Re: [PATCH] tmio_mmc_dma: fix PIO fallback on SDHI

[Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>]

On 08/02/2013 03:35 PM, Guennadi Liakhovetski wrote:

>> I'm testing SH-Mobile SDHI driver in DMA mode with a  new DMA controller  using
>> 'bonnie++' and getting DMA error after which the tmio_mmc_dma.c code falls back
>> to PIO but all commands time out after that.  It turned out that the fallback
>> code calls tmio_mmc_enable_dma() with RX/TX channels already freed and pointers
>> to them cleared, so that the function bails out early instead  of clearing the
>> DMA bit in the CTL_DMA_ENABLE register.  Fixing the RX/TX channel check so that
>> it takes place only when enabling DMA helps with the PIO fallback.

>> Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>

>> ---
>> The patch is against Chris Ball's 'mmc.git' repo, 'master' branch.

>>   drivers/mmc/host/tmio_mmc_dma.c |    2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)

>> Index: mmc/drivers/mmc/host/tmio_mmc_dma.c
>> =================================>> --- mmc.orig/drivers/mmc/host/tmio_mmc_dma.c
>> +++ mmc/drivers/mmc/host/tmio_mmc_dma.c
>> @@ -25,7 +25,7 @@
>>
>>   void tmio_mmc_enable_dma(struct tmio_mmc_host *host, bool enable)
>>   {
>> -	if (!host->chan_tx || !host->chan_rx)
>> +	if (enable && !(host->chan_tx && host->chan_rx))
>>   		return;

> Ok, I see the problem and this does fix it. But it adds complexity to the
> driver - one more condition to an if.

    So what?

> Whereas, I think, it can be avoided if we just move calls to

> tmio_mmc_enable_dma(host, false);

> in tmio_mmc_start_dma_rx() and tmio_mmc_start_dma_tx() a couple of lines

    Well, not couple. :-P

> up - before clearing ->chan_rx and ->chan_tx pointers? That should work
> too at no cost.

    Well, it was my first variant. However, not knowing the hardware well, I 
deemed it not quite safe to clear the DMA bit before shutting down the DMA 
channels, so came up with the second version (which I consider more logically 
correct).

> I think that would be a better fix, could you, please, try?

    I tried it but mystically fixing the PIO fallback makes DMA error less 
frequent. IIRC, I didn't see DMA error with the first variant of the fix, so 
I'll have to retry it...

> Thanks
> Guennadi

WBR, Sergei