linux-kernel - Re: [PATCH] mmc: don't request CD IRQ until mmc_start

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Message-ID: <541B0CFC.1000005@wwwdotorg.org>
Date:	Thu, 18 Sep 2014 10:49:00 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Adrian Hunter <adrian.hunter@...el.com>,
	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Chris Ball <chris@...ntf.net>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephen Warren <swarren@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Alexandre Courbot <acourbot@...dia.com>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH] mmc: don't request CD IRQ until mmc_start_host()

On 09/18/2014 12:49 AM, Adrian Hunter wrote:
> On 09/18/2014 08:25 AM, Adrian Hunter wrote:
>> On 09/17/2014 10:57 PM, Stephen Warren wrote:
>>> On 09/17/2014 01:55 PM, Ulf Hansson wrote:
>>>> On 12 September 2014 19:18, Stephen Warren <swarren@...dotorg.org> wrote:
>>>>> From: Stephen Warren <swarren@...dia.com>
>>>>>
>>>>> As soon as the CD IRQ is requested, it can trigger, since it's an
>>>>> externally controlled event. If it does, delayed_work host->detect will
>>>>> be scheduled.
>>>>>
>>>>> Many host controller probe()s are roughly structured as:
>>>>>
>>>>> *_probe() {
>>>>>       host = sdhci_pltfm_init();
>>>>>       mmc_of_parse(host->mmc);
>>>>>       rc = sdhci_add_host(host);
>>>>>       if (rc) {
>>>>>           sdhci_pltfm_free();
>>>>>           return rc;
>>>>>       }
>>>>>
>>>>> In 3.17, CD IRQs can are enabled quite early via *_probe() ->
>>>>> mmc_of_parse() -> mmc_gpio_request_cd() -> mmc_gpiod_request_cd_irq().
>>>>>
>>>>> Note that in linux-next, mmc_of_parse() calls mmc_gpio*d*_request_cd()
>>>>> rather than mmc_gpio_request_cd(), and mmc_gpio*d*_request_cd() doesn't
>>>>> call mmc_gpiod_request_cd_irq(). However, this issue still exists for
>>>>> any other direct users of mmc_gpio_request_cd().
>>>>>
>>>>> sdhci_add_host() may fail part way through (e.g. due to deferred
>>>>> probe for a vmmc regulator), and sdhci_pltfm_free() does nothing to
>>>>> unrequest the CD IRQ nor cancel the delayed_work. sdhci_pltfm_free() is
>>>>> coded to assume that if sdhci_add_host() failed, then the delayed_work
>>>>> cannot (or should not) have been triggered.
>>>>>
>>>>> This can lead to the following with CONFIG_DEBUG_OBJECTS_* enabled, when
>>>>> kfree(host) is eventually called inside sdhci_pltfm_free():
>>>>>
>>>>> WARNING: CPU: 2 PID: 6 at lib/debugobjects.c:263
>>>>> debug_print_object+0x8c/0xb4()
>>>>> ODEBUG: free active (active state 0) object type: timer_list hint:
>>>>> delayed_work_timer_fn+0x0/0x18
>>>>>
>>>>> The object being complained about is host->detect.
>>>>>
>>>>> There's no need to request the CD IRQ so early; mmc_start_host() already
>>>>> requests it, and I *assume* that mmc_start_host() is called somehow for
>>>>> all host controllers. For SDHCI hosts at least, the typical call path
>>>>> that does this is: *_probe() -> sdhci_add_host() -> mmc_add_host() ->
>>>>> mmc_start_host(). So, remove the call to mmc_gpiod_request_cd_irq() from
>>>>> mmc_gpio_request_cd(). This matches mmc_gpio*d*_request_cd(), which
>>>>> already doesn't call mmc_gpiod_request_cd_irq().
>>>>>
>>>>> This solves the problem (eliminates the kernel error message above),
>>>>> since it guarantees that the IRQ can't trigger before mmc_start_host()
>>>>> is called.
>>>>>
>>>>> The critical point here is that once sdhci_add_host() calls
>>>>> mmc_add_host() -> mmc_start_host(), sdhci_add_host() is coded not to
>>>>> fail. In other words, if there's a chance that mmc_start_host() may have
>>>>> been called, and CD IRQs triggered, and the delayed_work scheduled,
>>>>> sdhci_add_host() won't fail, and so cleanup is no longer via
>>>>> sdhci_pltfm_free() (which doesn't free the IRQ or cancel the work queue)
>>>>> but instead must be via sdhci_remove_host(), which calls mmc_remove_host()
>>>>> -> mmc_stop_host(), which does free the IRQ and cancel the work queue.
>>>>>
>>>>> This fixes what I might conclude to be a mistake in commit 740a221ef0e5
>>>>> ("mmc: slot-gpio: Add GPIO descriptor based CD GPIO API"), which added the
>>>>> call from mmc_start_host() to mmc_gpiod_request_cd_irq(), but also added
>>>>> incorrectly added a call from mmc_gpio_request_cd() to
>>>>> mmc_gpiod_request_cd_irq().
>
> That comment is wrong.  mmc_gpio_request_cd() has always set up the irq.

Uggh, yes. I did misinterpret your patch again, so that one paragraph is 
just wrong.

Aside from that though, I do think my patch is a step in the correct 
direction. It just needs some thought how to avoid the other issue you 
mentioned - that some drivers rely on calling mmc_gpio_request_cd() 
after the call to mmc_start().

Perhaps the logic should not be to remove mmc_gpio_request_cd()'s call 
to mmc_gpiod_request_cd_irq(), but rather to make it conditional upon 
mmc_start_host() having already been called; I assume that state that 
can easily be checked to determine that.
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