Re: [PATCH] iommu/rockchip: fix page table allocation flags for v2 IOMMU

[Shawn Lin <shawn.lin@rock-chips.com>]

+ Simon

在 2026/03/31 星期二 15:50, Midgy BALON 写道:
> commit 2a7e6400f72b ("iommu: rockchip: Allocate tables from all
> available memory for IOMMU v2") removed GFP_DMA32 from
> iommu_data_ops_v2, reasoning that RK356x and RK3588 IOMMU v2 hardware
> supports up to 40-bit physical addresses for page tables.  However, the
> RK3568 IOMMU page-table walker uses a 32-bit AXI bus: it cannot access
> physical addresses above 4 GB regardless of the address encoding range.
> 
> On boards with more than 4 GB of RAM (e.g. 8 GB LPDDR4X), removing
> GFP_DMA32 causes two distinct failure modes:
> 
> 1. Direct allocation above 4 GB: iommu_alloc_pages_sz() may return
>     memory above 0x100000000.  The hardware page-table walker issues a
>     bus error trying to dereference those addresses, causing an IOMMU
>     fault on the first DMA transaction.
> 
> 2. SWIOTLB bounce-buffer poisoning: without GFP_DMA32, page tables land
>     above the SWIOTLB window.  dma_map_single() with DMA_BIT_MASK(32)
>     then bounces them into a buffer below 4 GB.  rk_dte_get_page_table()
>     returns phys_to_virt() of the bounce buffer address; PTEs are written
>     there; the next dma_sync_single_for_device(DMA_TO_DEVICE) copies the
>     original (zero) data back over the bounce buffer, silently erasing the
>     freshly written PTEs.  The IOMMU faults because every PTE reads as zero.
> 
> Restore GFP_DMA32 (and DMA_BIT_MASK(32)) for iommu_data_ops_v2, which
> currently only serves "rockchip,rk3568-iommu" in mainline.
> 
> Tested on Radxa ROCK 3B (RK3568, 8 GB LPDDR4X):
>    - MobileNetV1 via RKNN: 5.8 ms/inference (IOMMU mode)
>    - YOLOv5s 640x640 via RKNN: ~57 ms/inference (IOMMU mode)
>    - No IOMMU faults, correct inference results
> 
> Fixes: 2a7e6400f72b ("iommu: rockchip: Allocate tables from all available memory for IOMMU v2")
> Cc: stable@vger.kernel.org
> Cc: Jonas Karlman <jonas@kwiboo.se>
> Signed-off-by: Midgy BALON <midgy971@gmail.com>
> ---
>   drivers/iommu/rockchip-iommu.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/rockchip-iommu.c b/drivers/iommu/rockchip-iommu.c
> index 85f3667e797..8b45db29471 100644
> --- a/drivers/iommu/rockchip-iommu.c
> +++ b/drivers/iommu/rockchip-iommu.c
> @@ -1358,8 +1358,8 @@ static struct rk_iommu_ops iommu_data_ops_v2 = {
>   	.pt_address = &rk_dte_pt_address_v2,
>   	.mk_dtentries = &rk_mk_dte_v2,
>   	.mk_ptentries = &rk_mk_pte_v2,
> -	.dma_bit_mask = DMA_BIT_MASK(40),
> -	.gfp_flags = 0,
> +	.dma_bit_mask = DMA_BIT_MASK(32),
> +	.gfp_flags = GFP_DMA32,
>   };
>   
>   static const struct of_device_id rk_iommu_dt_ids[] = {