Re: [PATCH] arm64/dma-mapping: Add DMA_ATTR_ALLOC_SINGLE_PAGES support

[Doug Anderson]

Will,

On Mon, Mar 21, 2016 at 11:01 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Thu, Mar 03, 2016 at 02:54:26AM +0800, Yong Wu wrote:
>> Sometimes it is not worth for the iommu allocating big chunks.
>> Here we enable DMA_ATTR_ALLOC_SINGLE_PAGES which could help avoid to
>> allocate big chunks while iommu allocating buffer.
>>
>> More information about this attribute, please check Doug's commit[1].
>>
>> [1]: https://lkml.org/lkml/2016/1/11/720
>>
>> Cc: Robin Murphy <robin.murphy@xxxxxxx>
>> Suggested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
>> Signed-off-by: Yong Wu <yong.wu@xxxxxxxxxxxx>
>> ---
>>
>> Our video drivers may soon use this.
>>
>>  arch/arm64/mm/dma-mapping.c |  4 ++--
>>  drivers/iommu/dma-iommu.c   | 14 ++++++++++----
>>  include/linux/dma-iommu.h   |  4 ++--
>>  3 files changed, 14 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
>> index 331c4ca..3225e3ca 100644
>> --- a/arch/arm64/mm/dma-mapping.c
>> +++ b/arch/arm64/mm/dma-mapping.c
>> @@ -562,8 +562,8 @@ static void *__iommu_alloc_attrs(struct device *dev, size_t size,
>>               struct page **pages;
>>               pgprot_t prot = __get_dma_pgprot(attrs, PAGE_KERNEL, coherent);
>>
>> -             pages = iommu_dma_alloc(dev, iosize, gfp, ioprot, handle,
>> -                                     flush_page);
>> +             pages = iommu_dma_alloc(dev, iosize, gfp, ioprot, attrs,
>> +                                     handle, flush_page);
>>               if (!pages)
>>                       return NULL;
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index 72d6182..3569cb6 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -190,7 +190,8 @@ static void __iommu_dma_free_pages(struct page **pages, int count)
>>       kvfree(pages);
>>  }
>>
>> -static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
>> +static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp,
>> +                                          struct dma_attrs *attrs)
>>  {
>>       struct page **pages;
>>       unsigned int i = 0, array_size = count * sizeof(*pages);
>> @@ -203,6 +204,10 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
>>       if (!pages)
>>               return NULL;
>>
>> +     /* Go straight to 4K chunks if caller says it's OK. */
>> +     if (dma_get_attr(DMA_ATTR_ALLOC_SINGLE_PAGES, attrs))
>> +             order = 0;
>
> I have a slight snag with this, in that you don't consult the IOMMU
> pgsize_bitmap at any point, and assume that it can map pages at the
> same granularity as the CPU. The documentation for
> DMA_ATTR_ALLOC_SINGLE_PAGES seems to be weaker than that.

Interesting.  Is that something that exists in the real world?  ...or
something you think is coming soon?

I'd argue that such a case existed in the real world then probably
we're already broken.  Unless I'm misreading, existing code will
already fall all the way back to order 0 allocations.  ...so while
existing code might could work if it was called on a totally
unfragmented system, it would already break once some fragmentation
was introduced.

I'm not saying that we shouldn't fix the code to handle this, I'm just
saying that Yong Wu's patch doesn't appear to break any code that
wasn't already broken.  That might be reason to land his code first,
then debate the finer points of whether IOMMUs with less granularity
are likely to exist and whether we need to add complexity to the code
to handle them (or just detect this case and return an error).

>From looking at a WIP patch provided to me by Yong Wu, it looks as if
he thinks several more functions need to change to handle this need
for IOMMUs that can't handle small pages.  That seems to be further
evidence that the work should be done in a separate patch.


Doug