Re: [PATCH 1/1] [RFC] drm/ttm: Don't init dma32_zone on 64-bit systems

["Kuehling, Felix" <Felix.Kuehling-5C7GfCeVMHo@public.gmane.org>]

On 2019-02-21 2:59 a.m., Koenig, Christian wrote:
> On x86 with HIGHMEM there is no dma32 zone. Why do we need one on
>>> x86_64? Can we make x86_64 more like HIGHMEM instead?
>>>
>>> Regards,
>>>      Felix
>>>
>> IIRC with x86, the kernel zone is always smaller than any dma32 zone,
>> so we'd always exhaust the kernel zone before dma32 anyway.
>>
>> Not sure why we have dma32 on x86 without highmem, though. sounds
>> superflous but harmless.
> Well DMA32 denotes memory which is accessible by devices who can only do
> 32bit addressing. And IIRC we can actually do DMA32 to highmem since
> something like 2.4.*.
>
> Because of this it is actually irrelevant if you have highmem or not,
> what matters for DMA32 is if you have an IOMMU or not.

Are you saying we should have a dma32_zone even on x86 with HIGHMEM?


>
> So even on x86_64 you actually do need the DMA32 zone if you don't have
> an IOMMU which remaps all memory for devices which can't directly
> address it.

Why is DMA32 special in this way? For example AMD GFX8 GPUs support 
40-bit DMA. But we don't have a special zone for that.

How common is it to have devices that need DMA32 on an x86_64 system?

Regards,
   Felix


>
> Regards,
> Christian.
>
>> /Thomas
>>
>>
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