linux-kernel - Re: [PATCH] iommu/arm-smmu: Add clk_bulk_{prepare/unprepare} to system pm callbacks

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Date:   Tue, 27 Jul 2021 16:05:12 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Taniya Das <tdas@...eaurora.org>,
        srimuc <srimuc@...eaurora.org>, linux-arm-msm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] iommu/arm-smmu: Add clk_bulk_{prepare/unprepare} to
 system pm callbacks

Hi Robin,

On 2021-07-27 16:03, Robin Murphy wrote:
> On 2021-07-27 11:25, Robin Murphy wrote:
>> On 2021-07-27 10:33, Sai Prakash Ranjan wrote:
>>> Some clocks for SMMU can have parent as XO such as 
>>> gpu_cc_hub_cx_int_clk
>>> of GPU SMMU in QTI SC7280 SoC and in order to enter deep sleep states 
>>> in
>>> such cases, we would need to drop the XO clock vote in unprepare call 
>>> and
>>> this unprepare callback for XO is in RPMh (Resource Power 
>>> Manager-Hardened)
>>> clock driver which controls RPMh managed clock resources for new QTI 
>>> SoCs
>>> and is a blocking call.
>>> 
>>> Given we cannot have a sleeping calls such as clk_bulk_prepare() and
>>> clk_bulk_unprepare() in arm-smmu runtime pm callbacks since the iommu
>>> operations like map and unmap can be in atomic context and are in 
>>> fast
>>> path, add this prepare and unprepare call to drop the XO vote only 
>>> for
>>> system pm callbacks since it is not a fast path and we expect the 
>>> system
>>> to enter deep sleep states with system pm as opposed to runtime pm.
>>> 
>>> This is a similar sequence of clock requests (prepare,enable and
>>> disable,unprepare) in arm-smmu probe and remove.
>> 
>> Nope. We call arm_smmu_rpm_get(), which may resume the device, from 
>> atomic contexts. clk_prepare() may sleep. This doesn't work.
> 
> Urgh, or maybe I skimmed the commit message too lightly *and* managed
> to totally misread the patch, sorry :(
> 
> I'll wake up some more and try again later...
> 

No worries, we took our time looking through that many times before 
posting this :)

Thanks,
Sai

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