[PATCH 3/3] drm/amdgpu: add gtt_sys_limit

[Christian König]

Am 27.06.2017 um 04:57 schrieb Michel Dänzer:
> On 27/06/17 08:39 AM, John Brooks wrote:
>> On Mon, Jun 26, 2017 at 03:39:57PM +0200, Christian König wrote:
>>> From: Christian König <christian.koenig at amd.com>
>>>
>>> Limit the size of the GART table for the system domain.
>>>
>>> This saves us a bunch of visible VRAM, but also limitates the maximum BO size we can swap out.
>>>
>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>> Hmm.
>>
>> On my system, GTT is 4096MiB (1048576 pages). For this, the table takes up
>> 1048576*8 bytes = 8MiB. Reducing GTT to 256MiB (65536 pages) would reduce the
>> size of the table to 512 KiB. A relatively large saving, to be sure. But in the
>> grander scheme of things, is saving 7.5MiB (3% of visible VRAM @ 256M) worth
>> cutting GTT memory by a factor of 16?

The amount of GTT memory the application can use through the VM page 
tables stays the same.

Only the system VM is limited to 256MB and so saves us a whole bunch of 
space.

> I'm afraid not, especially since it would limit the maximum BO size to <
> 256MB, if I understand correctly. Pretty sure that would cause failures
> with real world apps.
Yes, exactly that's the major problem here. I should have put a WIP mark 
on the patch.

Christian.