Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support

From	Ulf Hansson <ulf.hansson@linaro.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support
Date	2016-09-14 14:00 +0200
Message-ID	<shh18-no-17@gated-at.bofh.it> (permalink)
References	(2 earlier) <sgVjX-1TH-7@gated-at.bofh.it> <sgWSJ-35y-15@gated-at.bofh.it> <shcEa-6cS-13@gated-at.bofh.it> <shfC1-86s-9@gated-at.bofh.it> <shg54-8gk-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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[...]

>>>
>>>> To solve this problem, I was thinking we could convert to use the
>>>> asynchronous pm_runtime_get() API, when trying to runtime resume the
>>>> device from atomic contexts.
>>>
>>> I'm not sure if this will work for DMA engine devices. If I understand
>>> correctly some client's of DMA engine device might rely on the DMA
>>> engine being configured and operational after queuing a request and
>>> they might lock up if the DMA engine device activation if postponed
>>> because of async runtime pm activation.
>>
>> I didn't know about this. If you have an example from the top of your
>> head, could you perhaps point me to it?
>
>
> No, I don't have any example. This is just my fear that there might be some
> hardware which works this way. I can imagine a driver, which queue dma
> engine
> request and then triggers 'start' command to its hw block. HW blocks usually
> uses some additional signal lines for DMA, so the HW block might check if
> all
> needed signals from DMA engine HW are ready. If not it will fail to start
> avoid lockup of starting without DMA engine HW being ready.

Well, I think this reasoning makes lots of sense! Clearly we wouldn't
be able to guarantee that there's isn't a glitch in an undefined HW
behaviour.

I will drop my suggested approach and try out another one.

>
> However I don't have much experience with DMA engine and drivers. I only
> helped in adding DMA engine support to Samsung UART driver and in the
> hardware manual there is information about additional lines between DMA
> controller and UART module, which are used in the DMA mode.

Thanks for sharing your experience!

Now, I should allow you to get back to the original review! :-)

Kind regards
Uffe

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[PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Marek Szyprowski <m.szyprowski@samsung.com> - 2016-09-13 14:50 +0200
  Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Ulf Hansson <ulf.hansson@linaro.org> - 2016-09-13 16:30 +0200
    Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Marek Szyprowski <m.szyprowski@samsung.com> - 2016-09-14 09:20 +0200
      Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Ulf Hansson <ulf.hansson@linaro.org> - 2016-09-14 12:30 +0200
        Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Marek Szyprowski <m.szyprowski@samsung.com> - 2016-09-14 13:00 +0200
          Re: [PATCH v3 2/2] iommu/exynos: Add proper runtime pm support Ulf Hansson <ulf.hansson@linaro.org> - 2016-09-14 14:00 +0200

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