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

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Message-ID: <a5595eae-0de9-e454-8a3d-5718512422ee@arm.com>
Date:   Fri, 29 Jan 2021 17:03:59 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     John Garry <john.garry@...wei.com>,
        Zhen Lei <thunder.leizhen@...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 2021-01-29 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. 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.