On 02/05/16 11:24, Ulf Hansson wrote: > On 28 April 2016 at 15:02, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote: >> On 28/04/16 14:46, Ulf Hansson wrote: >>> On 28 April 2016 at 13:02, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote: >>>> On 28/04/16 13:34, Ulf Hansson wrote: >>>>> On 21 April 2016 at 15:28, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote: >>>>>> The RPMB partition only allows certain commands. In particular, >>>>>> the tuning command (CMD21) is not allowed - refer JEDEC eMMC >>>>>> standard v5.1 section 6.2.2 Command restrictions. >>>>>> >>>>>> To avoid tuning for RPMB, switch to High Speed mode from HS200 >>>>>> or HS400 mode if re-tuning has been enabled. And switch back >>>>>> when leaving RPMB. >>>>> >>>>> I would rather just disable re-tuning during this period, instead of >>>>> changing the speed mode. >>>>> The primary reason to why, is because the latency it would introduce >>>>> to first switch to HS speed then back to HS200/400. >>>> >>>> I wouldn't expect RPMB accesses to be frequent enough for the latency to matter. >>>> >>>>> >>>>> My concern is not the throughput as I expect read/writes request to an >>>>> RPMB partition is rather small. >>>>> >>>>> Of course I realize that we need to take care when disable re-tuning. >>>>> Perhaps we can solve that by a re-try mechanism if the RPMB request >>>>> fails, and thus perform the re-tuning as part of the re-try? >>>> >>>> The interdependent nature of RPMB commands suggests that re-trying is not >>>> possible. It seems to me that you would have to make up a new set of >>>> commands and start again. i.e. return an error to the user so that they can >>>> start again. >>> >>> Ok. >>> >>> So perhaps returning -EAGAIN could work!? >> >> I don't think existing code would expect that. Doesn't seem like level of >> service I would expect from the kernel. >> >> And the concern is, that being an error path, it gets overlooked. > > I guess you are right. > >> >>> >>>> >>>> Another dependency is that we always need to re-tune after host runtime >>>> suspend, which is why we always hit this problem when RPMB is accessed. So >>> >>> Just to make sure I understand correctly; I would imagine you hit the >>> problem *only* when the RPMB partition was already selected, right? >> >> Yes >> >>> >>> Because that would then skip the switch command, and you will >>> therefore try to re-tune after the partition has already been switched >>> to? >> >> Yes >> >>> >>>> to avoid errors you would either need to disable runtime PM when the RPMB >>>> partition is selected (which might be a long time if we don't get an access >>>> to another partition), or always switch back to the main partition (not sure >>>> if that would mess up the RPMB command sequence though). >>> >>> I wouldn't mind that we switch back to the main partition when we have >>> served an RPMB IOCTL request. Of course not in between every mmc >>> request, in case of using the MULTI IOCTL. >>> >>> That would prevent the next regular mmc request on the main partition >>> to not have to switch partition and thus get decreased latency. >> >> Doesn't stop us getting CRC errors because the eMMC needs tuning while in >> the RPMB partition though. > > That's true. My idea was that we should return -EAGAIN as error code > to user space for these scenarios, but I guess it's not a good idea. > > I have given your suggested approach some more thinking. Besides the > latency impact, don't you think it's rather risky to switch speed > modes back an forth? > We don't know whether cards+controllers are really able to cope with > that, even if they should? Yes there is some risk, although that is what we already have to do for re-tuning in the HS400 case. Also I would expect it to fail straight away if it was going to i.e. people testing their systems would know. Given that we don't have a solution at all at the moment, that seemed acceptable. > > Perhaps we could instead force a re-tune to be done before switching > to the RPMB partition and thus also before each RPMB request? That > decreases/removes the probability of getting a CRC errors because of a > needed re-tune, right? Yes re-tuning beforehand should work. I would suggest switching straight back afterwards as well to avoid the possibility of going out of tune while still switched to RPMB. I am happy to implement that, if you agree. -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
