Re: [PATCH] mmc: fix race condition between dma and omap_hsmmc callback

[Venkatraman S <svenkatr@ti.com>]

On Tue, Apr 13, 2010 at 3:17 PM, Adrian Hunter <adrian.hunter@nokia.com> wrote:
> Venkatraman S wrote:
>>
>> This patch addresses the possible race condition between the dma
>> callback and hsmmc callback.
>
> Can you explain the problem in more detail?  If the final DMA interrupt
> comes before TC then all should be well.
   Actually it isn't, with descriptor loading. If the DMA callback arrives
"too early", the MMC TC is missed sometimes. The last transfer in the log
below is actually stalled waiting for TC. This happens more often when the
transfer is large (> 300 blocks)

>If it comes after, then we need
> to be sure that the DMA has finished - particularly in the "read" case.
> Neither the existing code nor this patch seems to address that issue.
>
   With this patch the assumption is that MMC TC correctly signals the
end of read / write as requested. I don't know if there are any specific reasons
to believe otherwise.

> Also, how can we be sure the final DMA interrupt doesn't race with the
> start of the next request?
>
Till the time omap_hsmmc_xfer_done is called, the next request is not
expected to arrive.
The requests are implicitly serialised by host->sem, which is released
only on the DMA callback.
The code releases the semaphore only after freeing the DMA channel.
---------------------------------------------------------------------------------------------

mmc0: starting CMD17 arg 00000066 flags 000000b5
mmc0:     blksz 512 blocks 1 flags 00000200 tsac 100 ms nsac 0
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD17, argument 0x00000066
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 1
mmci-omap-hs mmci-omap-hs.0: final dma callback
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 2
mmc0: req done (CMD17): 0: 00000900 00000000 00000000 00000000
mmc0:     512 bytes transferred: 0
mmc0: starting CMD18 arg 000025df flags 000000b5
mmc0:     blksz 512 blocks 32 flags 00000200 tsac 100 ms nsac 0
mmc0:     CMD12 arg 00000000 flags 0000049d
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD18, argument 0x000025df
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 1
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 2
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD12, argument 0x00000000
mmci-omap-hs mmci-omap-hs.0: final dma callback
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 3
mmc0: req done (CMD18): 0: 00000900 00000000 00000000 00000000
mmc0:     16384 bytes transferred: 0
mmc0:     (CMD12): 0: 00000b00 00000000 00000000 00000000
mmc0: starting CMD18 arg 000025ff flags 000000b5
mmc0:     blksz 512 blocks 192 flags 00000200 tsac 100 ms nsac 0
mmc0:     CMD12 arg 00000000 flags 0000049d
mmci-omap-hs mmci-omap-hs.0: new sglist 9fd00000 len =23
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD18, argument 0x000025ff
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 1
mmci-omap-hs mmci-omap-hs.0: final dma callback
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 2
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD12, argument 0x00000000
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 3
mmc0: req done (CMD18): 0: 00000900 00000000 00000000 00000000
mmc0:     98304 bytes transferred: 0
mmc0:     (CMD12): 0: 00000b00 00000000 00000000 00000000
mmc0: starting CMD18 arg 000026bf flags 000000b5
mmc0:     blksz 512 blocks 512 flags 00000200 tsac 100 ms nsac 0
mmc0:     CMD12 arg 00000000 flags 0000049d
mmci-omap-hs mmci-omap-hs.0: new sglist 9fd00000 len =26
mmci-omap-hs mmci-omap-hs.0: mmc0: CMD18, argument 0x000026bf
mmci-omap-hs mmci-omap-hs.0: IRQ Status is 1
mmci-omap-hs mmci-omap-hs.0: final dma callback