Subject: [PATCH v4 1/1] iommu-api: Add map_sg/unmap_sg functions

From: [email-masked] (Olav Haugan)
Date: Fri, 01 Aug 2014 09:44:33 -0700

Hi Will,

On 8/1/2014 1:22 AM, Will Deacon wrote:
> Hi Olav,
> 
> On Fri, Aug 01, 2014 at 01:54:44AM +0100, Olav Haugan wrote:
>> Mapping and unmapping are more often than not in the critical path.
>> map_sg and unmap_sg allows IOMMU driver implementations to optimize
>> the process of mapping and unmapping buffers into the IOMMU page tables.
>>
>> Instead of mapping a buffer one page at a time and requiring potentially
>> expensive TLB operations for each page, this function allows the driver
>> to map all pages in one go and defer TLB maintenance until after all
>> pages have been mapped.
>>
>> Additionally, the mapping operation would be faster in general since
>> clients does not have to keep calling map API over and over again for
>> each physically contiguous chunk of memory that needs to be mapped to a
>> virtually contiguous region.
> 
> Just a couple of minor comments, but I think this is almost there now.
> 
>> Signed-off-by: Olav Haugan <[email-masked]>
>> ---
>>  drivers/iommu/iommu.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>  include/linux/iommu.h | 28 ++++++++++++++++++++++++++++
>>  2 files changed, 72 insertions(+)
>>
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 1698360..1d5dc2e 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -1088,6 +1088,50 @@ size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
>>  }
>>  EXPORT_SYMBOL_GPL(iommu_unmap);
>>  
>> +int iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
>> +			struct scatterlist *sg, unsigned int nents,
>> +			int prot, unsigned long flags)
>> +{
> 
> What do you anticipate passing in the flags parameter? I assume it's
> something specific to the scatterlist, since we can't provide this to
> iommu_map as it stands?

Initially the flags argument is planned to be used by clients to
indicate to the driver that no TLB operation is necessary. This allows
clients to for example map/unmap multiple scatter-gather lists without
doing expensive TLB invalidate operations for each call but just do this
at the last mapping/unmapping call instead. I believe Rob Clark was
looking for this feature and I can see the benefit for our use cases also.

>> +	int ret = 0;
>> +	unsigned long offset = 0;
>> +
>> +	if (unlikely(domain->ops->map_sg == NULL)) {
>> +		unsigned int i;
>> +		struct scatterlist *s;
>> +
>> +		for_each_sg(sg, s, nents, i) {
>> +			phys_addr_t phys = page_to_phys(sg_page(s));
>> +			size_t page_len = s->offset + s->length;
>> +
>> +			ret = iommu_map(domain, iova + offset, phys, page_len,
>> +					prot);
>> +			if (ret)
>> +				goto fail;
>> +
>> +			offset += page_len;
>> +		}
>> +	} else {
>> +		ret = domain->ops->map_sg(domain, iova, sg, nents, prot, flags);
>> +	}
>> +	goto out;
>> +
>> +fail:
>> +	/* undo mappings already done in case of error */
>> +	iommu_unmap(domain, iova, offset);
> 
> I think this would be cleaner if you stuck it in the loop above and removed
> all these labels:
> 
>   if (ret) {
> 	iommu_unmap(...);
> 	break;
>   }

Sure, I can do that.

Thanks,

Olav

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