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

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Message-ID: <1375075224.25230.1.camel@linux-fkkt.site>
Date:	Mon, 29 Jul 2013 07:20:24 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	David Miller <davem@...emloft.net>
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

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.

	Regards
		Oliver



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