Re: [PATCH] mmc: block: enable cache-flushing when mmc cache is on

[Ulf Hansson <ulf.hansson@linaro.org>]

On Tue, 29 Mar 2022 at 11:08, Michael Wu <michael@allwinnertech.com> wrote:
>
> On 28/03/2022 19:38, Ulf Hansson wrote:
> > On Mon, 28 Mar 2022 at 12:11, Michael Wu <michael@allwinnertech.com> wrote:
> >>
> >> On 25/03/2022 18:13, Ulf Hansson wrote:
> >>> On Fri, 25 Mar 2022 at 06:46, Michael Wu <michael@allwinnertech.com> wrote:
> >>>>
> >>>> On 24/03/2022 19:27, Ulf Hansson wrote:
> >>>>> On Thu, 17 Mar 2022 at 10:14, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> >>>>>>
> >>>>>> On Wed, 16 Mar 2022 at 17:08, Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>>>>>>
> >>>>>>> On 16.3.2022 16.46, Christian Löhle wrote:
> >>>>>>>>> So we are not going to let the block layer know about SD cache?
> >>>>>>>>> Or is it a separate change?
> >>>>>>>>
> >>>>>>>> I have some code for this laying around, but as it requires reading, parsing and writing Function Registers,
> >>>>>>>> in particular PEH, it's a lot of boilerplate code to get the functionality, but I'll clean it up and send a patch in the coming weeks.
> >>>>>>>>
> >>>>>>>
> >>>>>>> We have the sd cache flush.  We would presumably just need to call blk_queue_write_cache()
> >>>>>>> for the !mmc_card_mmc(card) case e.g.
> >>>>>>>
> >>>>>>>            if (mmc_has_reliable_write(card)) {
> >>>>>>>                    md->flags |= MMC_BLK_REL_WR;
> >>>>>>>                    enable_fua = true;
> >>>>>>>            }
> >>>>>>>
> >>>>>>>            if (mmc_cache_enabled(card->host))
> >>>>>>>                    enable_cache = true;
> >>>>>>>
> >>>>>>>            blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
> >>>>>>
> >>>>>> To me, this seems like the most reasonable thing to do.
> >>>>>>
> >>>>>> However, I have to admit that it's not clear to me, if there was a
> >>>>>> good reason to why commit f4c5522b0a88 ("mmc: Reliable write
> >>>>>> support.") also added support for REQ_FLUSH (write back cache) and why
> >>>>>> not only REQ_FUA. I assumed this was wrong too, right?
> >>>>>>
> >>>>
> >>>> Hi Ulf,
> >>>>
> >>>> 1. I've found the reason. If we only enable REQ_FUA, there won't be any
> >>>> effect -- The block layer won't send any request with FUA flag to the
> >>>> driver.
> >>>> If we want REQ_FUA to take effect, we must enable REQ_FLUSH. But on the
> >>>> contrary, REQ_FLUSH does not rely on REQ_FUA.
> >>>> In the previous patch(commit f4c5522b0a88 ("mmc: Reliable write
> >>>> support.")), REQ_FLUSH was added to make REQ_FUA effective. I've done
> >>>> experiments to prove this.
> >>>
> >>> Thanks for doing the research and for confirming.
> >>>
> >>> Note that this is also pretty well documented in
> >>> Documentation/block/writeback_cache_control.rst.
> >>
> >> Thanks for reminding. I'm clear now.
> >>
> >>>
> >>>>
> >>>> 2. Why block layer requires REQ_FLUSH to make REQ_FUA effective? I did
> >>>> not find the reason. Does anyone know about this? Thank you.
> >>>
> >>> The REQ_FLUSH indicates that the storage device has a write back
> >>> cache, which also can be flushed in some device specific way.
> >>>
> >>> The REQ_FUA (Force Unit Access), tells that the data can be written to
> >>> the storage device, in a way that when the I/O request is completed,
> >>> the data is fully written to the device (the data must not be left in
> >>> the write back cache). In other words, REQ_FUA doesn't make sense
> >>> unless REQ_FLUSH is supported too.
> >>>
> >>
> >> Thank you for your answer.
> >>
> >>> $subject patch should also conform to this pattern.
> >>
> >> I'm not sure if I understood this in a right way... Did you mean I
> >> should modify the subject of this mail/patch?
> >
> > No, I just meant that the code in the patch should conform to this.
>
> No problem. Please have a look at the code below.
>
> >
> > If REQ_FUA is set, REQ_FLUSH must be set too.
> >
> >>
> >>>
> >>> However, it's still questionable to me whether we want to support
> >>> REQ_FUA through the eMMC reliable write command - in case we also have
> >>> support for managing the eMMC cache separately. It looks to me that
> >>> the reason why we currently support REQ_FUA, is because back in the
> >>> days when there was only the eMMC reliable write command available, it
> >>> was simply the best we could do. But it was never really a good fit.
> >>>
> >>> I am starting to think that we may consider dropping REQ_FUA, if we
> >>> have the option to manage the eMMC cache separately - no matter
> >>> whether the eMMC reliable write command is supported or not. In this
> >>> case, REQ_FLUSH is sufficient and also a better match to what we
> >>> really can support.
> >>
> >> Hi Ulf,
> >> As to dropping REQ_FUA, I don't know if it is a good idea, but generally
> >> we are facing three possible situations:
> >>
> >> 1. If both cache and reliable-write are available, both REQ_FUA and
> >> REQ_FLUSH can be supported at the same time. In this case, with
> >> available cache, the behavior of reliable-write is to write eMMC while
> >> skipping cache, which is consistent with the current kernel's definition
> >> of REQ_FUA. What's more, most eMMCs now support both cache and
> >> reliable-write command.
> >
> > Yes, this seems reasonable.
> >
> >
> >> 2. If only reliable-write is available, REQ_FUA should not be supported,
> >> which is consistent with the current standard in another way. But I
> >> don't think eMMCs that only support reliable-write can be easily found
> >> nowadays.
> >
> > If we drop REQ_FUA for this case, I am worried that we might break use
> > cases for those older eMMC devices.
> >
> > So, no, let's keep REQ_FUA and REQ_FLUSH if reliable-write is supported.
>
> OK. Let's keep them.
>
> >
> >> 3. If only cache is available, we just use REQ_FLUSH. It is not in
> >> conflict with keeping REQ_FUA.
> >
> > Right.
> >
> >>
> >> Maybe, is it more reasonable to reserve FUA and use if/else to pick it
> >> up or down, considering the compatibility? I mean, in most cases, FUA
> >> and FLUSH are complementary. So it seems more feasible with branch to
> >> choose.
> >
> > Let's summarize what I think we should do then:
> >
> >   if (reliable-write supported) {
> >       enable_fua = true;
> >       enable_cache = true;
> > }
> >
> > if (mmc_cache_enabled)
> >       enable_cache = true;
> >
> > blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
> >
> > Does this seem reasonable to you?
>
> Yes. Let me attach the complete code here:
>
>      if (md->flags & MMC_BLK_CMD23 &&
>          ((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN) ||
>          card->ext_csd.rel_sectors)) {
>          md->flags |= MMC_BLK_REL_WR;
>          enable_fua = true;
>          enable_cache = true;
>      }
>
>      if (mmc_cache_enabled(card->host))
>          enable_cache = true;
>
>      blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
>
>
> If this is good, I'll submit a patch-v2 soon.

Yes, this looks good to me!

Kind regards
Uffe