[PATCH v2 5/5] iommu: Allow default domain type to be set on the kernel command line

[Robin Murphy]

On 21/03/17 17:21, Will Deacon wrote:
> On Tue, Mar 21, 2017 at 04:45:27PM +0100, Joerg Roedel wrote:
>> On Fri, Mar 10, 2017 at 08:49:36PM +0000, Will Deacon wrote:
>>> @@ -1014,8 +1027,8 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
>>>  	 * IOMMU driver.
>>>  	 */
>>>  	if (!group->default_domain) {
>>> -		group->default_domain = __iommu_domain_alloc(dev->bus,
>>> -							     IOMMU_DOMAIN_DMA);
>>> +		group->default_domain =
>>> +			__iommu_domain_alloc(dev->bus, iommu_def_domain_type);
>>
>> It would be good to have a fall-back here if we are talking to an IOMMU
>> driver that uses default domains, but does not support identity-mapped
>> domains (yet). Exynos and Rockchip IOMMU drivers seem to fall into this
>> category. A dev_warn() also makes sense in case allocating a identity
>> domain fails.
> 
> Sure, something like the diff below?
> 
> Will
> 
> --->8
> 
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 42a842e3f95f..f787626a745d 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1027,10 +1027,19 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
>  	 * IOMMU driver.
>  	 */
>  	if (!group->default_domain) {
> -		group->default_domain =
> -			__iommu_domain_alloc(dev->bus, iommu_def_domain_type);
> +		struct iommu_domain *dom;
> +
> +		dom = __iommu_domain_alloc(dev->bus, iommu_def_domain_type);
> +		if (!dom) {
> +			dev_warn(dev,
> +				 "failed to allocate default IOMMU domain of type %u; falling back to IOMMU_DOMAIN_DMA",
> +				 iommu_def_domain_type);

Conversely, that's going to be noisy if iommu_def_domain_type was
IOMMU_DOMAIN_DMA to begin with. I think it makes sense to warn if the
user asked for a specific default domain type on the command line and
that didn't work, but maybe not to bother otherwise. Plus, if they asked
for passthrough, then not allocating a default domain at all is probably
closer to the desired result than installing a DMA ops domain would be.

Robin.

> +			dom = __iommu_domain_alloc(dev->bus, IOMMU_DOMAIN_DMA);
> +		}
> +
> +		group->default_domain = dom;
>  		if (!group->domain)
> -			group->domain = group->default_domain;
> +			group->domain = dom;
>  	}
>  
>  	ret = iommu_group_add_device(group, dev);
>