Re: [PATCH 12/13 v5] OMAP: GPIO: Use dev_pm_ops instead of sys_dev_class

From: Kevin Hilman <khilman@deeprootsystems.com>
To: "Basak, Partha" <p-basak2@ti.com>
Cc: "Varadarajan, Charulatha" <charu@ti.com>,
	"linux-omap@vger.kernel.org" <linux-omap@vger.kernel.org>,
	"paul@pwsan.com" <paul@pwsan.com>,
	"Cousson, Benoit" <b-cousson@ti.com>,
	"Nayak, Rajendra" <rnayak@ti.com>
Subject: Re: [PATCH 12/13 v5] OMAP: GPIO: Use dev_pm_ops instead of sys_dev_class
Date: Tue, 10 Aug 2010 11:10:03 -0700	[thread overview]
Message-ID: <87fwym5og4.fsf@deeprootsystems.com> (raw)
In-Reply-To: <B85A65D85D7EB246BE421B3FB0FBB59301E80AC458@dbde02.ent.ti.com> (Partha Basak's message of "Tue, 10 Aug 2010 18:07:01 +0530")

"Basak, Partha" <p-basak2@ti.com> writes:

[...]

>> > In the idle path (interrupt disabled context), PM runtime APIs cannot
>> > be used as they are not mutex-free functions. Hence omap_device APIs
>> > are used in the idle and resume after idle path.
>> 
>> This needs much more fleshing out.
>> 
>> Exactly what mutexes are causing the problems here.  As pointed out in
>> previous discussions, the ones in the PM runtime core should not be a
>> problem in this path.  Therefore, I'll assume the problems are coming
>> from the mutexes when the device code (mach-omap2/gpio.c) calls into the
>> hwmod layer.  More on this in comments on the next patch.
>> 
>
> Sorry, this has not been documented correctly. The issue has more to
> do unconditional enabling of interrupts. We have received a patch from
> you on using pm_runtime functions in Idle path. We will try on GPIO
> and revert back.

OK

>
>> > To summarize,
>> > 1. pm_runtime_get_sync() for any gpio bank is called when one of the
>> gpios
>> >    is requested on the bank, in which, no other gpio is being used (when
>> >    mod_usage becomes non-zero)
>> > 2. omap_device_enable() is called during gpio resume after idle, only
>> >    if the particular bank is being used (if mod_usage is non-zero)
>> 
>> context is saved/restored in the idle path, but...
>> 
>> > 3. pm_runtime_put_sync() is called when the last used gpio in that
>> >    gpio bank is freed (when mod_usage becomes zero)
>> 
>> in this path, the bank is now runtime suspended, but context has not
>> been saved for it.  That should be fine, since this bank is no longer
>> used, but now let's assume there was an off-mode transition and context
>> is lost.  Then, gpio_request() is called which will trigger
>> a pm_runtime_get_sync() and gpio_bank_runtime_resume() will be called.
>> 
>> In this case, it's not terribly clear that runtime_resume is doing sane
>> things if context has just been lost.  Seems like runtime_resume should
>> be a nop in this case since any re-init will be be done in gpio_request().
>
> Runtime_suspend/resume for GPIO is not doing any save/restore
> context. In that sense, they are NOP. Context save/restore is taken
> care of only in the Idle path based on target power state and last
> power state respectively.

OK, I didn't explain the problem I'm suspecting very well.  Imagine this
sequence of events:

- mod_usage becomes zero
- pm_runtime_put_sync()
- gpio_bank_runtime_suspend()  [ no context is saved ]
  [ off-mode transition, context is lost]
- gpio_request()
- pm_runtime_get_sync()
- gpio_bank_runtime_resume()

In this path, no context is saved, and no context is restored, which is
what I would expect, since there's no need to save context if nobody is
using that gpio bank anymore.   However, gpio_bank_runtime_resume() is
doing lots of reads/writes and read-modify-writes on GPIO bank registers
that may have undefined contents after a context loss.

The point is that the GPIO register twiddling in
gpio_bank_runtime_resume() does not seem to be needed if there are no
users of that GPIO bank.

[...]

>> >  static void omap3_enable_io_chain(void)
>> >  {
>> >  	int timeout = 0;
>> > @@ -395,15 +385,17 @@ void omap_sram_idle(void)
>> >  	/* PER */
>> >  	if (per_next_state < PWRDM_POWER_ON) {
>> >  		omap_uart_prepare_idle(2);
>> > -		omap2_gpio_prepare_for_idle(per_next_state);
>> >  		if (per_next_state == PWRDM_POWER_OFF) {
>> >  			if (core_next_state == PWRDM_POWER_ON) {
>> >  				per_next_state = PWRDM_POWER_RET;
>> >  				pwrdm_set_next_pwrst(per_pwrdm, per_next_state);
>> >  				per_state_modified = 1;
>> > -			} else
>> > -				omap3_per_save_context();
>> > +			}
>> >  		}
>> > +		if (per_next_state == PWRDM_POWER_OFF)
>> > +			omap2_gpio_prepare_for_idle(true);
>> > +		else
>> > +			omap2_gpio_prepare_for_idle(false);
>> 
>> Why is this better than passing the next power state?
>
> This would keep the GPIO function omap2_gpio_prepare_for_idle agnostic of Power state definition dependencies.
>

And why is this better?  

Personally, I think the GPIO code should be told about the powerdomain
state so it can make it's own decision about whether or not to save
context.

Kevin