Zephyr drivers
Contents
^ OVERVIEW
2022 Q4 notes on Zephyr RTOS drivers, driver frameworks and driver design. Current projects include work to extend KX132-1211 Zephyr out-of-tree driver, plus work to craft an IIS2DH driver which supports in Zephyr RTOS context both I2C and SPI bus connections for this sensor.
^ Example Zephyr drivers
Example apps here offer varying amounts of insight into Zephyr RTOS' framework(s) for device driver development. Of particular interest 2022 Q4 are the ways in which developers use Zephyr device tree macros to obtain pointers to devices, in their mostly C based code, at compile time. Though not fully explained in Zephyr's extensive documentation it appears that device pointers which can be correctly passed to and used with API function device_is_ready() and macro DEVICE_DT_GET() involve pairings of macros, which must be expanded prior to these calls in order to compile correctly.
What's going on in LM77 temperature sensor Kconfig file, on line 10? Interesting Kconfig stanza contains $(dt_compat_enabled,lm77):
^ Zephyr Kernel Level Device Support
Interesting code excerpt from `zephy/kernel/device.c`, this following routine is the implementation behind a wrapper like function `device_is_read()`. But this routine, and sensibly so, doesn't say anything about how a given device is initialized. It only looks at two data members of a device instance data structure:
150
151 bool z_device_is_ready(const struct device *dev)
152 {
153 /*
154 * if an invalid device pointer is passed as argument, this call
155 * reports the `device` as not ready for usage.
156 */
157 if (dev == NULL) {
158 return false;
159 }
160
161 return dev->state->initialized && (dev->state->init_res == 0U);
162 }
163
On a somewhat unrelated note, a couple of important Zephyr documents, including Zephyr device tree macros used to obtain node identifiers:
- https://docs.zephyrproject.org/latest/build/dts/dt-vs-kconfig.html Device Tree versus Kconfig, a fairly short read
^ Understanding Zephyr SPI API
To make use of Zephyr API for SPI bus, we must in part understand how each of the three parameters passed to spi_read() and spi_write() are defined. Further, when we have a sensor with which we communicate in our Zephyr based app we need to understand how parts of this sensor's data structures in the firmware relate to the SPI bus to which the sensor is connected. Our sensor driver's code which talks with the sensor via SPI must pass certain SPI bus related pointers to Zephyr's SPI Application Programmers' Interface.
Another important SPI defining resource in Zephyr RTOS is the header file spi.h#L379. This file defines the small data structure named `spi_dt_spec`. It gives us the data member names for a SPI bus peripheral, as Zephyr sees them.
^ LPC55S69 flexcomm2 and hs_lspi pins conflict
zephyr/boards/arm/lpcxpresso55s69/lpcxpresso55s69-pinctrl.dtsi lines 21 - 28
Note that in spite of the comment above in the dot dtsi file from NXP, SPI communications with an IIS2DH sensor work while UART2 is also in use and working - TMH
^ STMicro device context data structure
Symbol `stmdev_ctx_t` is defined in file modules/hal/st/sensor/stmemsc/iis2dh_STdC/driver/iis2dh_reg.c. On github this file as of 2022-11-18 located at https://github.com/zephyrproject-rtos/hal_st/tree/master/sensor/stmemsc/iis2dh_STdC/driver'
Data structure `stmdev_ctx_t` defined as follows:
104 /** @addtogroup Interfaces_Functions
105 * @brief This section provide a set of functions used to read and
106 * write a generic register of the device.
107 * MANDATORY: return 0 -> no Error.
108 * @{
109 *
110 */
111
112 typedef int32_t (*stmdev_write_ptr)(void *, uint8_t, const uint8_t *, uint16_t);
113 typedef int32_t (*stmdev_read_ptr)(void *, uint8_t, uint8_t *, uint16_t);
114
115 typedef struct
116 {
117 /** Component mandatory fields **/
118 stmdev_write_ptr write_reg;
119 stmdev_read_ptr read_reg;
120 /** Customizable optional pointer **/
121 void *handle;
122 } stmdev_ctx_t;
Parameter lists for the register write and register read functions are nearly the same, with exception of the third parameter. In the register write function this parameter is qualified with C `const`, as it is only expected to be read by the function using it.
With the above definition, an instance of stmdev_cts_t is created for each device with device tree compatible property of 'st_iis2dh', and is assigned in `zephyr/drivers/sensor/iis2dh/iis2dh-i2c.c` as follows:
38 stmdev_ctx_t iis2dh_i2c_ctx = {
39 .read_reg = (stmdev_read_ptr) iis2dh_i2c_read,
40 .write_reg = (stmdev_write_ptr) iis2dh_i2c_write,
41 };
42
43 int iis2dh_i2c_init(const struct device *dev)
44 {
45 struct iis2dh_data *data = dev->data;
46 const struct iis2dh_device_config *config = dev->config;
47
48 if (!device_is_ready(config->i2c.bus)) {
49 LOG_ERR("Bus device is not ready");
50 return -ENODEV;
51 }
52
53 data->ctx = &iis2dh_i2c_ctx;
54 data->ctx->handle = (void *)dev;
55
56 return 0;
57 }
58 #endif /* DT_ANY_INST_ON_BUS_STATUS_OKAY(i2c) */
Pointer *dev has a member named 'data', which in turn has a member named 'ctx', which in turn gets assigned the address of a not named instance of `iis2dh_i2c_ctx`. This means that we have `dev->data->ctx->read_reg()` and `dev->data->ctx->write_reg` on successful completion of routine iis2dh_i2c_init(). This is reflected in modules/hal/st/sensor/stmemsc/iis2dh_STdC/driver/iis2dh_reg.c, in the two generalized register read and write functions:
. . .
https://docs.zephyrproject.org/latest/hardware/peripherals/i2c.html#c.i2c_burst_read_dt
^ C Preprocessor macros
