drm/amdgpu: add dummy read for some GCVM status registers

The GRBM register interface is now capable of bursting 1 cycle per
register wr->wr, wr->rd much faster than previous muticycle per
transaction done interface.  This has caused a problem where status
registers requiring HW to update have a 1 cycle delay, due to the
register update having to go through GRBM.

SW may operate on an incorrect value if they write a register and
immediately check the corresponding status register.

Registers requiring HW to clear or set fields may be delayed by 1 cycle.
For example,

1. write VM_INVALIDATE_ENG0_REQ mask = 5a
2. read VM_INVALIDATE_ENG0_ACK till the ack is same as the request mask = 5a
    a. HW will reset VM_INVALIDATE_ENG0_ACK = 0 until invalidation is complete
3. write VM_INVALIDATE_ENG0_REQ mask = 5a
4. read VM_INVALIDATE_ENG0_ACK till the ack is same as the request mask = 5a
    a. First read of VM_INVALIDATE_ENG0_ACK = 5a instead of 0
    b. Second read of VM_INVALIDATE_ENG0_ACK = 0 because
       the remote GRBM h/w register takes one extra cycle to be cleared
    c. In this case, SW will see a false ACK if they exit on first read

Affected registers (only GC variant)  |  Recommended Dummy Read
--------------------------------------+----------------------------
VM_INVALIDATE_ENG*_ACK                |  VM_INVALIDATE_ENG*_REQ
VM_L2_STATUS                          |  VM_L2_STATUS
VM_L2_PROTECTION_FAULT_STATUS         |  VM_L2_PROTECTION_FAULT_STATUS
VM_L2_PROTECTION_FAULT_ADDR_HI/LO32   |  VM_L2_PROTECTION_FAULT_ADDR_HI/LO32
VM_L2_IH_LOG_BUSY                     |  VM_L2_IH_LOG_BUSY
MC_VM_L2_PERFCOUNTER_HI/LO            |  MC_VM_L2_PERFCOUNTER_HI/LO
ATC_L2_PERFCOUNTER_HI/LO              |  ATC_L2_PERFCOUNTER_HI/LO
ATC_L2_PERFCOUNTER2_HI/LO             |  ATC_L2_PERFCOUNTER2_HI/LO

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

0 files changed, 0 insertions, 0 deletions