RADV_PERFTEST ACO vs SAM vs AMD RX5700xt

[dimko]

Hello Fellow Nerds.

RADV_PERFTEST variable, does it allow both ACO and SAM(resizable BAR/smart memory access)

Seems odd, both features enabled by same variable, and I don't see reasons why both can't be used together.

Also AMD RX5700xt, will it benefit from SAM?(went into great pains to set up my gentoo from single root partition into root/boot with EFI and got a LVL UP + enabled resizable bar in my EFI)

Can someone comment please?

[dimko]

https://docs.mesa3d.org/envvars.html

RADV_PERFTESTa comma-separated list of named flags, which do various things:

[agd5f]

SAM is automatically enabled by the kernel GPU driver when it is available/enabled by the bios. Mesa can do various optimizations on top of that but the base functionality will be used if it is available.

[dimko]

SAM is automatically enabled by the kernel GPU driver when it is available/enabled by the bios. Mesa can do various optimizations on top of that but the base functionality will be used if it is available.

Thanks.
Is there a source to that I could read more about?

[agd5f]

Thanks.
Is there a source to that I could read more about?

SAM just means that the CPU can directly access all of the VRAM on the GPU using the PCI BAR. This can be accomplished either by the OS or the sbios. On Linux we can resize the PCI BAR if there is enough MMIO space on the platform. Alternatively, the sbios can resize the BARs when it enumerates the PCI devices on the platform. In that case, the BAR is already resized when the OS loads. In either case, the CPU is capable of accessing the entire amount of VRAM on the GPU. When the application wants a CPU mapping of a buffer in VRAM, we never run into contention for BAR space. That covers the base functionality. Now on the user mode driver side, you can do some optimizations if you know that the full amount of VRAM is available to the CPU (e.g., you might make different decisions about whether or not you use a staging buffer when accessing resources with the CPU, etc.).