amdgpu: free_memory calculation can result in negative free memory

[kkartaltepe]

From

nvtop/src/extract_gpuinfo_amdgpu.c

Lines 714 to 719 in 6e91c74

	// TODO: Determine if we want to include GTT (GPU accessible system memory)
	SET_GPUINFO_DYNAMIC(dynamic_info, total_memory, memory_info.vram.total_heap_size);
	SET_GPUINFO_DYNAMIC(dynamic_info, used_memory, memory_info.vram.heap_usage);
	SET_GPUINFO_DYNAMIC(dynamic_info, free_memory, memory_info.vram.usable_heap_size - dynamic_info->used_memory);
	SET_GPUINFO_DYNAMIC(dynamic_info, mem_util_rate,
	(dynamic_info->total_memory - dynamic_info->free_memory) * 100 / dynamic_info->total_memory);

Dumping the values on my machine gives

Gpu: AMD Radeon 780M
Gpu clock: 800Mhz
Mem clock: 2800Mhz
Gpu util: 18%
Mem util: 112%
Mem total: 603979776 B
Mem used: 522059776 B
Mem free: -76898304 B

Perhaps this is an amdgpu issue though as I would expect the memory_info.vram.usable_heap_size > memory_info.vram.heap_usage, but this doesnt seem to be the case. Though im not sure this is exposed in the UI anywhere so maybe its not a big deal.

[kkartaltepe]

https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c#L752-L757 appears to be the source of the calculation, i think the pin memory here is getting double counted when free space is calculated this way. So it is most appropriate to just do total_heap_size - heap_usage.