Re: [PATCH] mmc: core: Allow to avoid REQ_FUA if the eMMC supports an internal cache

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

On Tue, 21 Mar 2023 at 11:36, Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 16/03/23 18:45, Ulf Hansson wrote:
> > REQ_FUA translates into so called "reliable writes" (atomic writes) for
> > eMMC cards, which is generally supported as it was introduced as a
> > mandatory feature already in the v4.3 (2007) of the eMMC spec. To fully
> > support the reliable writes (thus REQ_FUA), the mmc host driver needs to
> > support the CMD23 (MMC_CAP_CMD23) too, which is rather common nowadays.
> >
> > File systems typically uses REQ_FUA for writing their meta-data and other
> > important information. Ideally it should provide an increased protection
> > against data-corruption, during sudden power-failures. This said, file
> > systems have other ways to handle sudden power-failures too, like using
> > checksums to detect partly-written data, for example.
> >
> > It has been reported that the reliable writes are costly for some eMMCs,
> > leading to performance degradations. Exactly why, is in the implementation
> > details of the internals of the eMMC.
> >
> > Moreover, in the v4.5 (2011) of the eMMC spec, the cache-control was
> > introduced as an optional feature. It allows the host to trigger a flush of
> > the eMMC's internal write-cache. In the past, before the cache-control
> > feature was added, the reliable write acted as trigger for the eMMC, to
> > also flush its internal write-cache, even if that too remains as an
> > implementation detail of the eMMC.
> >
> > In a way to try to improve the situation with costly reliable writes and
> > REQ_FUA, let's add a new card quirk MMC_QUIRK_AVOID_REL_WRITE, which may be
> > set to avoid announcing the support for it. However, as mentioned above,
> > due to the specific relationship with the cache-control feature, we must
> > keep REQ_FUA unless that is supported too.
> >
> > Reported-by: Wenchao Chen <wenchao.chen666@gmail.com>
> > Acked-by: Bean Huo <beanhuo@micron.com>
> > Acked-by: Avri Altman <avri.altman@wdc.com>
> > Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
>
> Minor cosmetic suggestion below, but nevertheless:
>
> Acked-by: Adrian Hunter <adrian.hunter@intel.com>

Thanks!

>
> > ---
> >
> > Updated since the RFC:
> >       Added a card quirk to maintain the current behaviour. The quirk isn't
> >       set for any cards yet, which is needed (a patch on top) to move forward
> >       with this.
> >
> > ---
> >  drivers/mmc/core/block.c | 16 ++++++++++++----
> >  drivers/mmc/core/card.h  |  5 +++++
> >  include/linux/mmc/card.h |  1 +
> >  3 files changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
> > index 672ab90c4b2d..35292e36a1fb 100644
> > --- a/drivers/mmc/core/block.c
> > +++ b/drivers/mmc/core/block.c
> > @@ -2409,8 +2409,7 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
> >       struct mmc_blk_data *md;
> >       int devidx, ret;
> >       char cap_str[10];
> > -     bool cache_enabled = false;
> > -     bool fua_enabled = false;
> > +     bool cache_enabled, avoid_fua, fua_enabled = false;
> >
> >       devidx = ida_simple_get(&mmc_blk_ida, 0, max_devices, GFP_KERNEL);
> >       if (devidx < 0) {
> > @@ -2494,11 +2493,20 @@ static struct mmc_blk_data *mmc_blk_alloc_req(struct mmc_card *card,
> >           ((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN) ||
> >            card->ext_csd.rel_sectors)) {
> >               md->flags |= MMC_BLK_REL_WR;
> > +     }
> > +
> > +     /*
> > +      * REQ_FUA is supported through eMMC reliable writes, which has been
> > +      * reported to be a bit costly for some eMMCs. In these cases, let's
> > +      * rely on the flush requests (REQ_OP_FLUSH) instead, if we can use the
> > +      * cache-control feature too.
> > +      */
> > +     cache_enabled = mmc_cache_enabled(card->host);
> > +     avoid_fua = cache_enabled && mmc_card_avoid_rel_write(card);
> > +     if (md->flags & MMC_BLK_REL_WR && !avoid_fua) {
> >               fua_enabled = true;
> >               cache_enabled = true;
> >       }
>
> looks like this could be just:
>
>         fua_enabled = (md->flags & MMC_BLK_REL_WR) && !avoid_fua;
>
> with fua_enabled no longer needing initialization

Unless I misunderstand your point, that would work for fua_enabled,
but would not be sufficient for cache_enabled.

cache_enabled should be set if fua_enabled is set - and no matter
whether mmc_cache_enabled() returns true or not.

Did that make sense?

[...]

Kind regards
Uffe