linux-kernel - Re: [RFC PATCH] mmc: sdhci-of-at91: don't put device in suspend after probe

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Date:	Thu, 11 Feb 2016 10:46:08 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Ulf Hansson <ulf.hansson@...aro.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>
Cc:	Ludovic Desroches <ludovic.desroches@...el.com>
Subject: Re: [RFC PATCH] mmc: sdhci-of-at91: don't put device in suspend after probe

[...]

>> >
>> >> Currently, sdhci disables card detect interrupts when runtime suspended,
>> >> and drivers use a card-detect GPIO to wake-up.
>> >>
>> >
>> > It is what I have seen going through the sdhci layer. So next question is:
>> > is it normal to not take care of card detect interrupts? We keep enabled
>> > some IRQs probably for SDIO modules IRQ but not for card detection. I
>> > don't understand the reason.
>>
>> If SDIO IRQ is enabled the mmc controller needs to stay runtime
>> resumed (as it's the mmc controller that monitors the IRQ), unless you
>> can re-configure the SDIO IRQ as a wakeup. For example by re-routing
>> it to a GPIO irq.
>> Whether this wakeup configuration can stay the same between system PM
>> and runtime PM is SoC dependent.
>
> I don't know if we are talking about the same thing. In
> sdhci_runtime_suspend_host(), we set SDHCI_INT_CARD_INT as a kind of
> wakeup irq before considering the host as suspended
> (host->runtime_suspended = true), isn't it?

No, you have got this wrong. The card detect IRQ is disabled at
sdhci_runtime_suspend_host().

It's not related to SDIO irq, as in that case the controller can't be
runtime suspended (unless wakeup is supported).

>
> Why not adding SDHCI_INT_CARD_INSERT | SDHCI_INT_CARD_REMOVE?
>
> In my case, it allows me to use runtime PM to disable two clocks and
> keep one enabled (I need this one to get the irq) when suspending the device.

That seems like a very special case and normally not how runtime PM is used.

So, if you only want to disable|enable some clocks from runtime PM, I
suggest you keep that out of the library function
sdhci_runtime_suspend_host() and deal with that in your driver
instead.

>
>>
>> Regarding card detects in runtime PM:
>>
>> If you have an option to use GPIO IRQs or it's possible to configure
>> the card detect IRQ as a wakeup in any other way, runtime PM works
>> fine.
>>
>
> It will depend of the customer but I am not sure I'll want to use a pio
> as a gpio for this if there is a pio routed to the sdhci controller.

That all has to do with how the SoC is designed from power management
point of view.

In general, it's a good idea to have card detect on GPIO, as it allows
to put other unused parts of the silicon into a low power state.

>
>> Now, when the card detect *can't* be configured as a wakeup in runtime
>> suspend mode, there are two options.
>>
>> 1) Rely on using MMC_CAP_NEEDS_POLL.
>> 2) Prevent runtime PM.
>>
>> Which option that's preferred is SoC/ mmc controller dependent.
>> Although but please consider below recommendations.
>>
>> - In some cases using polling works really well, as the the mmc core
>> get fast/easy information about whether a card is inserted or not, via
>> the ->get_cd() host ops.
>>
>> - In some cases ->get_cd() is broken (or not implemented) and always
>> returns a value indicating a card is inserted. That means external
>> regulators for the card must be enabled and a card initialization
>> sequence needs to be executed to find out whether a card was *really*
>> inserted.
>>
>> So to conclude, if the controller supports card detection but without
>> wakeup support and the polling mode sucks, then it probably better to
>> prevent runtime PM. Otherwise polling is probably better.
>>
>
> It is weird to claim that I need polling since I have a working card
> detect.

It's not, if you really care about saving power.

Although, as I stated, which solution that's best, depends on the SoC.

[...]

So, we have a regression to fix here. I can propose a patch adopting
the above recommendations!?

That's solution doesn’t have to stay long term, as you can try to
optimize it later on to what suits your SoC the best.

Kind regards
Uffe