On Mon, Sep 21, 2020 at 02:50:56PM +0300, Andy Shevchenko wrote:
> 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() ?
Forgot one important aspect.
GpioIo() can be used as interrupt and in this case the IoRestriction mustn't be
OutputOnly. It also requires active_low flag from _DSD in cases where it's
needed (better to always provide than rely on above assumption made on OS
level).
--
With Best Regards,
Andy Shevchenko
