porting pmbus driver guide

Overview

We need to implement a PMBus sensor driver so that it can be accessed from userspace applications. The key tasks when porting the driver are to define the probe() function and determine how PMBus commands will be sent to the sensor.

Read Schematic

Device is on i2c8 (counting starts from 1 in schematic)

Scan Devices

Make sure the device address matches the schematic.
If not, determine whether the issue lies in the schematic or identify the incorrect device.

Write the Device Tree

Path:
<linux>/arch/arm/boot/dts/aspeed-<project>-ast2600-<board>-swb-i2c.dtsi

Device Tree Structure

Find the correct location to insert your device node:

- aspeed-<project>-<board>-evt.dts
    - aspeed-<project>-ast2600-<board>.dtsi
        - aspeed-<project>-ast2600-<board>-swb.dtsi
            - aspeed-<project>-ast2600-<board>-swb-i2c.dtsi

&i2c7 {
	status = "okay";
	...
	mpc42013@63 {
		compatible = "mps,mpc42013";
		reg = <0x63>;
	};

Kconfig

/home/alan/<linux>/drivers/hwmon/pmbus/Kconfig Define a config option so the kernel can recognize and enable the driver through menuconfig.

config SENSORS_MPC42013
	tristate "MPS MPC42013"
	help
	  If you say yes here you get hardware monitoring support for MPS
	  MPC42013.

	  This driver can also be built as a module. If so, the module will
	  be called mpc42013.

Default configuration file

y: built into kernel
m: compiled as a module Path: <linux>/arch/arm/configs/aspeed_<project>_defconfig

Makefile

Path: <linux>/drivers/hwmon/pmbus/Makefile Tell the compiler to build this driver if .config has CONFIG_SENSORS_MPC42013=y or =m.

obj-$(CONFIG_SENSORS_MPC42013)	+= mpc42013.o

driver code

path: <linux>/drivers/hwmon/pmbus/mpc42013.c

Code Structure

pmbus driver template

device table

MODULE_DEVICE_TABLE(i2c, mpc42013_id);

driver structure

static struct i2c_driver mpc42013_driver = {
    .driver = {
        .name = "mpc42013",
        .of_match_table = of_match_ptr(mpc42013_of_match),
    },
    .probe_new = mpc42013_probe,
    .remove = pmbus_do_remove,
    .id_table = mpc42013_id,
};
module_i2c_driver(mpc42013_driver);

Read datasheet

`.probe`

Flow
1. finish pmbus_driver_info struct
2. pmbus_do_probe(client, info)

driver info structure `pmbus_driver_info`

.pages: Specifies the number of pages supported by the device.
.format[PSC_VOLTAGE_IN] = direct: Defines the PMBus data format for input voltage; the format types are defined in pmbus.h.
- pmbus data format
  - linear (m=1)
  - direct
    - Fill in the $m$ , $b$ , and $R$ values according to the following formula: $X = \frac{1}{m} \cdot \left( Y \cdot 10^{-R} - b \right)$
    - code:
      info = devm_kmemdup(dev, &mpc42013_info, sizeof(*info), GFP_KERNEL); ... info->m[PSC_VOLTAGE_IN] = 8; info->b[PSC_VOLTAGE_IN] = 0; info->R[PSC_VOLTAGE_IN] = 0;
.func[0]: Enables supported PMBus commands using the functionality bitmask, also defined in pmbus.h.

`.read_word_data`

.read_word_data = mpc42013_read_word_data: Assigns the driver's word data read handler. This function is responsible for reading sensor values.
Inside the function: int raw = pmbus_read_word_data(client, page, phase, reg); This API sends a PMBus command to the device to read a 16-bit word from the specified register.

PMBus register constants, such as PMBUS_READ_VIN, are defined in pmbus.h.

Driver Doumentation

<linux>/Documentation/devicetree/bindings/hwmon/pmbus/mps,mpc42013,yaml

# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
%YAML 1.2
---

$id: http://devicetree.org/schemas/hwmon/pmbus/mps,mpc42013,yaml
$schema: http://devicetree.org/meta-schemas/core.yaml#

title: MPS MPC 42013

maintainers:
  -Alan Tseng <alan_tseng@example.com>

description: |
  MPC 42013 pmbus driver
 
properties:
  compatible:
    enum:
      - mps, mpc42013

  reg:
    maxItems: 1

Verify

Verify that the input and output values satisfy the following equation and fall within the value ranges specified in the schematic. $P=I*V$

Useful cmd

i2cget -fy 7 0x63 0x8c w

Find driver in which hwmon:

for d in /sys/class/hwmon/hwmon*; do
  echo -n "$d → "
  cat "$d/name"
done

for f in /sys/class/hwmon/hwmon19/*_label; do
  printf "%-10s : %s\n" "${f##*/%_label}" "$(cat "$f")"
done

Make Flash

Path: buildroot/

export IMAGE="<project>-fw-<board>-1.1.x.x.img"

dd if=./output/build/uboot-custom/u-boot.bin of=$IMAGE conv=notrunc bs=1 seek=65536

dd if=./output/build/linux-custom/arch/arm/boot/uImage of=$IMAGE conv=notrunc bs=1 seek=524288

dd if=./output/build/linux-custom/arch/arm/boot/dts/aspeed-<project>-<board>-evt.dtb of=$IMAGE conv=notrunc bs=1 seek=12689408

Upload

scp $IMAGE root@$BMC_IP:/var/
ssh root@$BMC_IP

dd if=/var/<project>-fw-<board>-1.1.x.x.img of=/dev/mtdblock0 bs=64k seek=0
reboot

Table

name	function bit mask	reg	sys
VIN	PMBUS_HAVE_VIN	PMBUS_READ_VIN	in1
VOUT	PMBUS_HAVE_VOUT	PMBUS_READ_VOUT	in2
POUT	PMBUS_HAVE_POUT	PMBUS_READ_POUT	power2
IOUT	PMBUS_HAVE_IOUT	PMBUS_READ_IOUT	curr2

Overview​

Read Schematic​

Scan Devices​

Write the Device Tree​

Device Tree Structure​

Kconfig​

Default configuration file​

Makefile​

driver code​

Code Structure​

device table​

driver structure​

Read datasheet​

.probe​

driver info structure pmbus_driver_info​

.read_word_data​

Driver Doumentation​

Verify​

Useful cmd​

Make Flash​

Upload​

Table​

Materials​