linux-kernel - Re: [PATCH v6 6/6] iommu/tegra-smmu: Add pagetable mappings to debugfs

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Date:   Tue, 14 Sep 2021 21:38:06 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     thierry.reding@...il.com, joro@...tes.org, will@...nel.org,
        vdumpa@...dia.com, jonathanh@...dia.com,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 6/6] iommu/tegra-smmu: Add pagetable mappings to
 debugfs

On Tue, Sep 14, 2021 at 10:20:30PM +0300, Dmitry Osipenko wrote:
> 14.09.2021 21:49, Nicolin Chen пишет:
> > On Tue, Sep 14, 2021 at 04:29:15PM +0300, Dmitry Osipenko wrote:
> >> 14.09.2021 04:38, Nicolin Chen пишет:
> >>> +static unsigned long pd_pt_index_iova(unsigned int pd_index, unsigned int pt_index)
> >>> +{
> >>> +	return ((dma_addr_t)pd_index & (SMMU_NUM_PDE - 1)) << SMMU_PDE_SHIFT |
> >>> +	       ((dma_addr_t)pt_index & (SMMU_NUM_PTE - 1)) << SMMU_PTE_SHIFT;
> >>> +}
> >>
> >> We know that IOVA is fixed to u32 for this controller. Can we avoid all
> >> these dma_addr_t castings? It should make code cleaner a tad, IMO.
> > 
> > Tegra210 actually supports 34-bit IOVA...
> > 
> 
> It doesn't. 34-bit is PA, 32-bit is VA.
> 
> Quote from T210 TRM:
> 
> "The SMMU is a centralized virtual-to-physical translation for MSS. It
> maps a 32-bit virtual address to a 34-bit physical address. If the
> client address is 40 bits then bits 39:32 are ignored."

If you scroll down by a couple of sections, you can see 34-bit
virtual addresses in section 18.6.1.2; and if checking one ASID
register, you can see it mention the extra two bits va[33:32].

However, the driver currently sets its geometry.aperture_end to
32-bit, and we can only get 32-bit IOVAs using PDE and PTE only,
so I think it should be safe to remove the castings here. I'll
wait for a couple of days and see if there'd be other comments
for me to address in next version.

> Even if it supported more than 32bit, then the returned ulong is 32bit,
> which doesn't make sense.

On ARM64 (Tegra210), isn't ulong 64-bit?