linux-kernel - Re: Relation between MMC_CAP_WAIT_WHILE_BUSY and card

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Message-ID: <CAPDyKFr1OhTCsgu64_3sBPdOaCk0HLHrhj+PJH_NB-ZY8srkNg@mail.gmail.com>
Date:   Tue, 1 Feb 2022 09:24:05 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Jaehoon Chung <jh80.chung@...sung.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: Relation between MMC_CAP_WAIT_WHILE_BUSY and card_busy()

On Mon, 31 Jan 2022 at 21:15, Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Mon, Jan 31, 2022 at 7:46 AM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> >
> > On Wed, 26 Jan 2022 at 04:46, Saravana Kannan <saravanak@...gle.com> wrote:
> > >
> > > I'm trying to understand the MMC suspend path a bit.
> > >
> > > I looked at the commit message of 6fa79651cc808f68db6f6f297be5a950ccd5dffb.
> > >
> > > IIUC, if MMC_CAP_WAIT_WHILE_BUSY is set then the mmc framework is
> > > going to depend on the card_busy() op to ensure correctness instead of
> > > using the S_A_TIMEOUT value from the card.
> >
> > MMC_CAP_WAIT_WHILE_BUSY indicates whether the mmc controller supports
> > IRQ based busy detection completion. In other words, the mmc host
> > driver can receive an IRQ when busy signaling is completed on DAT0 by
> > the eMMC card.
> >
> > However, to avoid waiting for the IRQ forever, there is a maximum
> > timeout that is specified by the mmc core, for the particular command
> > in question. For eMMC sleep, the S_A_TIMEOUT.
>
> Ah ok, thanks for the explanation.
>
> >
> > >
> > > But I see a lot of mmc host drivers that implement card_busy() but
> > > don't set the MMC_CAP_WAIT_WHILE_BUSY flag. That doesn't seem right to
> > > me if my understanding is correct.
> >
> > That's perfectly okay. MMC_CAP_WAIT_WHILE_BUSY is IRQ based, while the
> > ->card_busy() ops is used to poll for busy completion.
>
> Yeah, it makes sense now.
>
> One thing I noticed when playing with some hardware is that during
> suspend, when MMC_CAP_WAIT_WHILE_BUSY isn't set and we have a
> card_busy() implementation, we don't seem to be using card_busy() op
> and just always using the timeout from S_A_TIMEOUT. To be more
> specific, I'm talking about this code path:
> _mmc_suspend() -> mmc_sleep() -> mmc_delay() -> msleep()
>
> I'd think card_busy() could be used here if it's implemented. Is there
> a reason for not using it in this path?

That was exactly what commit 6fa79651cc80 ("mmc: core: Enable eMMC
sleep commands to use HW busy polling") implemented. The commit was
introduced in v5.14.

If it doesn't work, there is a bug somewhere.

[...]

Kind regards
Uffe