netdev - Re: Problem with non aligned DMA in usbnet on ARM

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Message-ID: <4C642AA9.1060404@parrot.com>
Date:	Thu, 12 Aug 2010 19:08:57 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	"mfuzzey@...il.com" <mfuzzey@...il.com>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Gary King <GKing@...dia.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-usb <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Greg KH <greg@...ah.com>
Subject: Re: Problem with non aligned DMA in usbnet on ARM

Martin Fuzzey a écrit :
>> We don't want to add support for this to DMA bounce.  DMA bounce is already
>> a pain in the backside and causes its own set of problems - please let it
>> die a long slow but quite death.
>>
>> If you want to see the kind of pain dmabounce causes, look at this long
>> standing and as yet unsolved bug:
>>
>>   http://bugzilla.kernel.org/show_bug.cgi?id=7760
>>
>>   
> Well I don't know the dmabounce code but why is using it likely to cause
> OOM problems (at least why more so than copying the buffer in the HCD or
> the usb core). In both cases there will be two copies of the buffer in
> memory which could I agree be a problem in memory constrained systems.
> But if we _do_ want to accept unaligned buffers from usb drivers I can't
> see  a way round that.
> 
memmove is our friend :
the buffer allocated in usbnet got an offset.
All you have to do it remove this offset and memmove the data. That what 
I did [1], and why it is better to do it in usb driver.


Matthieu




[1] http://article.gmane.org/gmane.linux.usb.general/28700

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