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

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

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.

>
> 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.

$subject patch should also conform to this pattern.

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.

>
> >> When it comes to patches for stable kernels. mmc_cache_enabled() was
> >> introduced quite recently in v5.13, so for older kernels that call
> >> needs to be replaced with something else.
> >>
> >> In any case, the relevant commits that can be considered as needs to
> >> be fixed seems like these:
> >> commit f4c5522b0a88 ("mmc: Reliable write support.")
> >> commit 881d1c25f765 ("mmc: core: Add cache control for eMMC4.5 device")
> >> commit 130206a615a9 ("mmc: core: Add support for cache ctrl for SD cards")
> >>
> >> [...]
> >
> > Michael, are you planning to send a v2 for this? Or are there any
> > parts that are still unclear to you?
>
> Dear Ulf, Sorry for the delay. I was trying to figure out the SD cache
> stuff, so a few day was taken...

No problem, I have been busy too. :-)

Kind regards
Uffe