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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Fri, 6 Jul 2018 21:52:21 +0800
Message-ID: <CAFgQCTsouCPy7GWGFzWbLuSUxnE2rWe5EGqsE5ivT+=ATw2MJg@mail.gmail.com>
Subject: Re: [PATCHv3 0/4] drivers/base: bugfix for supplier<-consumer
 ordering in device_kset
To:     lukas@wunner.de
Cc:     rafael@kernel.org, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Grygorii Strashko <grygorii.strashko@ti.com>,
        Christoph Hellwig <hch@infradead.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Dave Young <dyoung@redhat.com>, linux-pci@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-pm@vger.kernel.org,
        kishon@ti.com
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On Fri, Jul 6, 2018 at 4:36 PM Lukas Wunner <lukas@wunner.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!)
>
Thanks for your exploration, very clearly. I had tried, but failed
since these commits are contributed with different authors. I am not
familiar with ARM and dts, so had thought
really_probe()->devices_kset_move_last() is used to address a very
popular "supplier<-consumer" order issue in smart phone, based on the
configuration hard-coded in "bios(or counterpart in ARM).

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

> If so, it might be best to remove that call and model the dependency
> with a call to device_link_add() in mmc_regulator_get_supply().
> Another idea would be to automatically add a device_link in the
> driver core if -EPROBE_DEFER is returned.
>
Maybe the first one is better, as it is already used by other drivers.

Thanks,
Pingfan