linux syscon and regmap study note

-v0.1 2015.9.19 Sherlock draft

1. What is regmap and syscon

regmap was introduced by https://lwn.net/Articles/451789/
From my understanding, it provided us a set API to read/write non memory-map I/O
(e.g. I2C and SPI read/write) at first. Then after introduced regmap-mmio.c,
we can use regmap to access memory-map I/O.

code path: drivers/base/regmap/*

syscon was introduced by https://lkml.org/lkml/2012/9/4/568
It provides a set API to access a misc device(e.g. pci-sas subsystem registers
in P660) based on regmap, explicitly based on regmap-mmio.c I guess.

code path: drivers/mfd/*

2. arch of regmap and syscon

basic structure of regmap:

struct regmap: per base address per regmap
struct regmap_bus: include read/write callback, different “bus”
(e.g. I2C, SPI, mmio) have different regmap_bus
struct regmap_mmio_context: don’t know…
struct regmap_config: confiuration info.

regmap-mmio call flow:

/* drivers/base/regmap/regmap-mmio.c */__devm_regmap_init_mmio_clk    --> __devm_regmap_init        /* regmap_bus(regmap_mmio), config as input, create regmap */        --> __regmap_init        /* if don't have bus->read or bus->write */        --> map->reg_read = _regmap_bus_reg_read;        --> map->reg_write = _regmap_bus_reg_write;        ...        /* if have bus->read */            --> map->reg_read  = _regmap_bus_read;            --> map->bus->read/* drivers/base/regmap/regmap.c */regmap_read(struct regmap *map, unsigned int reg, unsigned int *val)    --> _regmap_read        /* _regmap_bus_reg_read */        --> map->reg_read(context, reg, val);        --> map->bus->reg_read(map->bus_context, reg, val)/* drivers/base/regmap/regmap.c */regmap_update_bits    --> _regmap_update_bits        --> _regmap_read    --> _regmap_write        --> map->reg_write

basic structure of syscon:

struct syscon:                 include a strutct regmap; an element in list belowstatic LIST_HEAD(syscon_list)syscon driver init a regmap:syscon->regmap = devm_regmap_init_mmio(dev, base, &syscon_regmap_config);

3. why we need a syscon to describe a misc device

To understand this, we shoudl search related discussion in community:
https://lists.ozlabs.org/pipermail/devicetree-discuss/2012-August/018704.html

From my understanding, syscon firstly registers a syscon dts node to syscon_list,
we could find this node when we try to access related registers.

4. how to use syscon to access a misc device

e.g.

need dts node:    pcie_sas: pcie_sas@0xb0000000 {        compatible = "hisilicon,pcie-sas-subctrl", "syscon";        reg = <0xb0000000 0x10000>;    };

use below function read/write:

    regmap_read(hisi_pcie->subctrl, PCIE_SUBCTRL_SYS_STATE4_REG +            0x100 * hisi_pcie->port_id, &val);    regmap_update_bits(pcie->subctrl, reg, bit_mask, mode << bit_shift);

use below function create struct regmap:

    hisi_pcie->subctrl =        syscon_regmap_lookup_by_compatible("hisilicon,pcie-sas-subctrl");

reference:

1. Documentation/devicetree/bindings/regmap/regmap.txt
2. ../mfd/mfd.txt
3. ./syscon.txt