Re: [PATCH v2] drm/tegra: Stop using iommu_present()

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 4/11/22 16:46, Robin Murphy wrote:
> Refactor the confusing logic to make it both clearer and more robust. If
> the host1x parent device does have an IOMMU domain then iommu_present()
> is redundantly true, while otherwise for the 32-bit DMA mask case it
> still doesn't say whether the IOMMU driver actually knows about the DRM
> device or not.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
> 
> v2: Fix logic for older SoCs and clarify.
> 
>  drivers/gpu/drm/tegra/drm.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
> index 9464f522e257..4f2bdab31064 100644
> --- a/drivers/gpu/drm/tegra/drm.c
> +++ b/drivers/gpu/drm/tegra/drm.c
> @@ -1092,6 +1092,19 @@ static bool host1x_drm_wants_iommu(struct host1x_device *dev)
>  	struct host1x *host1x = dev_get_drvdata(dev->dev.parent);
>  	struct iommu_domain *domain;
>  
> +	/* For starters, this is moot if no IOMMU is available */
> +	if (!device_iommu_mapped(&dev->dev))
> +		return false;
> +
> +	/*
> +	 * Tegra20 and Tegra30 don't support addressing memory beyond the
> +	 * 32-bit boundary, so the regular GATHER opcodes will always be
> +	 * sufficient and whether or not the host1x is attached to an IOMMU
> +	 * doesn't matter.
> +	 */
> +	if (host1x_get_dma_mask(host1x) <= DMA_BIT_MASK(32))
> +		return true;
> +
>  	/*
>  	 * If the Tegra DRM clients are backed by an IOMMU, push buffers are
>  	 * likely to be allocated beyond the 32-bit boundary if sufficient
> @@ -1122,14 +1135,13 @@ static bool host1x_drm_wants_iommu(struct host1x_device *dev)
>  	domain = iommu_get_domain_for_dev(dev->dev.parent);
>  
>  	/*
> -	 * Tegra20 and Tegra30 don't support addressing memory beyond the
> -	 * 32-bit boundary, so the regular GATHER opcodes will always be
> -	 * sufficient and whether or not the host1x is attached to an IOMMU
> -	 * doesn't matter.
> +	 * At the moment, the exact type of domain doesn't actually matter.
> +	 * Only for 64-bit kernels might this be a managed DMA API domain, and
> +	 * then only on newer SoCs using arm-smmu, since tegra-smmu doesn't
> +	 * support default domains at all, and since those SoCs are the same
> +	 * ones with extended GATHER support, even if it's a passthrough domain
> +	 * it can still work out OK.
>  	 */
> -	if (!domain && host1x_get_dma_mask(host1x) <= DMA_BIT_MASK(32))
> -		return true;
> -
>  	return domain != NULL;
>  }
>  
> @@ -1149,7 +1161,7 @@ static int host1x_drm_probe(struct host1x_device *dev)
>  		goto put;
>  	}
>  
> -	if (host1x_drm_wants_iommu(dev) && iommu_present(&platform_bus_type)) {
> +	if (host1x_drm_wants_iommu(dev)) {
>  		tegra->domain = iommu_domain_alloc(&platform_bus_type);
>  		if (!tegra->domain) {
>  			err = -ENOMEM;

Robin, thank you for the updated version. The patch looks okay to me.

Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

A bit later I'll also will give it a test, just to be sure because we
had problems with that function in the past.