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

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Date:	Tue, 12 Jul 2011 20:54:32 +0800
From:	Felix Fietkau <nbd@....name>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:	Felix Fietkau <nbd@...nwrt.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	Jouni Malinen <jmalinen@...eros.com>,
	Senthil Balasubramanian <senthilkumar@...eros.com>,
	"ath9k-devel@...ts.ath9k.org" <ath9k-devel@...ts.ath9k.org>,
	Vasanthakumar Thiagarajan <vasanth@...eros.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>
Subject: Re: [ath9k-devel] [PATCH v2 07/46] net/wireless: ath9k: fix DMA API usage

On 12.07.2011, at 17:55, Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:

> 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.
I only mentioned MIPS to describe the potential side effect of this change. From my current understanding of the DMA API, it would be wrong on other platforms as well. I believe the _for_device function needs to be used to transfer ownership of the buffer back to the device, before calling _for_cpu again later for another read.
This is definitely required in this case, because when the return code is -EINPROGRESS, the driver waits for the hardware to complete this buffer, and the next call has to fetch the memory again after the device has updated it.
This is handled properly by the current code without your change.

- Felix--
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