Documentation: Add documentation for GPIO toggling in ACPI AML

This document provides information about the different functions that a
driver can use for generating ACPI code for toggling GPIO. These
functions are expected to be implemented by the SoC. It also defines the
different constraints on use of Local variables in ACPI code while
implementing these functions.

BUG=chrome-os-partner:55988

Change-Id: Ibc03d766afb6d7b75bc0dc9f79920b561f1c4a78
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/17128
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

Documentation/acpi/gpio.md

@@ -0,0 +1,162 @@
+# GPIO toggling in ACPI AML for coreboot #
+
+# Table of contents #
+- Introduction
+- Platform Interface
+- Implementation details
+- Arguments and Local Variables Management
+
+# Introduction #
+
+ACPI provides platform-independent interfaces enabling the operating
+system to perform power management for devices as well as the entire
+system. An operating system can simply call into Method()s implemented
+by the interface to request different power management operations. In
+order to be able to perform these operations, an interface might
+require toggling of GPIOs. e.g. a touchscreen device interface might
+require toggling of reset-gpio in order to take the device out of
+reset or to put it back into reset.
+
+Thus, any coreboot driver that implements such an ACPI interface might
+require the ability to toggle GPIOs. However, toggling of GPIO is not
+the same across different platforms and it will require the driver to
+depend upon platform to do the required work. This document presents a
+simple interface that can be used by any coreboot driver to generate
+ACPI AML code for reading or toggling platform GPIOs.
+
+# Platform Interface #
+
+All platforms that use drivers requiring ACPI AML code for GPIO
+interactions need to be implement the following functions:
+1. Return GPIO Rx value if it is acting as input
+   int acpigen_soc_read_rx_gpio(unsigned int gpio_num)
+2. Return GPIO Tx value if it is acting as output
+   int acpigen_soc_get_tx_gpio(unsigned int gpio_num)
+3. Set GPIO Tx value to 1 if it is acting as output
+   int acpigen_soc_set_tx_gpio(unsigned int gpio_num)
+4. Set GPIO Tx value to 0 if it is acting as output
+   int acpigen_soc_clear_tx_gpio(unsigned int gpio_num)
+
+Each of the above functions takes as input gpio_num which is the gpio
+number that needs to be read or toggled and returns an integer which
+is:
+1. Error = -1
+2. Success = 0
+
+Above callback functions are chosen to be implemented in C rather than
+adding them as AML code callbacks for the following reasons:
+1. It is easier to add error prints in C which will inform the
+   developer that these callbacks are missing. It restricts debugging
+   to coreboot logs.
+2. GPIO conversion from number to register offset can be easily done
+   in C by reusing implemented functions rather than adding all the
+   logic to AML code or depending upon complicated macros to be added
+   to device-tree.
+3. Allows GPIO AML methods to be present under any device scope and
+   gives SoC the flexibility to call them without any restrictions.
+
+# Implementation Details #
+
+ACPI library in coreboot will provide weak definitions for all the
+above functions with error messages indicating that these functions
+are being used. This allows drivers to conditionally make use of GPIOs
+based on device-tree entries or any other config option. It is
+recommended that the SoC code in coreboot should provide
+implementations of all the above functions generating ACPI AML code
+irrespective of them being used in any driver. This allows mainboards
+to use any drivers and take advantage of this common infrastructure.
+
+Platforms are restricted to using Local5, Local6 and Local7 variables
+only in implementations of the above functions. Any AML methods called
+by the above functions do not have any such restrictions on use of
+Local variables in AML code. Local0 is to be used for all get/read
+functions to return values. This means that the driver code should not
+make any assumptions about the values in Local5, Local6 and Local7
+variables.
+
+ **Function**                   **Operation**                **Return**
+ acpigen_soc_read_rx_gpio     Generate ACPI AML code to      Error = -1
+                              read value of Rx in Local0.    Success = 0
+ acpigen_soc_get_tx_gpio      Generate ACPI AML code to      Error = -1
+                              get value of Tx in Local0.     Success = 0
+ acpigen_soc_set_tx_gpio      Generate ACPI AML code to      Error = -1
+                              set Tx to 1.                   Success = 0
+ acpigen_soc_clear_tx_gpio    Generate ACPI AML code to      Error = -1
+                              set Tx to 0.                   Success = 0
+
+
+Ideally, the operation column in the above table should use one or
+more functions implemented by the platform in AML code library (like
+gpiolib.asl). In the example below SPC0 and GPC0 need to be
+implemented by the SoC in AML code library and they can be used by
+acpi_soc_set_tx_gpio to read and set bit in the appropriate register
+for the GPIO.
+
+**acpigen_soc_set_tx_gpio**
+
+	uint64_t gpio_reg_offset = gpio_get_reg_offset(gpio_num);
+
+	/* Store (\_SB.GPC0(gpio_reg_offset, Local5) */
+	acpigen_write_store();
+	acpigen_emit_namestring(“\\_SB.GPC0”);
+	acpigen_write_integer(gpio_reg_offset);
+	acpigen_emit_byte(LOCAL5_OP);
+
+
+	/* Or (Local5, TX_BIT, Local5) */
+	acpigen_write_or(LOCAL5_OP, TX_BIT, LOCAL5_OP);
+
+	/* \_SB.SPC0(gpio_reg_offset, LOCAL5) */
+	acpigen_emit_namestring(“\\_SB.SPC0”);
+	acpigen_write_integer(gpio_reg_offset);
+	acpigen_emit_byte(LOCAL5_OP);
+
+	return 0;
+
+**acpigen_soc_get_tx_gpio**
+
+	uint64_t gpio_reg_offset = gpio_get_reg_offset(gpio_num);
+
+
+	/* Store (\_SB.GPC0(gpio_reg_offset, Local5) */
+	acpigen_write_store();
+	acpigen_emit_namestring(“\\_SB.GPC0”);
+	acpigen_write_integer(gpio_reg_offset);
+	acpigen_emit_byte(LOCAL5_OP);
+
+
+	/*
+	 * If (And (Local5, TX_BIT)) Store (One, Local0) Else Store (Zero,
+	 * Local0)
+	 */
+	acpigen_write_if_and(Local5, TX_BIT);
+	acpigen_write_store_args(ONE_OP, LOCAL0_OP);
+	acpigen_pop_len();
+	acpigen_write_else();
+	acpigen_write_store_args(ZERO_OP, LOCAL0_OP);
+	acpigen_pop_len();
+
+	return 0;
+
+
+These are reference implementations and the platforms are free to
+implement these functions in any way they like. Coreboot driver can
+then simply call into these functions to generate ACPI AML code to
+get/set/clear any GPIO. In order to decide whether GPIO operations are
+required, driver code can rely either on some config option or read
+device-tree to use any user-provided GPIOs.
+
+# Arguments and Local Variables Management #
+
+Platform-defined functions can call methods using the same calling
+conventions provided by AML code. However, use of Local Variables is
+restricted to Local5, Local6 and Local7 unless they call into some
+other method. Called method can use any Local variables, Local0 -
+Local7. In case of functions expected to return back value to the
+caller, this value is expected to be returned in Local0.
+
+Driver code should not make any assumptions about the contents of
+Local5, Local6 and Local7 across callbacks to SoC code. If it makes a
+read or get call to SoC, the return value should be used from Local0
+on return. However, if it makes a set or clear call to SoC, the value
+in Local0 is undefined.