pinctrl: sunxi: keep a shadow copy of the data register output latches (v6.8) - #365

On Allwinner SoCs, reading a bank's data register returns the pin level,
not the output latch, for pins that are muxed as inputs.  Writing a GPIO
therefore corrupts the output latches of all input-muxed pins in the
same bank: the read-modify-write in sunxi_pinctrl_gpio_set() reads back
their pin levels and writes those into their latches.

This breaks emulated open-drain lines (e.g. a bit-banged I2C bus from
i2c-gpio).  Such a line is released high by muxing it as input and
letting the pull-up raise it, so any concurrent GPIO write in the same
bank stores 1 into its latch.  Driving the line low afterwards is a
non-atomic data-then-mux sequence in sunxi_pinctrl_gpio_direction_output();
if the poisoning write lands between the two steps, the pin actively
drives high (push-pull) instead of low.  Observed in practice as sporadic
SCL/SDA high glitches on a T507 board bit-banging I2C on port E while
other PE GPIOs are toggled.

The bank spinlock cannot help: the racing write is a perfectly valid
whole-register RMW that faithfully writes back what the hardware
returned.  There are no set/clear registers on this IP to write a single
bit atomically.

Fix it the same way gpio-mmio handles hardware whose data register read
does not return the output latch: keep a shadow copy of each bank's
latches, base the read-modify-write on the shadow, and only write the
register.  The shadow is seeded from the hardware at probe time so pins
left in output mode by the bootloader keep their state.  Pins that reach
output mode through the gpiolib paths write their value (and thereby
their shadow bit) before the mux switch in
sunxi_pinctrl_gpio_direction_output(); pins muxed to gpio_out directly
through a pinmux node bypass that path, so sunxi_pmx_set() refreshes
their shadow bit from the latch (readable once the pin is in output
mode) to keep them driving their pre-existing level.

Fixes: df7b34f ("pinctrl: sunxi: Fix gpio_set behaviour")
Cc: stable@vger.kernel.org
Signed-off-by: Evgeny Boger <boger@wirenboard.com>