The Allwinner H616 SoC contains a mysterious bit at register offset 0x0
in the SRAM control block. If bit 16 is set (the reset value), the
temperature readings of the THS are way off, leading to reports about
200C, at normal ambient temperatures. Clearing this bits brings the
reported values down to the expected values.
The BSP code clears this bit in firmware (U-Boot), and has an explicit
comment about this, but offers no real explanation.

Experiments in U-Boot show that register 0x0 has no effect on the SRAM C
visibility: all tested bit settings still allow full read and write
access by the CPU to the whole of SRAM C. Only bit 24 of the register at
offset 0x4 makes all of SRAM C inaccessible by the CPU. So modelling
the THS switch functionality as an SRAM region would not reflect reality.

Since we should not rely on firmware settings, allow other code (the THS
driver) to access this register, by exporting it through the already
existing regmap. This mimics what we already do for the LDO control and
the EMAC register.

To avoid concurrent accesses to the same register at the same time, by
the SRAM switch code and the regmap code, use the same lock to protect
the access. The regmap subsystem allows to use an existing lock, so we
just need to hook in there.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240219153639.179814-2-andre.przywara@arm.com