[Nouveau] [PATCH 08/11] drm/nouveau: tegra: Skip IOMMU initialization if already attached

[Robin Murphy]

Hi Thierry,

On 16/09/2019 16:04, Thierry Reding wrote:
> From: Thierry Reding <treding at nvidia.com>
> 
> If the GPU is already attached to an IOMMU, don't detach it and setup an
> explicit IOMMU domain. Since Nouveau can now properly handle the case of
> the DMA API being backed by an IOMMU, just continue using the DMA API.
> 
> Signed-off-by: Thierry Reding <treding at nvidia.com>
> ---
>   .../drm/nouveau/nvkm/engine/device/tegra.c    | 19 +++++++------------
>   1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> index d0d52c1d4aee..fc652aaa41c7 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
> @@ -23,10 +23,6 @@
>   #ifdef CONFIG_NOUVEAU_PLATFORM_DRIVER
>   #include "priv.h"
>   
> -#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
> -#include <asm/dma-iommu.h>
> -#endif
> -
>   static int
>   nvkm_device_tegra_power_up(struct nvkm_device_tegra *tdev)
>   {
> @@ -109,14 +105,13 @@ nvkm_device_tegra_probe_iommu(struct nvkm_device_tegra *tdev)
>   	unsigned long pgsize_bitmap;
>   	int ret;
>   
> -#if IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU)
> -	if (dev->archdata.mapping) {
> -		struct dma_iommu_mapping *mapping = to_dma_iommu_mapping(dev);
> -
> -		arm_iommu_detach_device(dev);
> -		arm_iommu_release_mapping(mapping);
> -	}
> -#endif
> +	/*
> +	 * Skip explicit IOMMU initialization if the GPU is already attached
> +	 * to an IOMMU domain. This can happen if the DMA API is backed by an
> +	 * IOMMU.
> +	 */
> +	if (iommu_get_domain_for_dev(dev))
> +		return;

Beware of "iommu.passthrough=1" - you could get a valid default domain 
here yet still have direct/SWIOTLB DMA ops. I guess you probably want to 
double-check the domain type as well.

Robin.

>   
>   	if (!tdev->func->iommu_bit)
>   		return;
>