Re: [RFC][PATCH 1/2] Introduce the IOMMU_DEVICE flag.

[Olav Haugan <ohaugan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 10/6/2014 4:04 AM, Will Deacon wrote:
> Hi Varun,
>
> [adding the Qualcomm guys, as I have an open question below]
>
> On Mon, Oct 06, 2014 at 11:28:15AM +0100, Varun Sethi wrote:
>> This is used for indicating device memory type for a DMA transaction. IOMMU
>> driver would set up attributes indicationg access to device memory.
>>
>> Signed-off-by: Varun Sethi <Varun.Sethi-KZfg59tc24xl57MIdRCFDg@public.gmane.org>
>> ---
>>   include/linux/iommu.h |    1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>> index 20f9a52..0599fe1 100644
>> --- a/include/linux/iommu.h
>> +++ b/include/linux/iommu.h
>> @@ -28,6 +28,7 @@
>>   #define IOMMU_WRITE	(1 << 1)
>>   #define IOMMU_CACHE	(1 << 2) /* DMA cache coherency */
>>   #define IOMMU_EXEC	(1 << 3)
>> +#define IOMMU_DEVICE	(1 << 4) /* Indicates access to device memory */
>
> An alternative to this would be to make device-memory the default type for
> !IOMMU_CACHE mappings (i.e. MAIR index 0).
>
> I'd value feedback either way; the argument comes down to whether we should
> use normal non-cacheable or device-nGnRE as the default (!IOMMU_CACHE) memory
> type. The latter is likely to be significantly slower, but provides the
> ordering guarantees that you need for MSI delivery.

We would prefer to stay with normal non-cacheable as the default since 
this is our most common use case and this is what is the default now.

.Olav

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