netdev - Re: [PATCH] iommu/arm-smmu: don't unregister on shutdown

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Date:   Wed, 14 Dec 2022 21:25:45 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     iommu@...ts.linux.dev, Will Deacon <will@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Michael Walle <michael@...le.cc>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Claudiu Manoil <claudiu.manoil@....com>, netdev@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: don't unregister on shutdown

On 2022-12-14 17:34, Vladimir Oltean wrote:
> On Fri, Dec 09, 2022 at 11:24:32AM +0000, Robin Murphy wrote:
>>> Fixes: b06c076ea962 ("Revert "iommu/arm-smmu: Make arm-smmu explicitly non-modular"")
>>
>> I think that's semantically correct, but I'm pretty sure at that point it
>> would have been benign in practice - the observable splat will be a much
>> more recent fallout from me changing the iommu_device_unregister() behaviour
>> in 57365a04c921 ("iommu: Move bus setup to IOMMU device registration"). The
>> assumption therein is that unregister would only happen on probe failure,
>> before the IOMMU instance is in use, or on module unload, which would not be
>> allowed while active devices still hold module references. I overlooked that
>> the SMMU drivers were doing what they do, sorry about that.
> 
> Ok, I'll change the Fixes: tag, I didn't notice that iommu_device_unregister()
> changed in behavior only later, I just looked at current trees and tried
> to infer what went wrong.
> 
>> The change itself looks sensible. The point of this shutdown hook is simply
>> not to leave active translations in place that might confuse future software
>> after reboot/kexec; any housekeeping in the current kernel state is a waste
>> of time anyway. Fancy doing the same for SMMUv3 as well?
> 
> I can try, but I won't have hardware to test.
> 
> Basically the only thing truly relevant for shutdown from arm_smmu_device_remove()
> is arm_smmu_device_disable(), would you agree to a patch which changes
> things as below?
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 6d5df91c5c46..d4d8bfee9feb 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -3854,7 +3854,9 @@ static int arm_smmu_device_remove(struct platform_device *pdev)
>   
>   static void arm_smmu_device_shutdown(struct platform_device *pdev)
>   {
> -	arm_smmu_device_remove(pdev);
> +	struct arm_smmu_device *smmu = platform_get_drvdata(pdev);
> +
> +	arm_smmu_device_disable(smmu);
>   }
>   
>   static const struct of_device_id arm_smmu_of_match[] = {


Looks fine to me! I'll let Will decide if he'd still prefer to do the 
full remove-calls-shutdown reversal here as well for complete 
consistency, but I reckon the minimal diff is no bad thing :)

Cheers,
Robin.