Expand the GPUVM documentation to better describe the hardware functionality and use cases it serves.
Signed-off-by: Alex Deucher <alexander.deuc...@amd.com> --- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 41 +++++++++++++++++++------- 1 file changed, 31 insertions(+), 10 deletions(-) diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index 003aa9e47085..cb57a7bf5e2c 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -45,22 +45,43 @@ /** * DOC: GPUVM * - * GPUVM is similar to the legacy gart on older asics, however - * rather than there being a single global gart table - * for the entire GPU, there are multiple VM page tables active - * at any given time. The VM page tables can contain a mix - * vram pages and system memory pages and system memory pages + * GPUVM is the MMU functionality provided on the GPU. + * GPUVM is similar to the legacy GART on older asics, however + * rather than there being a single global GART table + * for the entire GPU, there can be multiple GPUVM page tables active + * at any given time. The GPUVM page tables can contain a mix + * VRAM pages and system pages (both memory and MMIO) and system pages * can be mapped as snooped (cached system pages) or unsnooped * (uncached system pages). - * Each VM has an ID associated with it and there is a page table - * associated with each VMID. When executing a command buffer, - * the kernel tells the ring what VMID to use for that command + * + * Each active GPUVM has an ID associated with it and there is a page table + * linked with each VMID. When executing a command buffer, + * the kernel tells the engine what VMID to use for that command * buffer. VMIDs are allocated dynamically as commands are submitted. * The userspace drivers maintain their own address space and the kernel * sets up their pages tables accordingly when they submit their * command buffers and a VMID is assigned. - * Cayman/Trinity support up to 8 active VMs at any given time; - * SI supports 16. + * The hardware supports up to 16 active GPUVMs at any given time. + * + * Each GPUVM is represented by a 1-2 or 1-5 level page table, depending + * on the ASIC family. GPUVM supports RWX attibutes on each page as well + * as other features such as encryption and caching attributes. + * + * VMID 0 is special. It is the GPUVM used for the kernel driver. In + * addition to an aperture managed by a page table, VMID 0 also has + * several other apertures. There is an aperture for direct access to VRAM + * and there is a legacy AGP aperture which just forwards accesses directly + * to the matching system physical addresses (or IOVAs when an IOMMU is + * present). These apertures provide direct access to these memories without + * incurring the overhead of a page table. VMID 0 is used by the kernel + * driver for tasks like memory management. + * + * GPU clients (i.e., engines on the GPU) use GPUVM VMIDs to access memory. + * For user applications, each application can have their own unqiue GPUVM + * address space. The application manages the address space and the kernel + * driver manages the GPUVM page tables for each process. If an GPU client + * accesses an invalid page, it will generate a GPU page fault, similar to + * accessing an invalid page on a CPU. */ #define START(node) ((node)->start) -- 2.38.1