Re: [RFC PATCH] mmc: core: Disable REQ_FUA if the eMMC supports an internal cache

[Ulf Hansson]

On Fri, 3 Mar 2023 at 10:39, Avri Altman <Avri.Altman@xxxxxxx> wrote:
>
> > On 02.03.23 3:43 PM, Ulf Hansson wrote:
> > > REQ_FUA is in general supported for eMMC cards, which translates into
> > > so called "reliable writes". To support these write operations, the
> > > CMD23 (MMC_CAP_CMD23), needs to be supported by the mmc host too,
> > > which is common but not always the case.
> > >
> > > For some eMMC devices, it has been reported that reliable writes are
> > > quite costly, leading to performance degradations.
> > >
> > > In a way to improve the situation, let's avoid announcing REQ_FUA
> > > support if the eMMC supports an internal cache, as that allows us to
> > > rely solely on flush-requests (REQ_OP_FLUSH) instead, which seems to be a
> > lot cheaper.
> > > Note that, those mmc hosts that lacks CMD23 support are already using
> > > this type of configuration, whatever that could mean.
> > >
> > > Reported-by: Wenchao Chen<wenchao.chen666@xxxxxxxxx>
> > > Signed-off-by: Ulf Hansson<ulf.hansson@xxxxxxxxxx>
> > Acked-by: Bean Huo <beanhuo@xxxxxxxxxx>
> Acked-by: Avri Altman <avri.altman@xxxxxxx>

Thanks!

>
> Another option might be, allowing to report "broken_fua",
> should the platform owner chooses to, much like scsi does per sdev.

Are you suggesting a static or dynamic configuration option?

For mmc, we also have the card quirks that may be used to configure
the support for FUA, based upon what would work best for the card. Is
that what you were thinking of?

>
> Thanks,
> Avri

Kind regards
Uffe