linux-kernel - Re: [PATCH v3 2/3] perf/smmuv3: Add a MODULE_SOFTDEP() to indicate dependency on SMMU

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Date:   Sat, 30 Jan 2021 09:34:55 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Joerg Roedel" <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v3 2/3] perf/smmuv3: Add a MODULE_SOFTDEP() to indicate
 dependency on SMMU



On 2021/1/30 1:03, Robin Murphy wrote:
> On 2021-01-29 15:34, John Garry wrote:
>> On 29/01/2021 15:12, Robin Murphy wrote:
>>> On 2021-01-27 11:32, Zhen Lei wrote:
>>>> The MODULE_SOFTDEP() gives user space a hint of the loading sequence. And
>>>> when command "modprobe arm_smmuv3_pmu" is executed, the arm_smmu_v3.ko is
>>>> automatically loaded in advance.
>>>
>>> Why do we need this? If probe order doesn't matter when both drivers are built-in, why should module load order?
>>>
>>> TBH I'm not sure why we even have a Kconfig dependency on ARM_SMMU_V3, given that the drivers operate completely independently :/
>>
>> Can that Kconfig dependency just be removed? I think that it was added under the idea that there is no point in having the SMMUv3 PMU driver without the SMMUv3 driver.
> 
> A PMCG *might* be usable for simply counting transactions to measure device activity regardless of its associated SMMU being enabled.

If that's the case, the SOFTDEP really shouldn't be added. I wasn't trying to make sure they were loaded in order, just to make sure that the SMMU was not forgotten to load.

> Either way, it's not really Kconfig's job to decide what makes sense (beyond the top-level "can this driver *ever* be used on this platform" visibility choices). Imagine if we gave every PCI/USB/etc. device driver an explicit ?dependency on at least one host controller driver being enabled...
> 
> Robin.
> 
> .
>