[PATCH v3 39/57] tpm: cr50: Add ACPI support

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Sun, Sep 06, 2020 at 03:43:47PM -0600, Simon Glass wrote:
> Generate ACPI information for this device so that Linux can use it
> correctly.

> +	ret = acpi_device_write_interrupt_or_gpio(ctx, (struct udevice *)dev,
> +						  "ready-gpios");
> +	if (ret < 0)
> +		return log_msg_ret("irq_gpio", ret);

I looked a bit closer at the acpi_table.c and would like to emphasize on
lessons learn from BIOS mistakes found in the wild with GPIOs.

Because GPIO resources are quite badly described in ACPI (it seems MS failed to
deliver GPIO abstraction to ACPI and to Windows API), there are some corner
cases, in order to mitigate which we need to consider the following to avoid
potential glitches and misconfiguration:

- GpioIo() doesn't have any means of Active Low / High setting, the _DSD must
  be provided to mitigate this.

- GpioIo() doesn't properly communicate the initial state of the output pin,
  thus Linux assumes the simple rule:

  Pull Bias	  Polarity	Requested...

  Implicit	  x		AS IS (assumed firmware configured for us)
  Explicit	  x (no _DSD)	as Pull Bias (Up == High, Down == Low),
  		  		assuming non-active (Polarity = !Pull Bias)

  Down		  Low		as low, assuming active
  Down		  High		as high, assuming non-active
  Up		  Low		as low, assuming non-active
  Up		  High		as high, assuming active

Hopefully this helps (and maybe can be added to some documentation).

P.S. Why I2cSerialBus() and not I2cSerialBusV2() ?

-- 
With Best Regards,
Andy Shevchenko