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

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

On 19/10/17 14:44, Adrian Hunter wrote:
> On 18/10/17 09:16, Adrian Hunter wrote:
>> On 11/10/17 16:58, Ulf Hansson wrote:
>>> On 11 October 2017 at 14:58, Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>> On 11/10/17 15:13, Ulf Hansson wrote:
>>>>> On 10 October 2017 at 15:31, Adrian Hunter <adrian.hunter@intel.com> 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.
>>
>> Have you tested the latest version now?
>>
> 
> Ping?

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