[PATCH v2] dmaengine: fsl-edma: add PM suspend/resume support

Li Yang leoli at freescale.com
Mon Aug 17 12:10:46 PDT 2015

On Mon, Aug 17, 2015 at 2:22 AM, Yao Yuan <yao.yuan at freescale.com> wrote:
> Hi Nigel,
> On Mon, Aug 17, 2015 at 2:49 PM, Nigel Cunningham < nigel at nigelcunningham.com.au > wrote:
>> On 17/08/15 13:59, Yao Yuan wrote:
>> > On Sat, Aug 15, 2015 at 7:48 AM, pku.leo < pku.leo at gmail.com > wrote:
>> >> On Fri, Aug 14, 2015 at 1:24 AM, Yao Yuan <yao.yuan at freescale.com> wrote:
>> >>> Hi Leo,
>> >>>
>> >>> Thanks for your review.
>> >>> About those two methods for DMA suspend that you have mentioned.
>> We
>> >> have a lot of the discussions in other DMA driver like DMA for
>> >> Freescale PowerPC.
>> >>> Finally, we think the device which used the DMA transmission service
>> >>> should
>> >> cancel the transmission service in its suspend.
>> >>> So DMA in suspend should be idle.
>> >> If that's the case you should clearly state this in the commit
>> >> message and in code, although I don't know if it is safe to make such
>> >> assumption.  There could be user of the DMA that doesn't track the
>> completion of transfers.
>> > I think it should be safe. In my opinion, even some client(the user of
>> > the DMA) forget to cancel its DMA transmission, It will just lead to PM failed
>> but no other system and data risk.
>> > Although we should first fix the behavior of the client.
>> > Once you are no need the DMA transmission, why not stop it?
>> >
>> > Is it right?
>> Think of it from the end user perspective. Would you like your laptop (or
>> whatever) to refuse to suspend because of this condition? The user may well
>> expect that closing the lid on their laptop will reliably lead to it suspending to
>> ram. Returning a failure here could result in a loss of data if the condition is not
>> detected and the machine subsequently runs out of power.
> Yes, the user may well expect that closing the lid on their laptop will reliably lead to it suspending to ram.
> So the client(the user of the DMA) must  to PAUSE or terminate the DMA transmission.
> We need to rely on client doing the right thing here.
> The DMA should not make a decision instead of client.
> If the DMA is not idle in DMA suspend, it should be the client's issue.
> We don't know what the client really want to do, so just return the non-success value.

The problem here is that neither the client nor the DMA controller
driver should easily decide to stop the suspend entrance and rollback.
I don't think the non-idle situation is serious enough to cause a
rollback.  You should do whatever can be done with the DMA
controller(such as stop the controller and leave whatever to be done
to the wake up) and continue with the suspend.


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