[PATCH 1/4] iommu/io-pgtable-arm: use inner shareable for Mali LPAE on a non-coherent GPU

[Young-Jun Hong <nirvanah@nirvanahyj.com>]

io-pgtable marks every ARM_MALI_LPAE PTE outer shareable (SH = 2). Outer
shareability is only meaningful when the GPU sits behind a coherent
interconnect, i.e. when the page-table walk is coherent (cfg.coherent_walk,
which panfrost sets from the dma-coherent DT property). The vendor kbase
driver on a non-coherent Exynos 5422 (COHERENCY_NONE) never uses outer
shareable: its PTEs are non-shareable or inner shareable.

On Exynos 5422 (Mali-T628 MP6, ARMv7 LPAE, not IO-coherent) the outer
shareable attribute hangs the whole SoC when a large GPU buffer is first
used under GPU bandwidth load and memory pressure: 0 of 22 runs survived
with SH = 2, 3 of 3 with inner shareable. Non-shareable also avoids the
hang but breaks global atomics between the two core groups, so use inner
shareable for a non-coherent Mali; coherent configurations are unchanged.
Sustained GPU copy bandwidth also rises from 6.3 to 7.7 GB/s.

Tested on Samsung ARTIK 10 (Exynos 5422) with panfrost and Mesa Rusticl:
hang reproducer clean, OpenCL accuracy suite 96/96, no GPU faults over
several days of use.

The analysis, the code and this changelog were produced with Claude
Code (Anthropic), a coding assistant.

Fixes: 7618e4790982 ("iommu/io-pgtable-arm: Improve attribute handling")
Cc: stable@vger.kernel.org
Signed-off-by: Young-Jun Hong <nirvanah@nirvanahyj.com>
---
 drivers/iommu/io-pgtable-arm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index 476c0e25631a..0e7b2edd9ca1 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -565,6 +565,13 @@ static arm_lpae_iopte arm_lpae_prot_to_pte(struct arm_lpae_io_pgtable *data,
 	 */
 	if (prot & IOMMU_CACHE && data->iop.fmt != ARM_MALI_LPAE)
 		pte |= ARM_LPAE_PTE_SH_IS;
+	else if (data->iop.fmt == ARM_MALI_LPAE && !data->iop.cfg.coherent_walk)
+		/*
+		 * A non-coherent Mali must not mark its accesses outer
+		 * shareable; inner shareable keeps atomics consistent across
+		 * its core groups.
+		 */
+		pte |= ARM_LPAE_PTE_SH_IS;
 	else
 		pte |= ARM_LPAE_PTE_SH_OS;
 
-- 
2.47.3