AMD Smart Access Memory

[mluetkedreim]

Hi,

 

I just watched the new Video about the new AMD GPUs: https://www.youtube.com/watch?v=0xUioPsLRKA

Because I am currently developing an own bare metal driver for AMD Rx570 GPUs, I find minutes 7:18 to 8:00 of the video very interesting.

 

I also encountered the fact that the GPU has only a 256MB BAR of accessible VRAM at boot time. However, most GPUs have a PCI Express register for resizing the BAR to it's full size. The CPU can access all of the VRAM after resizing. This is done in Linux Kernel already since version 4 for every supported AMD GPU and not just the latest Radeon 6000 cards and I assume the same is done in the windows driver too!

You can find the function for resizing the framebuffer BAR in the Linux Kernel here: https://elixir.bootlin.com/linux/v4.16/source/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c#L610

The driver will read the amount of VRAM from a GPU register called "mmCONFIG_MEMSIZE" first and then call the BAR resize function: https://elixir.bootlin.com/linux/v4.16/source/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c#L324

The mechanism of BAR resizing is explained in detail in the PCI Express Specification (https://cupdf.com/document/pci-express-base-specification-revision-30.html) in chapter 7.22 Resizable Bar Capability on page 757. Because the VRAM BAR is already a 64-bit BAR (for the Rx570 at least) it can already operate with 4GB or more.

There is no need to access VRAM in 256MB slices afterwards!

 

Did I misunderstood the video?

There are some more source which states the same like the video (https://www.wepc.com/news/what-is-amd-smart-access-memory/), but I could not find any sources where they have their information from.

If I am right the Smart Access Memory is nothing new and the concept is already used since years with older GPUs and even Intel CPUs.

[BTGbullseye]

12 hours ago, mluetkedreim said:

If I am right the Smart Access Memory is nothing new and the concept is already used since years with older GPUs and even Intel CPUs.

Then how do you explain their reporting of up to double-digit percentages of performance increase by enabling it?

[mluetkedreim]

38 minutes ago, BTGbullseye said:

Then how do you explain their reporting of up to double-digit percentages of performance increase by enabling it?

I think I expressed myself a bit misleading. I think there is something new called Smart Access Memory and it provides a performance increase, but it works different and not like the video explains it.

[BTGbullseye]

11 hours ago, mluetkedreim said:

I think I expressed myself a bit misleading. I think there is something new called Smart Access Memory and it provides a performance increase, but it works different and not like the video explains it.

That I will agree to.