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Subject: Re: [PATCH 12/18] iommu/tegra-gart: Add IOMMU_DOMAIN_DMA support
To:     Robin Murphy <robin.murphy@arm.com>, hch@lst.de, joro@8bytes.org,
        linux@armlinux.org.uk
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References: <cover.1597931875.git.robin.murphy@arm.com>
 <516b33118d489e56499ff8c64c019709b744110c.1597931876.git.robin.murphy@arm.com>
From:   Dmitry Osipenko <digetx@gmail.com>
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Date:   Thu, 20 Aug 2020 23:16:02 +0300
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20.08.2020 18:08, Robin Murphy пишет:
> Now that arch/arm is wired up for default domains and iommu-dma,
> implement the corresponding driver-side support for DMA domains.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/iommu/tegra-gart.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
> index fac720273889..e081387080f6 100644
> --- a/drivers/iommu/tegra-gart.c
> +++ b/drivers/iommu/tegra-gart.c
> @@ -9,6 +9,7 @@
>  
>  #define dev_fmt(fmt)	"gart: " fmt
>  
> +#include <linux/dma-iommu.h>
>  #include <linux/io.h>
>  #include <linux/iommu.h>
>  #include <linux/moduleparam.h>
> @@ -145,16 +146,22 @@ static struct iommu_domain *gart_iommu_domain_alloc(unsigned type)
>  {
>  	struct iommu_domain *domain;

Hello, Robin!

Tegra20 GART isn't a real IOMMU, but a small relocation aperture. We
would only want to use it for a temporal mappings (managed by GPU
driver) for the time while GPU hardware is busy and working with a
sparse DMA buffers, the driver will take care of unmapping the sparse
buffers once GPU work is finished [1]. In a case of contiguous DMA
buffers, we want to bypass the IOMMU and use buffer's phys address
because GART aperture is small and all buffers simply can't fit into
GART for a complex GPU operations that involve multiple buffers [2][3].
The upstream GPU driver still doesn't support GART, but eventually it
needs to be changed.

[1]
https://github.com/grate-driver/linux/blob/master/drivers/gpu/drm/tegra/gart.c#L489

[2]
https://github.com/grate-driver/linux/blob/master/drivers/gpu/drm/tegra/gart.c#L542

[3]
https://github.com/grate-driver/linux/blob/master/drivers/gpu/drm/tegra/uapi/patching.c#L90

> -	if (type != IOMMU_DOMAIN_UNMANAGED)
> +	if (type != IOMMU_DOMAIN_UNMANAGED && type != IOMMU_DOMAIN_DMA)
>  		return NULL;

Will a returned NULL tell to IOMMU core that implicit domain shouldn't
be used? Is it possible to leave this driver as-is?