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? 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? Kind regards Uffe -- 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
