linux-kernel - Re: [RFC PATCH 2/2] iommu: rockchip: Handle system-wide and runtime PM

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Date:	Sat, 20 Dec 2014 21:01:06 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Kevin Hilman <khilman@...nel.org>,
	Tomasz Figa <tfiga@...omium.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	iommu@...ts.linux-foundation.org, Len Brown <len.brown@...el.com>,
	Pavel Machek <pavel@....cz>, Heiko Stuebner <heiko@...ech.de>,
	Joerg Roedel <joro@...tes.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Daniel Kurtz <djkurtz@...omium.org>
Subject: Re: [RFC PATCH 2/2] iommu: rockchip: Handle system-wide and runtime PM

Hi Kevin,

On Friday 19 December 2014 03:27:35 Rafael J. Wysocki wrote:
> On Thursday, December 18, 2014 11:28:58 PM Laurent Pinchart wrote:
> > Hi Kevin,
> 
> [cut]
> 
>>>>> It would be better to be able to reference count the DMA engine from
>>>>> the bus master IMO and arguably you can use the runtime PM framework
>>>>> for that. Namely, give bus masters someting like
>>>>> 
>>>>> 	pm_runtime_get_my_DMA_engine(bus_master_device)
>>>>> 	pm_runtime_put_my_DMA_engine(bus_master_device)
>>>>> 
>>>>> and let them call these as they see fit.
>>>> 
>>>> Please note that we're not talking about DMA engines here, but about
>>>> IOMMUs. DMA is involved through the DMA mapping API which hides the
>>>> IOMMU completely from the bus master drivers, not the DMA engine API.
>>>> 
>>>> Exposing the IOMMU is something we want to avoid, but DMA mapping
>>>> start/stop operations could certainly be implemented.
>>> 
>>> The problem with that is it only solves the IOMMU problem.  We have a
>>> more generic PM dependency problem of which this IOMMU example is only a
>>> subset, so I think we need a more generic solution.
>> 
>> I agree that a more generic solution is needed at least to support ACPI
>> _DEP, but that might not be optimal in the IOMMU use case as explained
>> above.
>
> Well, since we need it anyway, why don't we implement it and then figure out
> if anything more specific needs to be done for the IOMMU case?

Patches are welcome ;-)

-- 
Regards,

Laurent Pinchart

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