linux-kernel - Re: Relation between MMC_CAP_WAIT_WHILE_BUSY and card

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Date:   Mon, 31 Jan 2022 12:14:52 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
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, 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?

> >
> > If it's supposed to be "we'll use card_busy() if
> > MMC_CAP_WAIT_WHILE_BUSY isn't set", then why do we have some mmc host
> > drivers that have both?
> >
> > What am I misunderstanding?
>
> There are some additional complexity for the corresponding code. This
> has mostly ended up there because we also need to deal with mmc
> controller's HW limitations around this feature.
>
> For example, some mmc controllers have a HW limit for the length of
> the timeout that can be set. If the needed timeout is longer than what
> can be supported, we can't use IRQ based busy completion.
>
> Did this make it more clear?

Yes, it does. Much appreciated!

Thanks,
Saravana