linux-kernel - Re: [PATCH 1/1] iommu/arm-smmu-qcom: remove runtime pm enabling for TBU driver

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Message-ID: <dee2f394-c9c5-4099-8e05-ce0c8756887b@quicinc.com>
Date: Tue, 13 Aug 2024 15:56:23 +0800
From: Zhenhua Huang <quic_zhenhuah@...cinc.com>
To: Pranjal Shrivastava <praan@...gle.com>
CC: <robdclark@...il.com>, <will@...nel.org>, <robin.murphy@....com>,
        <joro@...tes.org>, <iommu@...ts.linux.dev>,
        <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <quic_c_gdjako@...cinc.com>, <dmitry.baryshkov@...aro.org>
Subject: Re: [PATCH 1/1] iommu/arm-smmu-qcom: remove runtime pm enabling for
 TBU driver



On 2024/8/13 15:20, Pranjal Shrivastava wrote:
> On Tue, Aug 13, 2024 at 10:37:33AM +0800, Zhenhua Huang wrote:
>>
>>
>> On 2024/8/12 21:25, Pranjal Shrivastava wrote:
>>> On Tue, Jul 30, 2024 at 06:30:43PM +0800, Zhenhua Huang wrote:
>>>> TBU driver has no runtime pm support now, adding pm_runtime_enable()
>>>> seems to be useless. Remove it.
>>>>
>>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>
>>>> ---
>>>>    drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 6 ------
>>>>    1 file changed, 6 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
>>>> index 36c6b36ad4ff..aff2fe1fda13 100644
>>>> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
>>>> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
>>>> @@ -566,7 +566,6 @@ static struct acpi_platform_list qcom_acpi_platlist[] = {
>>>>    static int qcom_smmu_tbu_probe(struct platform_device *pdev)
>>>>    {
>>>> -	struct device *dev = &pdev->dev;
>>>>    	int ret;
>>>>    	if (IS_ENABLED(CONFIG_ARM_SMMU_QCOM_DEBUG)) {
>>>> @@ -575,11 +574,6 @@ static int qcom_smmu_tbu_probe(struct platform_device *pdev)
>>>>    			return ret;
>>>>    	}
>>>> -	if (dev->pm_domain) {
>>>> -		pm_runtime_set_active(dev);
>>>> -		pm_runtime_enable(dev);
>>>
>>> I assumed that this was required to avoid the TBU from being powered
>>> down? If so, then I think we shall move it under the
>>
>> Hi Pranjal,
>>
>> In my sense, this was giving the TBU ability to power down when
>> necessary(through pm callbacks)? While I haven't seen any RPM impl for TBU
>> device.. hence having the doubt..
>>
>> Thanks,
>> Zhenhua
> 
> Apologies for being unclear. I just meant to ask if there was a reason
> to add pm_runtime_set_active & enable in the tbu probe previously? And I

It's also my doubt and hope Georgi can help clarify :)
Actually I assume this part of codes was copied from arm-smmu driver
https://elixir.bootlin.com/linux/v6.11-rc3/source/drivers/iommu/arm/arm-smmu/arm-smmu.c#L2264 
.. but for TBU, seems no user ?

> *assumed* that it was to set the device state as RPM_ACTIVE to avoid it
> being RPM_SUSPENDED after enabling pm_runtime

Yeah, it's normal sequence from doc: 
https://elixir.bootlin.com/linux/v6.11-rc3/source/Documentation/power/runtime_pm.rst#L574

> 
> I agree that there are no pm_runtime_suspend/resume calls within the TBU
> driver. I'm just trying to understand why was pm_runtime enabled here
> earlier (since it's not implemented) in order to ensure that removing it
> doesn't cause further troubles?

See above my assumption, need Georgi to comment but.

> 
> I see Georgi added it as a part of
> https://lore.kernel.org/all/20240704010759.507798-1-quic_c_gdjako@quicinc.com/
> 
> But I'm unsure why was it required to fix that bug?

I'm just thinking it is dead code and want to see if my understanding is 
correct.

Thanks,
Zhenhua

> 
>>
>>> previous if condition, i.e. CONFIG_ARM_SMMU_QCOM_DEBUG?
>>>
>>> If not, we can remove it give that the TBU would be powered ON as needed
>>>
>>>> -	}
>>>> -
>>>>    	return 0;
>>>>    }
>>>> -- 
>>>> 2.7.4
>>>>
>>>>
>>>
>>> Thanks,
>>> Pranjal
> 
> Thanks,
> Pranjal