[RFC 0/2] iommu: arm-smmu: Inherit SMR and CB config during init

From: Bjorn Andersson
Date: Wed Jun 05 2019 - 17:13:06 EST

Qualcomm devices typically boot with some sort of splash screen on the display,
or for some devices (i.e. recent Qualcomm laptops) an EFI framebuffer. For this
the bootloader allocates a static framebuffer, configures the display hardware
to output this on the display, sets up the SMMU for the display hardware and
jumps to the kernel.

But as the arm-smmu disables all SMRs the display hardware will no longer be
able to access the framebuffer and the result is that the device resets.

The given proposal reads back the SMR state at boot and for marks these
contexts as busy. This ensures that the display hardware will have continued
access to the framebuffer. Once a device is attached we try to match it to the
predefined stream mapping, so that e.g. the display driver will inherit the
particular SMRs/CBs.

This has the positive side effect that on some Qualcomm platforms, e.g. QCS404,
writes to the SMR registers are ignored. But as we inherit the preconfigured
mapping from the bootloader we can use the arm-smmu driver on these platforms.

Bjorn Andersson (2):
iommu: arm-smmu: Handoff SMR registers and context banks
iommu: arm-smmu: Don't blindly use first SMR to calculate mask

drivers/iommu/arm-smmu-regs.h | 2 +
drivers/iommu/arm-smmu.c | 100 +++++++++++++++++++++++++++++++---
2 files changed, 93 insertions(+), 9 deletions(-)