linux-kernel - Re: [PATCH] dmaengine: edma: Remove dynamic TPTC power management feature

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Date:	Thu, 28 Jan 2016 11:00:50 +0200
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Tony Lindgren <tony@...mide.com>
CC:	<vinod.koul@...el.com>, <dan.j.williams@...el.com>,
	<dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <nsekhar@...com>,
	<t-kristo@...com>
Subject: Re: [PATCH] dmaengine: edma: Remove dynamic TPTC power management
 feature

On 01/27/2016 05:54 PM, Tony Lindgren wrote:
> * Peter Ujfalusi <peter.ujfalusi@...com> [160127 01:12]:
>> The dynamic or on demand pm_runtime does not work correctly on am335x and
>> am437x due to interference with hwmod.
> 
> Hmm care expand a bit what is the problem with this "interference"?

The idea was to enable/power on only the TPTCs which is actually in use and
leave the unused ones off. Which is is nice and all, but...
The original implementation did the pm_runtime calls for the tptcs from the
edma tpcc driver instance and the main issue was that I did the pm_runtime
calls in the edma-tpcc pm callbacks as well.
Since omap hwmod/device also handles pm_runtime on behalf of the drivers we
got nasty issues, kernel crash, warnings on suspend/resume.

Then I did implemented the on demand power management in a totally different
way, still keeping only tptcs enabled which is in use.
In this way all the omap hwmod/device incoherency was gone and things looked
fine, but it turned out that on second suspend we are not able to wake up the
board.
I and Tero debugged this a bit and it turns out that we need to kepp all tptcs
enabled and powered, otherwise the HW will not going to be able to complete
the transition, breaking suspend/resume.

With pm_runtime_enable() + get_sync() on all tptcs we can suspend and resume
w/o problems and they will be disabled/enabled by omap hwmod/device code,
following nicely the power state of the system.

As a note: I did tried the suspend/resume with the old code with dra7, but it
turned out that on dra7 SW has no control over the tptc power state, it
follows the system in HW.

In short: The implementation was flawed and even if the implementation is
correct the HW will lock up if we do on demand tptc power management.

-- 
Péter