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

[Ulf Hansson]

On 10 October 2017 at 15:31, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> 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.

No I can't, but I don't see why that matters.

My point is, if we want to go down that road by avoiding the CMD13
polling, that needs to be a separate change, which we can test and
confirm on its own.

>
>>
>>>
>>> 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.

This is already proven, because this how the existing mmc async
request mechanism works.

In ->post_req() callbacks, host drivers may do dma_unmap_sg(), which
is something that could be costly and therefore it's better to start a
new request before, such these things can go on in parallel.

Kind regards
Uffe