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
