linux-kernel - Re: [PATCH v7 1/6] base: power: runtime: Export pm_runtime_get/put

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Date:   Tue, 13 Feb 2018 12:00:37 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Tomasz Figa <tfiga@...omium.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>, joro@...tes.org,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Will Deacon <will.deacon@....com>,
        Rob Clark <robdclark@...il.com>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Greg KH <gregkh@...uxfoundation.org>, sboyd@...eaurora.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v7 1/6] base: power: runtime: Export
 pm_runtime_get/put_suppliers

On 13/02/18 07:44, Tomasz Figa wrote:
> Hi Vivek,
> 
> On Wed, Feb 7, 2018 at 7:31 PM, Vivek Gautam
> <vivek.gautam@...eaurora.org> wrote:
>> The device link allows the pm framework to tie the supplier and
>> consumer. So, whenever the consumer is powered-on the supplier
>> is powered-on first.
>>
>> There are however cases in which the consumer wants to power-on
>> the supplier, but not itself.
>> E.g., A Graphics or multimedia driver wants to power-on the SMMU
>> to unmap a buffer and finish the TLB operations without powering
>> on itself.
> 
> This sounds strange to me. If the SMMU is powered down, wouldn't the
> TLB lose its contents as well (and so no flushing needed)?

Depends on implementation details - if runtime PM is actually 
implemented via external clock gating (in the absence of fine-grained 
power domains), then "suspended" TLBs might both retain state and not 
receive invalidation requests, which is really the worst case.

> Other than that, what kind of hardware operations would be needed
> besides just updating the page tables from the CPU?

Domain attach/detach also require updating SMMU hardware state (and 
possibly TLB maintenance), but don't logically require the master device 
itself to be active at the time.

Robin.