3] net/usb/r815x: replace USB buffer from stack to
 DMA-able

From: Oliver Neukum <oneukum@...e.de>
Date: Mon, 29 Jul 2013 07:20:24 +0200

> On Sat, 2013-07-27 at 20:21 -0700, David Miller wrote:
>> From: Hayes Wang <hayeswang@...ltek.com>
>> Date: Thu, 25 Jul 2013 15:59:02 +0800
>> 
>> > Some USB buffers use stack which may not be DMA-able.
>> > Use the buffers from kmalloc to replace those one.
>> > 
>> > Signed-off-by: Hayes Wang <hayeswang@...ltek.com>
>> 
>> I don't think it's reasonable to kmalloc() a small integer every time
>> you want to use a USB message transfer to read or write chip
>> registers.
>> 
>> Instead, add a scratch buffer to struct r8152 which is allocated once
>> at driver attach time and which you can use for the transfers.
>> 
>> I think you only need an array of two u32's so something like:
>> 
>> 	u32		transfer_buf[2];
>> 
>> ought to be sufficient.
> 
> We cannot do that. It would violate the rules about DMA coherency.
> We must not touch the same cacheline while DMA is in operation.

Good point, then it'll need to be:

	u32		*transfer_buf;

and allocated appropriately.
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