SPI

Introduction

SPI is a high-speed, full-duplex, synchronous serial communication
interface for connecting microcontrollers, sensors, storage devices,
etc.

Face-RK3399 SPI leads to a SPI2(reusable GPIO) for external use.

Face-RK3399 development board provides the SPI4 (single chip optional)
interface, and the specific position is as follows:


How SPI works

SPI works in a master-slave mode, which typically has one master
device and one or more slave devices, requiring at least four wires,
respectively:

   CS          #Slice selection signal
   SCLK        #Clock signal
   MOSI        #Master device data output and slave device data input
   MISO        #Master device data input and slave device data output

The Linux kernel uses a combination of "CPOL" and "CPHA" to represent
the four working modes of the current SPI:

   CPOL＝0，CPHA＝0		SPI_MODE_0
   CPOL＝0，CPHA＝1		SPI_MODE_1
   CPOL＝1，CPHA＝0		SPI_MODE_2
   CPOL＝1，CPHA＝1		SPI_MODE_3

CPOL : Represents the state of the initial level of the clock signal,
0 is the low level and 1 is the high level.

CPHA : Is sampling along which clock, 0 is sampling along the first
clock and 1 is sampling along the second clock.

The waveforms of SPI's four working modes are as follows:


Drive coding

The following W25Q128FV Flash module as an example of a simple
introduction to the preparation of SPI driver.

Makefile/Kconfig

Add the corresponding driver file configuration in
"kernel/drivers/spi/Kconfig":

   config SPI_FIREFLY
          tristate "Firefly SPI demo support "
          default y
           help
             Select this option if your Firefly board needs to run SPI demo.

Add the corresponding driver file name in
"kernel/drivers/spi/Makefile":

   obj-$(CONFIG_SPI_FIREFLY)              += spi-firefly-demo.o

Select the added driver file in config, such as:

     │ Symbol: SPI_FIREFLY [=y]
     │ Type  : tristate
     │ Prompt: Firefly SPI demo support
     │   Location:
     │     -> Device Drivers
     │       -> SPI support (SPI [=y])
     │   Defined at drivers/spi/Kconfig:704
     │   Depends on: SPI [=y] && SPI_MASTER [=y]

DTS configuration

Add SPI driver node description in
"kernel/arch/arm64/boot/dts/rockchip/rk3399-firefly-demo.dtsi":

   &spi4 {
       status = "disabled";
       max-freq = <48000000>;
       spidev@00 {
           status = "disabled";
           compatible = "linux,spidev";
           reg = <0x00>;
           spi-max-frequency = <48000000>;
       };
   };

status: Set okay if you want to enable SPI, or disable if not.

compatible: The attribute here must be compatible with the member of
the structure in the driver: of_device_id.

reg: This is consistent with spi-demo@00, set to 0x00 in this example.

spi-max-frequency: Set the highest frequency used by spi. RK3399
supports up to 48000000.

Define SPI drivers

Create a new driver file in     "kernel/drivers/spi/", such as: "spi-
firefly-demo.c".

Before defining the SPI driver, should define the variable
"of_device_id". "*of_device_id" is used to call the device information
defined in the DTS in the driver. The definition is as follows:

   static struct of_device_id firefly_match_table[] = {{ .compatible = "linux,spidev",},{},};

The values of "compatible" here are consistent with those in the DTS.

"spi_driver" is defined as follows:

   static struct spi_driver firefly_spi_driver = {
       .driver = {
           .name = "firefly-spi",
           .owner = THIS_MODULE,
           .of_match_table = firefly_match_table,},
       .probe = firefly_spi_probe,
   };

Registration of SPI equipment

"spi_register_driver(&firefly_spi_driver)" was registered into kernel
by "Static int __init spidev_init(void)".

If the kernel is successfully matched on startup, the SPI core will
configure SPI's parameters (mode, speed, etc.) and call
"firefly_spi_probe".

Read-write SPI date

"Firefly_spi_probe" uses two interface operations to read the ID of
"W25Q128FV":

The firefly_spi_read_w25x_id_0 interface uses spi_transfer and
spi_message to send data.

The firefly_spi_read_w25x_id_1 interface uses the SPI interface
spi_write_then_read to read and write data.

After success, it will print:

   root@rk3399_firefly_face:/ # dmesg | grep firefly-spi
   [    1.006235] firefly-spi spi0.0: Firefly SPI demo program
   [    1.006246] firefly-spi spi0.0: firefly_spi_probe: setup mode 0, 8 bits/w, 48000000 Hz max
   [    1.006298] firefly-spi spi0.0: firefly_spi_read_w25x_id_0: ID = ef 40 18 00 00
   [    1.006361] firefly-spi spi0.0: firefly_spi_read_w25x_id_1: ID = ef 40 18 00 00

Open SPI demo

The Spi of Face-RK3399 is not opened by default. User can open it in
"rk3399-firefly-face.dtsi":

   &spi4 {
       status = "disabled";
       max-freq = <48000000>;
       spidev@00 {
      -    status = "disabled";
      +    status = "okay"
   		compatible = "linux,spidev";
           reg = <0x00>;
           spi-max-frequency = <48000000>;
       };

Common SPI interface

Here are the common SPI API definitions:

   void spi_message_init(struct spi_message *m);
   void spi_message_add_tail(struct spi_transfer *t, struct spi_message *m);
   int spi_sync(struct spi_device *spi, struct spi_message *message) ;
   int spi_write(struct spi_device *spi, const void *buf, size_t len);
   int spi_read(struct spi_device *spi, void *buf, size_t len);
   ssize_t spi_w8r8(struct spi_device *spi, u8 cmd);
   ssize_t spi_w8r16(struct spi_device *spi, u8 cmd);
   ssize_t spi_w8r16be(struct spi_device *spi, u8 cmd);
   int spi_write_then_read(struct spi_device *spi, const void *txbuf, unsigned n_tx, void *rxbuf, unsigned n_rx);

Please refer origin code to get usage: "/kernel/driver/spi/spidev.c".

FAQs

Q1: SPI data transfer exception?

A1 :

 Make sure the IOMUX configuration of SPI 4 pins is correct. Confirm
that when TX sends data, TX pins have normal waveform, CLK frequency
is correct, CS signal is pulled down, and mode matches the device.