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

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Date:   Wed, 15 Sep 2021 15:19:27 -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 Wed, Sep 15, 2021 at 03:09:48PM +0300, Dmitry Osipenko wrote:
> 15.09.2021 07:38, Nicolin Chen пишет:
> > 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].
> 
> Thanks for the pointer. It says that only certain memory clients allow
> to combine 4 ASIDs to form 34bit VA space. In this case the PA space is
> split into 4GB areas and there are additional bitfields which configure
> the ASID mapping of each 4GB area. Still each ASID is 32bit.

True.

> This is what TRM says:
> 
> "For the GPU and other clients with 34-bit address interfaces, the ASID
> registers are extended to point to four ASIDs. The SMMU supports 4GB of
> virtual address space per ASID, so mapping addr[33:32] into ASID[1:0]
> extends the virtual address space of a client to 16GB."
> 
> > 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.
> 
> You will need to read the special "ASID Assignment Register" which
> supports 4 sub-ASIDs to translate the PA address into the actual VA. By
> default all clients are limited to a single ASID and upstream kernel
> doesn't support programming of 34bit VAs. So doesn't worth the effort to
> fully translate the VA, IMO.

Yea. It'd be easier to just stay in 32-bit. I will remove those
castings in the next version, waiting for Thierry taking a look
at this v6 first.