Subject: Re: [PATCH 3/3] iommu/io-pgtable-arm: Allow coherent walks for Mali

From: Steven Price <[email-masked]>
Date: Thu, 12 Sep 2019 11:53:52 +0100

On 11/09/2019 15:42, Robin Murphy wrote:
> Midgard GPUs have ACE-Lite master interfaces which allows systems to
> integrate them in an I/O-coherent manner. It seems that from the GPU's
> viewpoint, the rest of the system is its outer shareable domain, and it
> will only emit snoop signals for outer shareable accesses. As such,
> setting the TTBR_SHARE_OUTER bit does indeed get coherent pagetable
> walks working nicely.
> 
> Making data accesses coherent seems to be more of a challenge...
> 
> Signed-off-by: Robin Murphy <[email-masked]>

Reviewed-by: Steven Price <[email-masked]>

Note the terminology in the GPU is *very* confusing here. Midgard refers
to the system's inner shareable domain as "outer shareable", and uses
"inner shareable" to mean purely within the GPU.

For data access kbase sets up a different default MEMATTR if ACE is
available:

	/* Set to implementation defined, outer caching */
	#define AS_MEMATTR_LPAE_OUTER_IMPL_DEF        0x88ull
[...]
	#define AS_MEMATTR_INDEX_DEFAULT_ACE           3
[...]
	/* Outer coherent, inner implementation defined policy */
	#define AS_MEMATTR_INDEX_OUTER_IMPL_DEF        3

Steve

> ---
>  drivers/iommu/io-pgtable-arm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 77f41c9dd9be..2794d4661339 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -1061,6 +1061,9 @@ arm_mali_lpae_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
>  	cfg->arm_mali_lpae_cfg.transtab = virt_to_phys(data->pgd) |
>  					  ARM_MALI_LPAE_TTBR_READ_INNER |
>  					  ARM_MALI_LPAE_TTBR_ADRMODE_TABLE;
> +	if (cfg->coherent_walk)
> +		cfg->arm_mali_lpae_cfg.transtab |= ARM_MALI_LPAE_TTBR_SHARE_OUTER;
> +
>  	return &data->iop;
>  
>  out_free_data:
> 

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel