GPIO toggling in ACPI AML for coreboot

Table of contents

  • Introduction
  • Platform Interface
  • Helper routines
  • Implementation details
  • Arguments and Local Variables Management


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.

Helper routines

In order to relieve drivers of the task of implementing the same code for enabling/disabling Tx GPIOs based on the GPIO polarity, helper routines are provided which implement this common code and can be used directly in the driver routines:

  1. Enable Tx GPIO int acpigen_enable_tx_gpio(struct acpi_gpio gpio)
  2. Disable Tx GPIO int acpigen_disable_tx_gpio(struct acpi_gpio gpio)

Both the above functions take as input struct acpi_gpio type and return -1 on error and 0 on success. These helper routines end up calling the platform specific acpigen_soc_{set,clear}_tx_gpio functions internally. Thus, all the ACPI AML calling conventions for the platform functions apply to these helper functions as well.

  1. Get Rx GPIO int acpigen_get_rx_gpio(struct acpi_gpio gpio)

This function takes as input, an struct acpi_gpio type and outputs AML code to read the logical value of a gpio (after taking its polarity into consideration), into the Local0 variable. It calls the platform specific acpigen_soc_read_rx_gpio() to actually read the raw Rx gpio value.

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.


uint64_t gpio_reg_offset = gpio_get_reg_offset(gpio_num);

/* Store (\_SB.GPC0(gpio_reg_offset, Local5) */

/* Or (Local5, TX_BIT, Local5) */
acpigen_write_or(LOCAL5_OP, TX_BIT, LOCAL5_OP);

/* \_SB.SPC0(gpio_reg_offset, LOCAL5) */

return 0;


uint64_t gpio_reg_offset = gpio_get_reg_offset(gpio_num);

/* Store (\_SB.GPC0(gpio_reg_offset, Local5) */

 * 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_write_store_args(ZERO_OP, LOCAL0_OP);

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.