Re: [PATCH v5] ARM: omap: edma: add suspend suspend/resume hooks

[Nishanth Menon <nm@ti.com>]

On 11/08/2013 10:14 AM, Joel Fernandes wrote:
> On 11/08/2013 12:28 AM, Kevin Hilman wrote:
> [..]
>>>> Also, I believe it was already suggested by Nishanth, but the late/early
>>>> callbacks are probably more appropriate here than the noirq callbacks.
>>>> Unless there's a *really* good reason to use the noirq callbacks, they
>>>> should be avoided.
>>>>
>>>> That being said, I wonder if the whole approach here is the right one.
>>>> I know you're basing your stuff on some TI tree, but that doesn't make
>>>> it the right way (usually, it's the opposite, but I digress...)  ;)
>>>>
>>>> IMO, EDMA should be done like we currently do I2C and not implement
>>>> suspend/resume at all.  Instead, the driver should do runtime PM done on
>>>
>>> But a potential problem with this is powering edma on or off between xfers may
>>> slow things down quite a bit because xfers happen so much often and there is
>>> some overhead in the pm_runtime calls which adds up overtime when dealing with
>>> something as frequent as EDMA. Also we would lose the global EDMA context right
>>> so we'd have to restore the context every time during runtime PM (?).
>>
>> Have a look at the autosuspend feature of runtime PM.  (c.f.
>> Documentation/power/pm_runtime.txt)
> 
> Sure, will do that. Thanks :)
> 
> Just one more silly question, in very frequent operations is there no over-head
> even when using autosuspend? Because when I traced last time (without
> autosuspend), the depth of pm runtime calls were quite a lot but this could also
> be because of the OMAP implementation.

that is the entire purpose of autosuspend - the back2back calls have
almost no overhead (other than to see that device suspend was not
invoked) - this is pretty fast. you can tweak autosuspend timeout for
the right configuration to meet up with what you need.


-- 
Regards,
Nishanth Menon