linux-kernel - Re: [PATCH v3 2/2] iommu/arm-smmu-v3: Replace vsmmu_size/type with get_viommu

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Message-ID: <aIEkZoTOSlQ0nMKd@Asurada-Nvidia>
Date: Wed, 23 Jul 2025 11:05:26 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Pranjal Shrivastava <praan@...gle.com>
CC: <jgg@...dia.com>, <will@...nel.org>, <joro@...tes.org>,
	<robin.murphy@....com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] iommu/arm-smmu-v3: Replace vsmmu_size/type with
 get_viommu_size

On Wed, Jul 23, 2025 at 01:37:53PM +0000, Pranjal Shrivastava wrote:
> On Mon, Jul 21, 2025 at 01:04:44PM -0700, Nicolin Chen wrote:
> > @@ -1273,6 +1279,10 @@ tegra241_cmdqv_init_vintf_user(struct arm_vsmmu *vsmmu,
> >  	phys_addr_t page0_base;
> >  	int ret;
> >  
> > +	/* Unsupported type was rejected in tegra241_cmdqv_get_vintf_size() */
> > +	if (WARN_ON(vsmmu->core.type != IOMMU_VIOMMU_TYPE_TEGRA241_CMDQV))
> > +		return -EOPNOTSUPP;
> > +
> 
> Nit: I don't think we'd expect a call to this if the vintf_size returned
> 0? I see that in iommufd_viommu_alloc_ioctl, we already have a check:

It's added in the previous patch where I explained that this is
to detect data corruption. When something like that happens, it
would be often illogical.

> And call ops->viommu_init only when the above isn't met. Thus,
> if we still end up calling ops->viommu_init, shouldn't we BUG_ON() it?
> I'd rather have the core code handle such things (since the driver is
> simply implementing the ops) and BUG_ON() something that's terribly
> wrong..

BUG_ON is discouraged following the coding style:
https://docs.kernel.org/process/coding-style.html#use-warn-rather-than-bug

> I can't see any ops->viommu_init being called elsewhere atm, let me
> know if there's a different path that I missed..

I see it as a precaution that should never get triggered. But in
case that it happens, I don't want it to proceed further wasting
precious HW resource given that this function allocates a VINTF.

Nicolin