Re: [PATCH v7 2/2] drm/lima: driver for ARM Mali4xx GPUs

Qiang Yu yuq825@gmail.com writes:

• Mali 4xx GPUs have two kinds of processors GP and PP. GP is for OpenGL vertex shader processing and PP is for fragment shader processing. Each processor has its own MMU so prcessors work in virtual address space.
• There's only one GP but multiple PP (max 4 for mali 400 and 8 for mali 450) in the same mali 4xx GPU. All PPs are grouped togather to handle a single fragment shader task divided by FB output tiled pixels. Mali 400 user space driver is responsible for assign target tiled pixels to each PP, but mali 450 has a HW module called DLBU to dynamically balance each PP's load.
• User space driver allocate buffer object and map into GPU virtual address space, upload command stream and draw data with CPU mmap of the buffer object, then submit task to GP/PP with a register frame indicating where is the command stream and misc settings.
• There's no command stream validation/relocation due to each user process has its own GPU virtual address space. GP/PP's MMU switch virtual address space before running two tasks from different user process. Error or evil user space code just get MMU fault or GP/PP error IRQ, then the HW/SW will be recovered.
• Use GEM+shmem for MM. Currently just alloc and pin memory when gem object creation. GPU vm map of the buffer is also done in the alloc stage in kernel space. We may delay the memory allocation and real GPU vm map to command submission stage in the furture as improvement.
• Use drm_sched for GPU task schedule. Each OpenGL context should have a lima context object in the kernel to distinguish tasks from different user. drm_sched gets task from each lima context in a fair way.

mesa driver can be found here before upstreamed: https://gitlab.freedesktop.org/lima/mesa

v7:

• remove lima_fence_ops with default value
• move fence slab create to device probe
• check pad ioctl args to be zero
• add comments for user/kernel interface

Thanks for adding the comments! That helps a lot. I feel pretty good about the ABI at this point.

Reviewed-by: Eric Anholt eric@anholt.net