Re: [PATCH] drm/nouveau: do not move buffers when not needed

[Maarten Lankhorst <maarten.lankhorst@canonical.com>]

Op 04-09-13 03:24, Ben Skeggs schreef:
> On Mon, Jul 15, 2013 at 6:39 PM, Maarten Lankhorst
> <maarten.lankhorst@canonical.com> wrote:
>> Op 15-07-13 08:05, Ben Skeggs schreef:
>>> On Fri, Jul 12, 2013 at 10:45 PM, Maarten Lankhorst
>>> <maarten.lankhorst@canonical.com> wrote:
>>>> I have no idea what this bogus restriction on placement is, but it breaks decoding 1080p
>>>> VDPAU at boot speed. With this patch applied I only need to bump the vdec clock to
>>>> get real-time 1080p decoding. It prevents a lot of VRAM <-> VRAM buffer moves.
>>> It's not bogus, and is required for pre-GF8 boards with VRAM > BAR size.
>>>
>>> What configuration does the buffer that's getting moved here have
>>> exactly?  The placement restriction isn't necessary on GF8, the rest
>>> of the restrictions may currently be required still however.
>>>
>>> = vdpau on NVC0 with tiling. I upload the raw bitstream to a tiling bo. This is ok because
>> the vm hides all the tiling translations, and the engines will read the raw bitstream correctly.
> Why would you be doing such a thing in the first place?  It seems
> pointless, and quite possibly counter-productive to use a tiled layout
> for a linear data structure...
Initially I just allocated everything I didn't need to access directly tiled, and it seems I did the same for
the bitstream bo. I only found out later about the bug with excessive moves causing a major slowdown.

>> 8<---
>> This prevents buffer moves from being done on NV50+, where remapping is not needed because
>> the bar has its own VM, instead of only having the first BAR1-size chunk of VRAM accessible.
>> nouveau_bo_tile_layout is always 0 on < NV_50.
>>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
>> ---
>> diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c
>> index d506da5..762bfcd 100644
>> --- a/drivers/gpu/drm/nouveau/nouveau_bo.c
>> +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c
>> @@ -1346,14 +1361,13 @@ nouveau_ttm_fault_reserve_notify(struct ttm_buffer_object *bo)
>>         struct nouveau_device *device = nv_device(drm->device);
>>         u32 mappable = pci_resource_len(device->pdev, 1) >> PAGE_SHIFT;
>>
>> -       /* as long as the bo isn't in vram, and isn't tiled, we've got
>> -        * nothing to do here.
>> +       /*
>> +        * if the bo is not in vram, or remapping can be done (nv50+)
>> +        * do not worry about placement, any location is valid
>>          */
>> -       if (bo->mem.mem_type != TTM_PL_VRAM) {
>> -               if (nv_device(drm->device)->card_type < NV_50 ||
>> -                   !nouveau_bo_tile_layout(nvbo))
>> -                       return 0;
>> -       }
>> +       if (nv_device(drm->device)->card_type >= NV_50 ||
>> +           bo->mem.mem_type != TTM_PL_VRAM)
>> +               return 0;
> I get what you're trying to do here, and we should definitely avoid
> the "mappable vram" check on GF8, but I suspect this condition is too
> broad.  I'll think about it more after I finish reviewing the rest of
> the patches on the list..
>
I think this relaxed check is fine. If it's !VRAM, the host can always access it because it has direct access to the
pages without needing anything from the gpu. On >= NV50 the move can always be skipped too because the
memory is mapped to the vm, and always accessible.

~Maarten