SPI

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 letter
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 XM25QU128C Flash module as an example of a simple introduction to the preparation of SPI driver.

Hardware connection

The hardware connection between AIO-3588L and XM25QU128C is shown in the following table:

Makefile/Kconfig

Add the corresponding driver file configuration in kernel-5.10/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-5.10/drivers/spi/Makefile:

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

Configure the DTS nodes

Add SPI driver node description in kernel-5.10/arch/arm64/boot/dts/rockchip/rk3588-firefly-demo.dtsi, as shown below:

/* Firefly SPI demo */
&spi1{
    spi_demo: spi_demo@00{
        compatible = "firefly,rk3588-spi";
        status = "okay";
        reg = <0x00>;
        spi-max-frequency = <50000000>;
        //spi-cpha;   /* SPI mode: CPHA=1 */
        //spi-cpol; 	/* SPI mode: CPOL=1 */
        //spi-cs-high;
    };
};

&spidev1 {
    status = "disabled";
};

status : set okay if you want to enable SPI, or disable if not.
spi-demo@00 : since CS0 is used in this example, it is set to 00; if CS1 is used, it is set to 01.
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 here. AIO-3588L supports up to 48000000.
spi-cpha，spi-cpol : the working mode of spi is set here. The working mode of the module spi used in this example is SPI_MODE_0 or SPI_MODE_3. Here we choose SPI_MODE_0. If SPI_MODE_3 is used, open spi-cpha and spi-cpol in spi_demo.

Define SPI drivers

Create a new driver file in kernel-5.10/drivers/spi/, such as: spi-firefly-demo.c.

Before defining the SPI driver, the user first defines the variable of_device_id. Of_device_id is used to call the device information defined in the DTS file in the driver. The definition is as follows:

static struct of_device_id firefly_match_table[] = { {.compatible = "firefly,rk3588-spi",},{},};

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

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

Static int __init firefly_spi_init(void) registers SPI driver with kernel: spi_register_driver(&firefly_spi_driver);

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 data

Firefly_spi_probe USES two interface operations to read the ID of XM25QU128C:
The firefly_spi_read_xm25x_id_0 interface directly USES spi_transfer and spi_message to transmit data.
The firefly_spi_read_xm25x_id_1 interface USES the SPI interface spi_write_then_read to read and write data.

After success, it will print:

console:/ $  dmesg | grep spi
[    1.791786] [    T1] firefly-spi spi1.0: Firefly SPI demo program
[    1.791788] [    T1] firefly spi demo
[    1.791795] [    T1] firefly-spi spi1.0: firefly_spi_probe: setup mode 0, 8 bits/w, 50000000 Hz max 
[    1.791797] [    T1] spi demo mode ; 0     
[    1.791838] [    T1] firefly_spi_read_xm25x_id_0 ID = 20 41 18
[    1.791875] [    T1] firefly_spi_read_xm25x_id_1 ID = 20 41 18

Open SPI demo

spi-firefly-demo is not opened by default. If necessary, the demo driver can be opened with the following patch:

--- a/kernel-5.10/arch/arm64/boot/dts/rockchip/rk3588-firefly-demo.dtsi
+++ b/kernel-5.10/arch/arm64/boot/dts/rockchip/rk3588-firefly-demo.dtsi
@@ -64,7 +64,7 @@ /* Firefly SPI demo */
 &spi1 {spi_demo: spi-demo@00{
 -                status = "disabled";
 +                status = "okay";
                  compatible = "firefly,rk3588-spi";
                  reg = <0x00>;
                  spi-max-frequency = <50000000>;

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);

Interface usage

Linux provides a SPI user interface with limited functionality. If IRQ or other kernel driver interfaces are not required, consider using spidev interface to write user-level programs to control SPI devices. The corresponding path in the AIO-3588L development board is /dev/spidev1.0.

spidev corresponding driver code is kernel-5.10/drivers/spi/spidev.c.

The config in the kernel needs to select SPI_SPIDEV:

 │ Symbol: SPI_SPIDEV [=y]
 │ Type  : tristate
 │ Prompt: User mode SPI device driver support
 │   Location:
 │     -> Device Drivers
 │       -> SPI support (SPI [=y])
 │   Defined at drivers/spi/Kconfig:684
 │   Depends on: SPI [=y] && SPI_MASTER [=y]

DTS configuration is as follows:

&spi1{
    status = "okay";
    pinctrl-0 = <&spi1m2_cs0 &spi1m2_pins>;
    max-freq = <50000000>;
    spidev1: spidev@00{
        compatible = "rockchip,spidev";
        status = "okay";
        reg = <0x0>;
        spi-max-frequency = <50000000>;
    };
};

Please refer to kernel-5.10/Documentation/spi/spidev.rst for detailed instructions.

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.

SPI

Introduction