[PATCH v6 00/25] iommu: Make default_domain's mandatory

[Baolu Lu]

On 2023/8/15 1:30, Jason Gunthorpe wrote:
> On Mon, Aug 14, 2023 at 04:43:23PM +0800, Baolu Lu wrote:
> 
>>> This is on github:https://github.com/jgunthorpe/linux/commits/iommu_all_defdom
>> It seems that after this series, all ARM iommu drivers are able to
>> support the IDENTITY default domain, hence perhaps we can remove below
>> code?
> Yes, but this code is still used
> 
>> If I remember it correctly, the background of this part of code is
>> that some arm drivers didn't support IDENTITY domain, so fall back to
>> DMA domain if IDENTITY domain allocation fails.
> Not quite..
> 
> 	if (req_type)
> 		return __iommu_group_alloc_default_domain(group, req_type);
> 
> req_type == 0 can still happen because it depends on what
> def_domain_type returns, which is still 0 in alot of cases
> 
> 	/* The driver gave no guidance on what type to use, try the default */
> 	dom = __iommu_group_alloc_default_domain(group, iommu_def_domain_type);
> 	if (dom)
> 		return dom;
> 
> So we try the default which might be IDENTITY/DMA/DMA_FQ - still have
> to do this.
> 
> 	/* Otherwise IDENTITY and DMA_FQ defaults will try DMA */
> 	if (iommu_def_domain_type == IOMMU_DOMAIN_DMA)
> 		return NULL;
> 	dom = __iommu_group_alloc_default_domain(group, IOMMU_DOMAIN_DMA);
> 	if (!dom)
> 		return NULL;
> 
> 	pr_warn("Failed to allocate default IOMMU domain of type %u for group %s - Falling back to IOMMU_DOMAIN_DMA",
> 		iommu_def_domain_type, group->name);
> 
> And this hunk is primarily a fallback in case the DMA_FQ didn't
> work. Then we try normal DMA.
> 
> That it also protected against not implementing IDENTITY is a side
> effect, so I think we have to keep all of this still.

Okay, fair enough. Thanks for the explanation.

Best regards,
baolu