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

From: Felix Fietkau <[email-masked]>
Date: Tue, 12 Jul 2011 12:36:06 +0800

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<[email-masked]>
> ---
>   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.

- Felix
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