linux-kernel - Re: [PATCH v2 2/2] mmc: sdhci-npcm: Add NPCM SDHCI driver

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Message-ID: <CAHp75Vd5DzkCW0Gpouv+0Or=Yhjp_KdFGP-jXkpHD=UZrG2ajA@mail.gmail.com>
Date:   Wed, 7 Dec 2022 15:25:27 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Tomer Maimon <tmaimon77@...il.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>, ulf.hansson@...aro.org,
        avifishman70@...il.com, tali.perry1@...il.com, joel@....id.au,
        venture@...gle.com, yuenn@...gle.com, benjaminfair@...gle.com,
        skhan@...uxfoundation.org, davidgow@...gle.com,
        pbrobinson@...il.com, gsomlo@...il.com, briannorris@...omium.org,
        arnd@...db.de, krakoczy@...micro.com, openbmc@...ts.ozlabs.org,
        linux-mmc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mmc: sdhci-npcm: Add NPCM SDHCI driver

On Wed, Dec 7, 2022 at 3:01 PM Tomer Maimon <tmaimon77@...il.com> wrote:
> On Mon, 5 Dec 2022 at 16:33, Adrian Hunter <adrian.hunter@...el.com> wrote:
> > On 5/12/22 16:17, Andy Shevchenko wrote:
> > > On Mon, Dec 5, 2022 at 4:14 PM Andy Shevchenko
> > > <andy.shevchenko@...il.com> wrote:
> > >> On Mon, Dec 5, 2022 at 3:41 PM Adrian Hunter <adrian.hunter@...el.com> wrote:
> > >>> On 5/12/22 15:25, Andy Shevchenko wrote:
> > >>>> On Mon, Dec 5, 2022 at 1:20 PM Tomer Maimon <tmaimon77@...il.com> wrote:

...

> > >>>> devm_ is problematic in your case.
> > >>>> TL;DR: you need to use clk_get_optional() and clk_put().
> > >>>
> > >>> devm_ calls exactly those, so what is the issue?
> > >>
> > >> The issue is the error path or removal stage where it may or may be
> > >> not problematic. To be on the safe side, the best approach is to make
> > >> sure that allocated resources are being deallocated in the reversed
> > >> order. That said, the
> > >>
> > >> 1. call non-devm_func()
> > >> 2. call devm_func()
> > >>
> > >> is wrong strictly speaking.
> > >
> > > To elaborate more, the
> > >
> > > 1. call all devm_func()
> > > 2. call only non-devm_func()
> > >
> > > is the correct order.
> >
> > 1. WRT pltfm_host->clk, that is what is happening
> > 2. WRT other resources that is simply not always possible because not every resource is wrapped by devm_
> > e.g. mmc_alloc_host() / mmc_free_host()
> I little confused about what to decide, should I use only
> non-devm_func because mmc_alloc_host() / mmc_free_host() is not
> warrped with devm_?

It is up to you how to proceed. I pointed out the problem with your
code which may or may not be fatal.

If you want to solve it, there are several approaches:
1) get rid of devm_ completely;
2) properly shuffle the ordering in ->probe(), so all devm_ calls are
followed by non-devm_;
3) wrap non-devm_ cals to become managed (see
devm_add_action_or_reset() approach);
4) fix SDHCI / MMC layer by providing necessary devm_ calls and/or fix
sdhci_pltfm_register() to handle the clock.

Personally, the list order is from the least, what I prefer, to the
most (i.o.w. I would like to see rather 4) than 1) to be implemented).

> > > Hence in this case the driver can be worked around easily (by
> > > shuffling the order in ->probe() to call devm_ first), but as I said
> > > looking into implementation of the _unregister() I'm pretty sure that
> > > clock management should be in sdhci-pltfm, rather than in all callers
> > > who won't need the full customization.
> > >
> > > Hope this helps to understand my point.
> > >
> > >>>> Your ->remove() callback doesn't free resources in the reversed order
> > >>>> which may or, by luck, may not be the case of all possible crashes,
> > >>>> UAFs, races, etc during removal stage. All the same for error path in
> > >>>> ->probe().
> > >>
> > >> I also pointed out above what would be the outcome of neglecting this rule.

...

> > >>>>>> Why can't you use sdhci_pltfm_register()?
> > >>>>> two things are missing in sdhci_pltfm_register
> > >>>>> 1. clock.
> > >>>>
> > >>>> Taking into account the implementation of the corresponding
> > >>>> _unregister() I would add the clock handling to the _register() one.
> > >>>> Perhaps via a new member of the platform data that supplies the name
> > >>>> and index of the clock and hence all clk_get_optional() / clk_put will
> > >>>> be moved there.
> Do you mean to add it to sdhci_pltfm_register function? if yes I
> believe it will take some time to modify sdhci_pltfm_register
> I prefer not to use sdhci_pltfm_register.

In the Linux kernel we are trying hard to avoid code duplication. Why
do you need it to be open coded? (Yes, I heard you, but somebody
should fix the issues with that funcion at some point, right?)

> > >>>>> 2. Adding SDHCI_CAN_DO_8BIT capability according the eMMC capabilities.
> > >>>>
> > >>>> All the same, why can't platform data be utilised for this?

-- 
With Best Regards,
Andy Shevchenko