linux-kernel - Re: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual ARM MMU-500 usage

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Message-ID: <eb0ffc7e-f41b-d17c-6a90-049335098cd2@nvidia.com>
Date:   Tue, 30 Jun 2020 17:44:23 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Krishna Reddy <vdumpa@...dia.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        Thierry Reding <treding@...dia.com>,
        "Yu-Huan Hsu" <YHsu@...dia.com>, Sachin Nikam <Snikam@...dia.com>,
        Pritesh Raithatha <praithatha@...dia.com>,
        Timo Alho <talho@...dia.com>,
        Bitan Biswas <bbiswas@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        Bryan Huntsman <bhuntsman@...dia.com>,
        "nicoleotsuka@...il.com" <nicoleotsuka@...il.com>
Subject: Re: [PATCH v8 1/3] iommu/arm-smmu: add NVIDIA implementation for dual
 ARM MMU-500 usage


On 30/06/2020 17:32, Jon Hunter wrote:
> On 30/06/2020 17:23, Krishna Reddy wrote:
>>>> +struct arm_smmu_device *nvidia_smmu_impl_init(struct arm_smmu_device 
>>>> +*smmu) {
>>>> +	unsigned int i;
>> ....
>>>> +	for (i = 1; i < MAX_SMMU_INSTANCES; i++) {
>>>> +		struct resource *res;
>>>> +
>>>> +		res = platform_get_resource(pdev, IORESOURCE_MEM, i);
>>>> +		if (!res)
>>>> +			break;
>>
>>> Currently this driver is only supported for Tegra194 which I understand has 3 SMMUs. Therefore, I don't feel that we should fail silently here, I think it is better to return an error if all 3 cannot be initialised.
>>
>> Initialization of all the three SMMU instances is not necessary here.
> 
> That is not what I am saying.
> 
>> The driver can work with all the possible number of instances 1, 2 and 3 based on the DT config though it doesn't make much sense to use it with 1 instance.
>> There is no silent failure here from driver point of view. If there is misconfig in DT, SMMU faults would catch issues.
> 
> I disagree and you should return a proper error here.

OK, well I see what you are saying, but if we intended to support all 3
for Tegra194, then we should ensure all 3 are initialised correctly.

My concern here is testing, because when things break in upstream I am
usually the one that tracks it down. Not having clear warning/error
messages when something is not initialised as expected makes it harder.

It would be better to query the number of SMMUs populated in device-tree
and then ensure that all are initialised correctly.

Jon

-- 
nvpublic