netdev - Re: [ath9k-devel] [PATCH v2 07/46] net/wireless: ath9k: fix DMA API usage

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Date:	Tue, 12 Jul 2011 11:55:41 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Felix Fietkau <nbd@...nwrt.org>
Cc:	netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
	Jouni Malinen <jmalinen@...eros.com>,
	Senthil Balasubramanian <senthilkumar@...eros.com>,
	ath9k-devel@...ts.ath9k.org,
	Vasanthakumar Thiagarajan <vasanth@...eros.com>,
	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org
Subject: Re: [ath9k-devel] [PATCH v2 07/46] net/wireless: ath9k: fix DMA
 API usage

On Tue, Jul 12, 2011 at 12:36:06PM +0800, Felix Fietkau wrote:
> On 2011-07-11 8:52 AM, Michał Mirosław wrote:
> >Also constify buf_addr for ath9k_hw_process_rxdesc_edma() to verify
> >assumptions --- dma_sync_single_for_device() call can be removed.
> >
> >Signed-off-by: Michał Mirosław<mirq-linux@...e.qmqm.pl>
> >---
> >  drivers/net/wireless/ath/ath9k/ar9003_mac.c |    4 ++--
> >  drivers/net/wireless/ath/ath9k/ar9003_mac.h |    2 +-
> >  drivers/net/wireless/ath/ath9k/recv.c       |   10 +++-------
> >  3 files changed, 6 insertions(+), 10 deletions(-)
> >
> >diff --git a/drivers/net/wireless/ath/ath9k/recv.c b/drivers/net/wireless/ath/ath9k/recv.c
> >index 70dc8ec..c5f46d5 100644
> >--- a/drivers/net/wireless/ath/ath9k/recv.c
> >+++ b/drivers/net/wireless/ath/ath9k/recv.c
> >@@ -684,15 +684,11 @@ static bool ath_edma_get_buffers(struct ath_softc *sc,
> >  	BUG_ON(!bf);
> >
> >  	dma_sync_single_for_cpu(sc->dev, bf->bf_buf_addr,
> >-				common->rx_bufsize, DMA_FROM_DEVICE);
> >+				common->rx_bufsize, DMA_BIDIRECTIONAL);
> >
> >  	ret = ath9k_hw_process_rxdesc_edma(ah, NULL, skb->data);
> >-	if (ret == -EINPROGRESS) {
> >-		/*let device gain the buffer again*/
> >-		dma_sync_single_for_device(sc->dev, bf->bf_buf_addr,
> >-				common->rx_bufsize, DMA_FROM_DEVICE);
> >+	if (ret == -EINPROGRESS)
> >  		return false;
> >-	}
> >
> >  	__skb_unlink(skb,&rx_edma->rx_fifo);
> >  	if (ret == -EINVAL) {
> I have strong doubts about this change. On most MIPS devices,
> dma_sync_single_for_cpu is a no-op, whereas
> dma_sync_single_for_device flushes the cache range. With this
> change, the CPU could cache the DMA status part behind skb->data and
> that cache entry would not be flushed inbetween calls to this
> functions on the same buffer, likely leading to rx stalls.

You're suggesting a platform implementation bug then. If the platform is not
cache-coherent, it should invalidate relevant CPU cache lines for sync_to_cpu
and unmap cases. Do other devices show such symptoms on MIPS systems?

I'm not familiar with the platform internals, so we should ask MIPS people.

[added Cc: linux-mips]

Best Regards,
Michał Mirosław
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