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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Sat, 21 Dec 2019 18:46:37 -0500
To:     Tom Murphy <murphyt7@tcd.ie>
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Subject: Re: [PATCH 1/8] iommu/vt-d: clean up 32bit si_domain assignment
Message-ID: <20191221234635.GA99623@rani.riverdale.lan>
References: <20191221150402.13868-1-murphyt7@tcd.ie>
 <20191221150402.13868-2-murphyt7@tcd.ie>
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On Sat, Dec 21, 2019 at 03:03:53PM +0000, Tom Murphy wrote:
> In the intel iommu driver devices which only support 32bit DMA can't be
> direct mapped. The implementation of this is weird. Currently we assign
> it a direct mapped domain and then remove the domain later and replace
> it with a domain of type IOMMU_DOMAIN_IDENTITY. We should just assign it
> a domain of type IOMMU_DOMAIN_IDENTITY from the begging rather than
> needlessly swapping domains.
> 
> Signed-off-by: Tom Murphy <murphyt7@tcd.ie>
> ---
>  drivers/iommu/intel-iommu.c | 88 +++++++++++++------------------------
>  1 file changed, 31 insertions(+), 57 deletions(-)
> 
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 0c8d81f56a30..c1ea66467918 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -5640,7 +5609,12 @@ static int intel_iommu_add_device(struct device *dev)
>  	domain = iommu_get_domain_for_dev(dev);
>  	dmar_domain = to_dmar_domain(domain);
>  	if (domain->type == IOMMU_DOMAIN_DMA) {
> -		if (device_def_domain_type(dev) == IOMMU_DOMAIN_IDENTITY) {
> +		/*
> +		 * We check dma_mask >= dma_get_required_mask(dev) because
> +		 * 32 bit DMA falls back to non-identity mapping.
> +		 */
> +		if (device_def_domain_type(dev) == IOMMU_DOMAIN_IDENTITY &&
> +				dma_mask >= dma_get_required_mask(dev)) {
>  			ret = iommu_request_dm_for_dev(dev);
>  			if (ret) {
>  				dmar_remove_one_dev_info(dev);
> -- 
> 2.20.1
> 

Should this be dma_direct_get_required_mask? dma_get_required_mask may
return DMA_BIT_MASK(32) -- it callbacks into intel_get_required_mask,
but I'm not sure what iommu_no_mapping(dev) will do at this point?