From: nicolas.ferre@atmel•com (Nicolas Ferre)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v2] net/macb: Use non-coherent memory for rx buffers
Date: Wed, 5 Dec 2012 16:12:39 +0100 [thread overview]
Message-ID: <50BF6467.5060701@atmel.com> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B70D9@saturn3.aculab.com>
On 12/05/2012 10:35 AM, David Laight :
>> If I understand well, you mean that the call to:
>>
>> dma_sync_single_range_for_device(&bp->pdev->dev, phys,
>> pg_offset, frag_len, DMA_FROM_DEVICE);
>>
>> in the rx path after having copied the data to skb is not needed?
>> That is also the conclusion that I found after having thinking about
>> this again... I will check this.
>
> You need to make sure that the memory isn't in the data cache
> when you give the rx buffer back to the MAC.
> (and ensure the cpu doesn't read it until the rx is complete.)
> I've NFI what that dma_sync call does - you need to invalidate
> the cache lines.
The invalidate of cache lines is done by
dma_sync_single_range_for_device(, DMA_FROM_DEVICE) so I need to keep it.
>> For the CRC, my driver is not using the CRC offloading feature for the
>> moment. So no CRC is written by the device.
>
> I was thinking it would matter if the MAC wrote the CRC into the
> buffer (even though it was excluded from the length).
> It doesn't - you only need to worry about data you've read.
>
>>> I was wondering if the code needs to do per page allocations?
>>> Perhaps that is necessary to avoid needing a large block of
>>> contiguous physical memory (and virtual addresses)?
>>
>> The page management seems interesting for future management of RX
>> buffers as skb fragments: that will allow to avoid copying received data.
>
> Dunno - the complexities of such buffer loaning schemes often
> exceed the gain of avoiding the data copy.
> Using buffers allocated to the skb is a bit different - since
> you completely forget about the memory once you pass the skb
> upstream.
>
> Some quick sums indicate you might want to allocate 8k memory
> blocks and split into 5 buffers.
Well, for the 10/100 MACB interface, I am stuck with 128 Bytes buffers!
So this use of pages seems sensible.
On the other hand, it is true that I may have to reconsider the GEM
memory management (it one is able to cover 128-10KB rx DMA buffers)...
Best regards,
--
Nicolas Ferre
next prev parent reply other threads:[~2012-12-05 15:12 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-23 13:50 [PATCH] net/macb: Use non-coherent memory for rx buffers Nicolas Ferre
2012-11-23 16:12 ` Joachim Eastwood
2012-11-26 10:44 ` Nicolas Ferre
2012-12-03 12:14 ` [PATCH v2] " Nicolas Ferre
2012-12-03 12:43 ` David Laight
2012-12-03 13:21 ` Nicolas Ferre
2012-12-03 14:25 ` David Laight
2012-12-04 17:16 ` Nicolas Ferre
2012-12-05 9:35 ` David Laight
2012-12-05 15:12 ` Nicolas Ferre [this message]
2012-12-05 15:22 ` David Laight
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