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

[Ulf Hansson]

On 11 October 2017 at 14:58, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> On 11/10/17 15:13, Ulf Hansson wrote:
>> 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.
>
> OK I will make a patch that takes care of both issues.  That will also mean
> the request is not completed in the ->done() callback because ->post_req()
> must precede block layer completion.

Right.

Actually completing the request in the ->done callback, may still be
possible, because in principle it only needs to inform the other
prepared request that it may start, before it continues to post
process/completes the current one.

However, by looking at for example how mmci.c works, it actually holds
its spinlock while it calls mmc_request_done(). The same spinlock is
taken in the ->request() function, but not in the ->post_req()
function. In other words, completing the request in the ->done()
callback, would make mmci to keep the spinlock held throughout the
post processing cycle, which then prevents the next request from being
started.

So my conclusion is, let's start a as you suggested, by not completing
the request in ->done() as to maintain existing behavior. Then we can
address optimizations on top, which very likely will involve doing
changes to host drivers as well.

Kind regards
Uffe