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

On 07/11/2011 09:36 PM, Felix Fietkau wrote:

On 2011-07-11 8:52 AM, Michał Mirosław wrote:

quoted

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<redacted>
---
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.

At the very least, it would need heavy testing.  It took a very long time to get
the ath9k DMA issues (mostly?) resolved...so we shouldn't go mucking in this
code on theory...

Thanks,
Ben

- Felix
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at http://vger.kernel.org/majordomo-info.html

-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com
--
To unsubscribe from this list: send the line "unsubscribe linux-wireless" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html