[v2,3/3] iommu/io-pgtable-arm: Allow coherent walks for Mali

Commit Message

Robin Murphy Sept. 30, 2019, 2:11 p.m. UTC

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 so
even when snoop signals are wired up, they are only emitted for outer
shareable accesses. As such, setting the TTBR_SHARE_OUTER bit does
indeed get coherent pagetable walks working nicely for the coherent
T620 in the Arm Juno SoC.

Exploiting coherency for data accesses is more of a challenge, since
not only do we need to get the GPU MMU attributes right but we'd also
have to avoid pgprot_writecombine creating an attribute mismatch on
the CPU side, so we won't try wiring that up just yet.

Reviewed-by: Steven Price <[e-mail]>
Signed-off-by: Robin Murphy <[e-mail]>
---
 drivers/iommu/io-pgtable-arm.c | 3 +++
 1 file changed, 3 insertions(+)

Comments

Rob Herring (Arm) Sept. 30, 2019, 6:36 p.m. UTC | #1

On Mon, Sep 30, 2019 at 9:11 AM Robin Murphy <[e-mail]> 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 so
> even when snoop signals are wired up, they are only emitted for outer
> shareable accesses. As such, setting the TTBR_SHARE_OUTER bit does
> indeed get coherent pagetable walks working nicely for the coherent
> T620 in the Arm Juno SoC.
>
> Exploiting coherency for data accesses is more of a challenge, since
> not only do we need to get the GPU MMU attributes right but we'd also
> have to avoid pgprot_writecombine creating an attribute mismatch on
> the CPU side, so we won't try wiring that up just yet.
>
> Reviewed-by: Steven Price <[e-mail]>
> Signed-off-by: Robin Murphy <[e-mail]>
> ---
>  drivers/iommu/io-pgtable-arm.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Rob Herring <[e-mail]>

Patch

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index ca51036aa53c..217c1fc06c69 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -1063,6 +1063,9 @@  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: