Re: [PATCH 4/6] drm: enable big page mapping for small pages when IOMMU is available

[Vince Hsu <vinceh-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On 04/17/2015 02:26 PM, Alexandre Courbot wrote:
> On Thu, Apr 16, 2015 at 8:06 PM, Vince Hsu <vinceh@nvidia.com> wrote:
>> Some platforms have IOMMU to map non-contiguous physical memory into
>> contiguous GPU virtual address. We can use this feature to enable big pages
>> mapping on scattered small pages. To achieve that, we also need changes in
>> subdev/mmu as well.
>>
>> Signed-off-by: Vince Hsu <vinceh@nvidia.com>
>> ---
>>   drm/nouveau/nouveau_bo.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/drm/nouveau/nouveau_bo.c b/drm/nouveau/nouveau_bo.c
>> index 77326e344dad..da76ee1121e4 100644
>> --- a/drm/nouveau/nouveau_bo.c
>> +++ b/drm/nouveau/nouveau_bo.c
>> @@ -221,6 +221,11 @@ nouveau_bo_new(struct drm_device *dev, int size, int align,
>>          if (drm->client.vm) {
>>                  if (!(flags & TTM_PL_FLAG_TT) && size > 256 * 1024)
>>                          nvbo->page_shift = drm->client.vm->mmu->lpg_shift;
>> +
>> +               if ((flags & TTM_PL_FLAG_TT) &&
>> +                               drm->client.vm->mmu->iommu_capable &&
>> +                               (size % (1 << drm->client.vm->mmu->lpg_shift)) == 0)
>> +                       nvbo->page_shift = drm->client.vm->mmu->lpg_shift;
> I wonder if we should not just use the same size heuristics as for VRAM above?
>
> Here, unless your buffer size is an exact multiple of the big page
> size (128K for GK20A), you will not use big pages at all. In effect,
> many buffers will be rejected for this reason. A behavior like "if the
> buffer size of more than 256KB, increase the size of the buffer to the
> next multiple of 128K and use big pages" would probably yield better
> results.
I'm told that the user space would align the buffer to the big page 
size. So I made this patch based on that impression. I'm not pretty 
familiar with the user space behavior, so sorry if I say something wrong 
here.

And are we allowed to extend the buffer size even the exporter doesn't 
know about that?

>>          }
>>
>>          nouveau_bo_fixup_align(nvbo, flags, &align, &size);
>> @@ -1641,6 +1646,10 @@ nouveau_bo_vma_add(struct nouveau_bo *nvbo, struct nvkm_vm *vm,
>>              (nvbo->bo.mem.mem_type == TTM_PL_VRAM ||
>>               nvbo->page_shift != vma->vm->mmu->lpg_shift))
>>                  nvkm_vm_map(vma, nvbo->bo.mem.mm_node);
>> +       else if (nvbo->bo.mem.mem_type == TTM_PL_TT &&
>> +               vma->vm->mmu->iommu_capable &&
>> +               nvbo->page_shift == vma->vm->mmu->lpg_shift)
>> +               nvkm_vm_map(vma, nvbo->bo.mem.mm_node);
> Sorry, I don't understand why this is needed, could you explain?
Originally the buffer can only be mapped when the memory type is VRAM or 
the page is small. So I added another condition to map big page for TT.

Thanks,
Vince





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