linux-kernel - Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer ordering in device

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Date:   Fri, 6 Jul 2018 10:47:32 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Dave Young <dyoung@...hat.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Kishon Vijay Abraham I <kishon@...com>
Subject: Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer
 ordering in device_kset

On Fri, Jul 6, 2018 at 10:36 AM, Lukas Wunner <lukas@...ner.de> wrote:
> [cc += Kishon Vijay Abraham]
>
> On Thu, Jul 05, 2018 at 11:18:28AM +0200, Rafael J. Wysocki wrote:
>> OK, so calling devices_kset_move_last() from really_probe() clearly is
>> a mistake.
>>
>> I'm not really sure what the intention of it was as the changelog of
>> commit 52cdbdd49853d doesn't really explain that (why would it be
>> insufficient without that change?)
>
> It seems 52cdbdd49853d fixed an issue with boards which have an MMC
> whose reset pin needs to be driven high on shutdown, lest the MMC
> won't be found on the next boot.
>
> The boards' devicetrees use a kludge wherein the reset pin is modelled
> as a regulator.  The regulator is enabled when the MMC probes and
> disabled on driver unbind and shutdown.  As a result, the pin is driven
> low on shutdown and the MMC is not found on the next boot.
>
> To fix this, another kludge was invented wherein the GPIO expander
> driving the reset pin unconditionally drives all its pins high on
> shutdown, see pcf857x_shutdown() in drivers/gpio/gpio-pcf857x.c
> (commit adc284755055, "gpio: pcf857x: restore the initial line state
> of all pcf lines").
>
> For this kludge to work, the GPIO expander's ->shutdown hook needs to
> be executed after the MMC expander's ->shutdown hook.
>
> Commit 52cdbdd49853d achieved that by reordering devices_kset according
> to the probe order.  Apparently the MMC probes after the GPIO expander,
> possibly because it returns -EPROBE_DEFER if the vmmc regulator isn't
> available yet, see mmc_regulator_get_supply().
>
> Note, I'm just piecing the information together from git history,
> I'm not responsible for these kludges.  (I'm innocent!)

Sure enough. :-)

In any case, calling devices_kset_move_last() in really_probe() is
plain broken and if its only purpose was to address a single, arguably
kludgy, use case, let's just get rid of it in the first place IMO.

> @Pingfan Liu, if you just remove the call to devices_kset_move_last()
> from really_probe(), does the issue go away?

I would think so from the description of the problem (elsewhere in this thread).

Thanks,
Rafael