linux-kernel - Re: [PATCH v11 3/7] iommu: Add verisilicon IOMMU driver

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Message-ID: <aXd4CXE0fqWiKJXl@willie-the-truck>
Date: Mon, 26 Jan 2026 14:19:53 +0000
From: Will Deacon <will@...nel.org>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>
Cc: joro@...tes.org, robin.murphy@....com, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, heiko@...ech.de,
	nicolas.dufresne@...labora.com, p.zabel@...gutronix.de,
	mchehab@...nel.org, iommu@...ts.linux.dev,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rockchip@...ts.infradead.org, linux-media@...r.kernel.org,
	kernel@...labora.com
Subject: Re: [PATCH v11 3/7] iommu: Add verisilicon IOMMU driver

On Mon, Jan 26, 2026 at 10:03:19AM +0100, Benjamin Gaignard wrote:
> 
> Le 23/01/2026 à 18:14, Will Deacon a écrit :
> > On Wed, Jan 21, 2026 at 02:50:18PM +0100, Benjamin Gaignard wrote:
> > > Le 21/01/2026 à 13:51, Will Deacon a écrit :
> > > > On Mon, Jan 19, 2026 at 03:03:44PM +0100, Benjamin Gaignard wrote:
> > > > > > > > > > > +static const struct iommu_ops vsi_iommu_ops = {
> > > > > > > > > > > +	.identity_domain = &vsi_identity_domain,
> > > > > > > > > > > +	.release_domain = &vsi_identity_domain,
> > > > > > > > > > > +	.domain_alloc_paging = vsi_iommu_domain_alloc_paging,
> > > > > > > > > > > +	.of_xlate = vsi_iommu_of_xlate,
> > > > > > > > > > > +	.probe_device = vsi_iommu_probe_device,
> > > > > > > > > > > +	.release_device = vsi_iommu_release_device,
> > > > > > > > > > > +	.device_group = generic_single_device_group,
> > > > > > > > > > > +	.owner = THIS_MODULE,
> > > > > > > > > > > +	.default_domain_ops = &(const struct iommu_domain_ops) {
> > > > > > > > > > > +		.attach_dev		= vsi_iommu_attach_device,
> > > > > > > > > > > +		.map_pages		= vsi_iommu_map,
> > > > > > > > > > > +		.unmap_pages		= vsi_iommu_unmap,
> > > > > > > > > > > +		.flush_iotlb_all	= vsi_iommu_flush_tlb_all,
> > > > > > > > > > This has no callers and so your unmap routine appears to be broken.
> > > > > > > > > It is a leftover of previous attempt to allow video decoder to clean/flush
> > > > > > > > > the iommu by using a function from the API.
> > > > > > > > > Now it is using vsi_iommu_restore_ctx().
> > > > > > > > > I while remove it in version 12.
> > > > > > > > Don't you still need some invalidation on the unmap path?
> > > > > > > In vsi_iommu_unmap_iova() page is invalided by calling vsi_mk_pte_invalid().
> > > > > > But that just writes an invalid descriptor and doesn't appear to invalidate
> > > > > > the TLB at all.
> > > > > > 
> > > > > > > That clear BIT(0) so the hardware knows the page is invalid.
> > > > > > > Do I have miss something here ?
> > > > > > Yes, the TLB structure needs to be invalidated so that the page-table
> > > > > > walker sees the new value that you have written in memory.
> > > > > > 
> > > > > > The rockchip driver gets this correct...
> > > > > Rockchip hardware have a ZAP_ONE_LINE register which didn't exist on Verisilicon
> > > > > hardware.
> > > > Presumably you have some sort of Verisilicon datasheet or downstream driver
> > > > from which you can infer the TLB invalidation runes?
> > > I have only this downstream driver:
> > > https://github.com/rockchip-linux/kernel/blob/develop-6.1/drivers/iommu/rockchip-iommu-av1d.c
> > > No datasheet...
> > > 
> > > > > I have tried to use VSI_MMU_BIT_FLUSH on VSI driver after unmapping iova
> > > > > but it doesn't work.
> > > > What do you mean by "doesn't work"? If it works without doing any
> > > > invalidation at all, then it's very peculiar that adding the invalidation
> > > > would introduce issues.
> > > I mean VSI_MMU_BIT_FLUSH register can't be used to invalid the TLB.
> > > I think the hardware iterates over the pages tables in memory and
> > > check the valid/invalid bit.
> > I bet it doesn't: that would be horrible for performance.
> > 
> > The hardware clearly has TLB invalidation support, as the downstream driver
> > that you linked above implements av1_iommu_flush_tlb_all() to poke it.
> > If the hardware has a TLB, then unmapping a page-table means you need to:
> > 
> > 1. Clear the valid bit from the descriptor in memory
> > 2. Have some sort of memory barrier
> > 3. Invalidate the TLB
> > 4. Wait for the invalidation to complete
> 
> That exactly what I had tried to do by calling vsi_iommu_flush_tlb_all() (minux the lock)
> after calling vsi_iommu_unmap_iova() in vsi_iommu_unmap() but that doesn't work
> and even make the system crash sometimes.

Then it sounds like you have some debugging to do...

I don't think we should elide the TLB invalidation just because you
couldn't get it to work.

Will