Linux Core SPI User's Guide

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Introduction

  • Serial interface
  • Synchronous
  • Master-slave configuration (driver supports only master mode)
  • Data Exchange - DMA/PIO


SOC Specific Information

SoC Family Driver
AM335x McSPI
AM437x McSPI
DRA7x McSPI
66AK2Gx McSPI
66AK2Lx Davinci
66AK2Hx Davinci
66AK2E Davinci

Features Not Supported

Below contains a list of features not supported by the Linux driver.
Note this isn't meant to be an exhaustive list and only takes into account features the SPI peripheral in the SoC is capable of but is currently not supported in the Linux driver.

SoCs using McSPI driver

SPI slave mode isn't supported

SoCs using Davinci Driver

SPI slave mode isn't supported

Kernel Configuration

The specific peripheral driver to enable depends on the SoC being used.

Enabling McSPI Driver

Device Drivers  --->
   [*] SPI support
      [*] McSPI driver for OMAP

Enabling DaVinci Driver

Device Drivers  --->
   [*] SPI support
      [*] Texas Instruments DaVinci/DA8x/OMAP-L/AM1x SoC SPI controller 

SPI Driver Usecases

There are numerous drivers that can be used to interact with a variety of hardware. From SPI based RTC to SPI based GPIO expander. A list of drivers along with their documentation can be found within the kernel sources. The below section attempts to provide information on SPI based chips that are located on TI's evms.

Flash Storage

Boards with SPI Flash

EVM Part # Flash Size
AM335x ICE EVM W25Q64 8 MB
K2E EVM N25Q128A11ESF40F 16 MB
K2HK EVM N25Q128A11ESF40F 16 MB
K2L EVM N25Q128A11ESF40F 16 MB

Kernel Configuration

Device Drivers  --->
   <*> Memory Technology Device (MTD) support  ---> 
       Self-contained MTD device drivers  ---> 
         <*> Support most SPI Flash chips (AT26DF, M25P, W25X, ...)

Reading/Writing to Flash

Determine SPI NOR Partition MTD Identifier

Within the kernel figuring out the mtd device number that is for a particular SPI NOR partition is simple. A user simply needs to view the list of mtd devices along with its name. Below command will provide this information:

cat /proc/mtd

An example of this output performed on the AM571x IDK EVM can be seen below.

dev:    size   erasesize  name
mtd0: 00040000 00010000 "QSPI.SPL"
mtd1: 00100000 00010000 "QSPI.u-boot"
mtd2: 00080000 00010000 "QSPI.u-boot-spl-os"
mtd3: 00010000 00010000 "QSPI.u-boot-env"
mtd4: 00010000 00010000 "QSPI.u-boot-env.backup1"
mtd5: 00800000 00010000 "QSPI.kernel"
mtd6: 01620000 00010000 "QSPI.file-system"

Note the names of these partitions, their sizes (in hex) and offsets (in hex) are determined within the specific board's device tree file.

Erasing
Erasing a NOR partition can be performed by using the below command:

flash_erase /dev/mtdX 0 0

Where X is the partition number.

Reading/Writing
Use the MTD interface provided for SPI flash on the EVM to validate the SPI driver interface.
The below step copies 8KiB from /dev/mtd2 partition (u-boot env) to /dev/mtd4 partition and reads
the 8KiB image from /dev/mtd4 to a file and checks the md5sum. The md5sum of test.img and test1.img should be same.

cd /tmp
dd if=/dev/mtd2 of=test.img bs=8k count=1
md5sum test.img
flash_eraseall /dev/mtd4
dd if=test.img of=/dev/mtd4 bs=8k count=1
dd if=/dev/mtd4 of=test1.img bs=8k count=1
md5sum test1.img

Linux Userspace Interface

In situations where a premade SPI driver doesn't exist or a user wants a simple means to send and receive SPI messages the spidev driver can be used. Spidev provides a user space accessible means to communicate with the SPI interface. Latest documentation regarding spidev driver can be found here.

Spidev allows users to interact with the spi interface in a variety of programming languages that can communicate with kernel ioctls.

Kernel Configuration

Device Drivers  --->
   [*] SPI support
      <*> User mode SPI device driver support

Device Tree

Below is an example of the device tree settings a user would use to enable the spidev driver. Like most drivers for a peripheral, the spidev driver is listed as a subnode of the main SPI peripheral driver.

&spi1 {
        status = "okay";
        pinctrl-names = "default";
        pinctrl-0 = <&spi1_pins_s0>;
        spidev@1 {
                spi-max-frequency = <24000000>;
                reg = <0>;
                compatible = "rohm,dh2228fv";
        };
};
  • Note that reg property for SPI subnodes are usually used to indicate the chip select to use when communicating with a particular driver.

Test Application

In the kernel sources, ./tools/spi/spidev_test.c is a test application within the kernel that can be cross compiled to show a C application interacting with the SPI peripheral.