netdev - Re: [PATCH 0/4] USB & USBNET: loose DMA SG check and support usbnet DMA SG

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Message-ID: <CACVXFVOsympDWLkGHZqzL4jOcxTLS662a18gsix9ZtZNr4tDQA@mail.gmail.com>
Date:	Wed, 31 Jul 2013 19:55:15 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Freddy Xin <freddy@...x.com.tw>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Grant Grundler <grundler@...gle.com>, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH 0/4] USB & USBNET: loose DMA SG check and support usbnet
 DMA SG

On Wed, Jul 31, 2013 at 7:12 PM, Oliver Neukum <oneukum@...e.de> wrote:
> On Wed, 2013-07-31 at 18:51 +0800, Ming Lei wrote:
>> Hi,
>>
>> This patchset allows drivers to pass sg buffers which size can't be divided
>> by max packet size of endpoint if the host controllers support this kind
>> of sg buffers.
>
> Cool. One question though. It makes sense to use sg only
> if all elements are in memory capable of DMA. It seems to be possible
> to have HCs which can do unlimited sg but are limited to 32bit DMA.

For current usbnet devices which supports TSO, one header need to add
before skb->data from network stack, so basically the first two elements aren't
OK for other HCs(EHCI/UHCI/...) to DMA from SG.


Thanks,
--
Ming Le
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