Re: [PATCH 2/2] iommu/arm-smmu: Add support for non-coherent page table mappings

[Vivek Gautam]

Hi Will,

On Tue, Jan 22, 2019 at 11:14 AM Will Deacon <will.deacon@xxxxxxx> wrote:
>
> On Mon, Jan 21, 2019 at 11:35:30AM +0530, Vivek Gautam wrote:
> > On Sun, Jan 20, 2019 at 5:31 AM Will Deacon <will.deacon@xxxxxxx> wrote:
> > > On Thu, Jan 17, 2019 at 02:57:18PM +0530, Vivek Gautam wrote:
> > > > Adding a device tree option for arm smmu to enable non-cacheable
> > > > memory for page tables.
> > > > We already enable a smmu feature for coherent walk based on
> > > > whether the smmu device is dma-coherent or not. Have an option
> > > > to enable non-cacheable page table memory to force set it for
> > > > particular smmu devices.
> > >
> > > Hmm, I must be missing something here. What is the difference between this
> > > new property, and simply omitting dma-coherent on the SMMU?
> >
> > So, this is what I understood from the email thread for Last level
> > cache support -
> > Robin pointed to the fact that we may need to add support for setting
> > non-cacheable
> > mappings in the TCR.
> > Currently, we don't do that for SMMUs that omit dma-coherent.
> > We rely on the interconnect to handle the configuration set in TCR,
> > and let interconnect
> > ignore the cacheability if it can't support.
>
> I think that's a bug. With that fixed, can you get what you want by omitting
> "dma-coherent"?

Based on the discussion on the first patch in this series [1], I can
update the series.
First thing can be -
if QUIRK_NO_DMA is set (i.e. the IOMMU _is_ coherent) then we use a
cacheable TCR;
So, we may need an additional check for this when setting the TCR.

For the second case -
IOMMUs that are *not* coherent, i.e ones that are omitting
'dma-coherent' property,
anyways have to access the page table directly from memory. We take
care of the CPU
side of this by allocating non-coherent memory, and making sure that we sync the
PTEs from map call.
Shouldn't we mark TCR for these IOMMUs as non-cacheable for inner and outer
cacheability attribute?


[1] https://lore.kernel.org/patchwork/patch/1032939/

Regards
Vivek

>
> Will



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