Re: [PATCH V8 00/14] mmc: Add Command Queue support

[Adrian Hunter <adrian.hunter@intel.com>]

On 10/10/17 16:08, Ulf Hansson wrote:
> [...]
> 
>>>>>
>>>>> I have also run some test on my ux500 board and enabling the blkmq
>>>>> path via the new MMC Kconfig option. My idea was to run some iozone
>>>>> comparisons between the legacy path and the new blkmq path, but I just
>>>>> couldn't get to that point because of the following errors.
>>>>>
>>>>> I am using a Kingston 4GB SDHC card, which is detected and mounted
>>>>> nicely. However, when I decide to do some writes to the card I get the
>>>>> following errors.
>>>>>
>>>>> root@ME:/mnt/sdcard dd if=/dev/zero of=testfile bs=8192 count=5000 conv=fsync
>>>>> [  463.714294] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  464.722656] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  466.081481] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  467.111236] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  468.669647] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  469.685699] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  471.043334] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  472.052337] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  473.342651] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  474.323760] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  475.544769] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  476.539031] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  477.748474] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>> [  478.724182] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>>>>
>>>>> I haven't yet got the point of investigating this any further, and
>>>>> unfortunate I have a busy schedule with traveling next week. I will do
>>>>> my best to look into this as soon as I can.
>>>>>
>>>>> Perhaps you have some ideas?
>>>>
>>>> The behaviour depends on whether you have MMC_CAP_WAIT_WHILE_BUSY. Try
>>>> changing that and see if it makes a difference.
>>>
>>> Yes, it does! I disabled MMC_CAP_WAIT_WHILE_BUSY (and its
>>> corresponding code in mmci.c) and the errors goes away.
>>>
>>> When I use MMC_CAP_WAIT_WHILE_BUSY I get these problems:
>>>
>>> [  223.820983] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  224.815795] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  226.034881] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  227.112884] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  227.220275] mmc0: Card stuck in wrong state! mmcblk0 mmc_blk_card_stuck
>>> [  228.686798] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  229.892150] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  231.031890] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> [  232.239013] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
>>> 5000+0 records in
>>> 5000+0 records out
>>> root@ME:/mnt/sdcard
>>>
>>> I looked at the new blkmq code from patch v10 13/15. It seems like the
>>> MMC_CAP_WAIT_WHILE_BUSY is used to determine whether the async request
>>> mechanism should be used or not. Perhaps I didn't looked close enough,
>>> but maybe you could elaborate on why this seems to be the case!?
>>
>> MMC_CAP_WAIT_WHILE_BUSY is necessary because it means that a data transfer
>> request has finished when the host controller calls mmc_request_done(). i.e.
>> polling the card is not necessary.
> 
> Well, that is a rather big change on its own. Earlier we polled with
> CMD13 to verify that the card has moved back to the transfer state, in
> case it was a write. And that was no matter of MMC_CAP_WAIT_WHILE_BUSY
> was set or not. Right!?

Yes

> 
> I am not sure it's a good idea to bypass that validation, it seems
> fragile to rely only on the busy detection on DAT line for writes.

Can you cite something from the specifications that backs that up, because I
couldn't find anything to suggest that CMD13 polling was expected.

> 
>>
>> Have you tried V9 or V10.  There was a fix in V9 related to calling
>> ->post_req() which could mess up DMA.
> 
> I have used V10.
> 
>>
>> The other thing that could go wrong with DMA is if it cannot accept
>> ->post_req() being called from mmc_request_done().
> 
> I don't think mmci has a problem with that, however why do you want to
> do this? Wouldn't that defeat some of the benefits with the async
> request mechanism?

Perhaps - but it would need to be tested.  If there are more requests
waiting, one optimization could be to defer ->post_req() until after the
next request is started.