On 11/10/17 16:58, Ulf Hansson wrote: > 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. It already does that. > > 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. It is in interrupt context also. So if the block layer work is on the same CPU it will anyway wait. > > 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. Ideally it should be possible to start the next already prepared request from the ->done() callback of the current request. Perhaps add a new host API.
