Am 17.01.20 um 18:07 schrieb Felix Kuehling:
On 2020-01-17 3:17 a.m., Christian König wrote:
Am 17.01.20 um 03:01 schrieb Felix Kuehling:
On 2020-01-16 8:09, Christian König wrote:
Coreboot seems to have a problem correctly setting up access to the
stolen VRAM
on KV/KB. Use the direct access only when necessary.
I'm not sure what you mean by "necessary".
Necessary for better performance.
Right. So your patch description says that sometimes better
performance is not necessary.
Well what I want to say is that it is not necessary for better
performance. If FB is small enough we can use the BAR as well.
The criteria is based on the size of the BAR, which doesn't really
have anything to do with performance.
Why? Most of the performance gain comes from not shuffling around VRAM
buffers for CPU access any more.
Additional to that there is a reduction in latency when accessing the
VRAM, but that usually doesn't matter for performance.
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
index 19d5b133e1d7..9da9596a3638 100644
--- a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
+++ b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
@@ -381,7 +381,8 @@ static int gmc_v7_0_mc_init(struct
amdgpu_device *adev)
adev->gmc.aper_size = pci_resource_len(adev->pdev, 0);
#ifdef CONFIG_X86_64
- if (adev->flags & AMD_IS_APU) {
+ if (adev->flags & AMD_IS_APU &&
+ adev->gmc.real_vram_size > adev->gmc.aper_size) {
CPU access to the whole VRAM isn't really necessary. I thought the
main motivation for accessing FB directly on APUs was better
performance. You're disabling that optimization on all APUs where
the FB is smaller than the aperture size.
Correct, yes. For some reason coreboot seems to explicitly setup the
memory used for the FB as write-protected.
The GPU can still read/write it normally cause it ignores that
protection, but the CPU can't change the FB memory any more with that
setup.
Can we test whether writing to FB works and make the decision based on
that?
Thought about that as well. But it is complicated to implement and we
would need to test the whole FB to be sure and that could take a while.
Christian.
Thanks,
Felix
No idea why they do this, most likely just an over conservative
protection of a reserved area of memory.
Regards,
Christian.
Regards,
Felix
adev->gmc.aper_base = ((u64)RREG32(mmMC_VM_FB_OFFSET)) <<
22;
adev->gmc.aper_size = adev->gmc.real_vram_size;
}
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