Re: [PATCH 2/3] iommu/io-pgtable-arm: Add IOMMU_LLC page protection flag

[Will Deacon]

On Fri, Jan 29, 2021 at 03:12:59PM +0530, Sai Prakash Ranjan wrote:
> On 2021-01-29 14:35, Will Deacon wrote:
> > On Mon, Jan 11, 2021 at 07:45:04PM +0530, Sai Prakash Ranjan wrote:
> > > Add a new page protection flag IOMMU_LLC which can be used
> > > by non-coherent masters to set cacheable memory attributes
> > > for an outer level of cache called as last-level cache or
> > > system cache. Initial user of this page protection flag is
> > > the adreno gpu and then can later be used by other clients
> > > such as video where this can be used for per-buffer based
> > > mapping.
> > > 
> > > Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
> > > ---
> > >  drivers/iommu/io-pgtable-arm.c | 3 +++
> > >  include/linux/iommu.h          | 6 ++++++
> > >  2 files changed, 9 insertions(+)
> > > 
> > > diff --git a/drivers/iommu/io-pgtable-arm.c
> > > b/drivers/iommu/io-pgtable-arm.c
> > > index 7439ee7fdcdb..ebe653ef601b 100644
> > > --- a/drivers/iommu/io-pgtable-arm.c
> > > +++ b/drivers/iommu/io-pgtable-arm.c
> > > @@ -415,6 +415,9 @@ static arm_lpae_iopte
> > > arm_lpae_prot_to_pte(struct arm_lpae_io_pgtable *data,
> > >  		else if (prot & IOMMU_CACHE)
> > >  			pte |= (ARM_LPAE_MAIR_ATTR_IDX_CACHE
> > >  				<< ARM_LPAE_PTE_ATTRINDX_SHIFT);
> > > +		else if (prot & IOMMU_LLC)
> > > +			pte |= (ARM_LPAE_MAIR_ATTR_IDX_INC_OCACHE
> > > +				<< ARM_LPAE_PTE_ATTRINDX_SHIFT);
> > >  	}
> > > 
> > >  	if (prot & IOMMU_CACHE)
> > > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > > index ffaa389ea128..1f82057df531 100644
> > > --- a/include/linux/iommu.h
> > > +++ b/include/linux/iommu.h
> > > @@ -31,6 +31,12 @@
> > >   * if the IOMMU page table format is equivalent.
> > >   */
> > >  #define IOMMU_PRIV	(1 << 5)
> > > +/*
> > > + * Non-coherent masters can use this page protection flag to set
> > > cacheable
> > > + * memory attributes for only a transparent outer level of cache,
> > > also known as
> > > + * the last-level or system cache.
> > > + */
> > > +#define IOMMU_LLC	(1 << 6)
> > 
> > On reflection, I'm a bit worried about exposing this because I think it
> > will
> > introduce a mismatched virtual alias with the CPU (we don't even have a
> > MAIR
> > set up for this memory type). Now, we also have that issue for the PTW,
> > but
> > since we always use cache maintenance (i.e. the streaming API) for
> > publishing the page-tables to a non-coheren walker, it works out.
> > However,
> > if somebody expects IOMMU_LLC to be coherent with a DMA API coherent
> > allocation, then they're potentially in for a nasty surprise due to the
> > mismatched outer-cacheability attributes.
> > 
> 
> Can't we add the syscached memory type similar to what is done on android?

Maybe. How does the GPU driver map these things on the CPU side?

Will