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

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Lukas,


On 2016-11-19 12:11, Lukas Wunner wrote:
> 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.

Thanks for checking this. You are right that I mixed up consumer and 
supplier
in point 8. I'm sorry for the confusion.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland