Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to control runtime pm

From: Lukas Wunner <lukas@wunner.de>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>,
	linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
	iommu@lists.linux-foundation.org,
	linux-samsung-soc@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
	Inki Dae <inki.dae@samsung.com>, Kukjin Kim <kgene@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Mark Brown <broonie@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>,
	Kevin Hilman <khilman@kernel.org>,
	Tobias Jakobi <tjakobi@math.uni-bielefeld.de>,
	Tomasz Figa <tomasz.figa@gmail.com>,
	Grant Likely <grant.likely@secretlab.ca>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Andrzej Hajda <a.hajda@samsung.com>,
	Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>
Subject: Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to control runtime pm
Date: Sat, 19 Nov 2016 12:11:57 +0100	[thread overview]
Message-ID: <20161119111157.GA364@wunner.de> (raw)
In-Reply-To: <4bc652ce-6cf9-f4df-4793-e126cf81079d@samsung.com>

On Tue, Nov 08, 2016 at 08:27:12AM +0100, Marek Szyprowski wrote:
> On 2016-11-07 22:47, Luis R. Rodriguez wrote:
> > If so
> > why? If this issue is present also on systems that only use ACPI is
> > this possibly due to an ACPI firmware bug  or the lack of some semantics
> > in ACPI to express ordering in a better way? If the issue is device
> > tree related only is this due to the lack of semantics in device tree
> > to express some more complex dependency ?
> 
> The main feature of device links that is used in this patch is enabling
> runtime pm dependency between Exynos SYSMMU controller (called it client
> device) and the device, for which it implements DMA address translation
> (called master device). The assumptions are following:
> 1. master device driver is completely unaware of the Exynos SYSMMU presence,
>    IOMMU is transparently hooked up and managed by DMA-mapping framework
> 2. SYSMMU belongs to the same power domain as it's master device
> 3. SYSMMU is optional, master device can fully operate without it, with
>    simple DMA address translation (DMA address == physical address)
> 4. Master device implements runtime pm, what in turn causes respective
>    power domain to be turned on/off
> 5. DMA-mapping and IOMMU frameworks provides no calls to notify SYSMMU
>    when its master device is performing DMA operations, so SYSMMU has
>    to be runtime active
> 6. Currently SYSMMU always sets its runtime pm status to active after
>    attaching to its master device to ensure proper hardware state. This
>    prevents power domain to be turned off, even when master device sets
>    its runtime pm status to suspended.
> 7. Exynos SYSMMU has to be runtime active at the same time when its
>    master device is runtime active to it to perform DMA operations and
>    allow the power domain to be turned off, when master device is
>    runtime suspended.
> 8. The terms of device links, Exynos SYSMMU is a 'consumer' and master
>    device is a 'supplier'.

You seem to have mixed up the consumer and supplier in point 8 above.
Your code is such that the SYSMMU is the supplier and the master device
is the consumer:

	device_link_add(dev, data->sysmmu, DL_FLAG_PM_RUNTIME);

Prototype of device_link_add:

	struct device_link *device_link_add(struct device *consumer,
				            struct device *supplier,
					    u32 flags);

Your code is correct, only point 8 above is wrong.

Best regards,

Lukas