215 lines
7.6 KiB
ReStructuredText
215 lines
7.6 KiB
ReStructuredText
==============================
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PXA2xx SPI on SSP driver HOWTO
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==============================
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This a mini HOWTO on the pxa2xx_spi driver. The driver turns a PXA2xx
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synchronous serial port into an SPI master controller
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(see Documentation/spi/spi-summary.rst). The driver has the following features
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- Support for any PXA2xx and compatible SSP.
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- SSP PIO and SSP DMA data transfers.
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- External and Internal (SSPFRM) chip selects.
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- Per slave device (chip) configuration.
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- Full suspend, freeze, resume support.
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The driver is built around a &struct spi_message FIFO serviced by kernel
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thread. The kernel thread, spi_pump_messages(), drives message FIFO and
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is responsible for queuing SPI transactions and setting up and launching
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the DMA or interrupt driven transfers.
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Declaring PXA2xx Master Controllers
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-----------------------------------
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Typically, for a legacy platform, an SPI master is defined in the
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arch/.../mach-*/board-*.c as a "platform device". The master configuration
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is passed to the driver via a table found in include/linux/spi/pxa2xx_spi.h::
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struct pxa2xx_spi_controller {
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u16 num_chipselect;
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u8 enable_dma;
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...
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};
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The "pxa2xx_spi_controller.num_chipselect" field is used to determine the number of
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slave device (chips) attached to this SPI master.
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The "pxa2xx_spi_controller.enable_dma" field informs the driver that SSP DMA should
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be used. This caused the driver to acquire two DMA channels: Rx channel and
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Tx channel. The Rx channel has a higher DMA service priority than the Tx channel.
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See the "PXA2xx Developer Manual" section "DMA Controller".
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For the new platforms the description of the controller and peripheral devices
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comes from Device Tree or ACPI.
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NSSP MASTER SAMPLE
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------------------
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Below is a sample configuration using the PXA255 NSSP for a legacy platform::
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static struct resource pxa_spi_nssp_resources[] = {
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[0] = {
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.start = __PREG(SSCR0_P(2)), /* Start address of NSSP */
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.end = __PREG(SSCR0_P(2)) + 0x2c, /* Range of registers */
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.flags = IORESOURCE_MEM,
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},
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[1] = {
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.start = IRQ_NSSP, /* NSSP IRQ */
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.end = IRQ_NSSP,
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.flags = IORESOURCE_IRQ,
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},
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};
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static struct pxa2xx_spi_controller pxa_nssp_master_info = {
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.num_chipselect = 1, /* Matches the number of chips attached to NSSP */
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.enable_dma = 1, /* Enables NSSP DMA */
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};
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static struct platform_device pxa_spi_nssp = {
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.name = "pxa2xx-spi", /* MUST BE THIS VALUE, so device match driver */
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.id = 2, /* Bus number, MUST MATCH SSP number 1..n */
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.resource = pxa_spi_nssp_resources,
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.num_resources = ARRAY_SIZE(pxa_spi_nssp_resources),
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.dev = {
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.platform_data = &pxa_nssp_master_info, /* Passed to driver */
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},
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};
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static struct platform_device *devices[] __initdata = {
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&pxa_spi_nssp,
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};
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static void __init board_init(void)
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{
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(void)platform_add_device(devices, ARRAY_SIZE(devices));
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}
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Declaring Slave Devices
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-----------------------
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Typically, for a legacy platform, each SPI slave (chip) is defined in the
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arch/.../mach-*/board-*.c using the "spi_board_info" structure found in
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"linux/spi/spi.h". See "Documentation/spi/spi-summary.rst" for additional
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information.
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Each slave device attached to the PXA must provide slave specific configuration
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information via the structure "pxa2xx_spi_chip" found in
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"include/linux/spi/pxa2xx_spi.h". The pxa2xx_spi master controller driver
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will uses the configuration whenever the driver communicates with the slave
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device. All fields are optional.
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::
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struct pxa2xx_spi_chip {
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u8 tx_threshold;
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u8 rx_threshold;
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u8 dma_burst_size;
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u32 timeout;
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};
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The "pxa2xx_spi_chip.tx_threshold" and "pxa2xx_spi_chip.rx_threshold" fields are
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used to configure the SSP hardware FIFO. These fields are critical to the
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performance of pxa2xx_spi driver and misconfiguration will result in rx
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FIFO overruns (especially in PIO mode transfers). Good default values are::
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.tx_threshold = 8,
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.rx_threshold = 8,
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The range is 1 to 16 where zero indicates "use default".
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The "pxa2xx_spi_chip.dma_burst_size" field is used to configure PXA2xx DMA
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engine and is related the "spi_device.bits_per_word" field. Read and understand
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the PXA2xx "Developer Manual" sections on the DMA controller and SSP Controllers
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to determine the correct value. An SSP configured for byte-wide transfers would
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use a value of 8. The driver will determine a reasonable default if
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dma_burst_size == 0.
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The "pxa2xx_spi_chip.timeout" fields is used to efficiently handle
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trailing bytes in the SSP receiver FIFO. The correct value for this field is
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dependent on the SPI bus speed ("spi_board_info.max_speed_hz") and the specific
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slave device. Please note that the PXA2xx SSP 1 does not support trailing byte
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timeouts and must busy-wait any trailing bytes.
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NOTE: the SPI driver cannot control the chip select if SSPFRM is used, so the
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chipselect is dropped after each spi_transfer. Most devices need chip select
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asserted around the complete message. Use SSPFRM as a GPIO (through a descriptor)
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to accommodate these chips.
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NSSP SLAVE SAMPLE
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-----------------
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For a legacy platform or in some other cases, the pxa2xx_spi_chip structure
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is passed to the pxa2xx_spi driver in the "spi_board_info.controller_data"
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field. Below is a sample configuration using the PXA255 NSSP.
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::
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static struct pxa2xx_spi_chip cs8415a_chip_info = {
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.tx_threshold = 8, /* SSP hardware FIFO threshold */
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.rx_threshold = 8, /* SSP hardware FIFO threshold */
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.dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
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.timeout = 235, /* See Intel documentation */
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};
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static struct pxa2xx_spi_chip cs8405a_chip_info = {
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.tx_threshold = 8, /* SSP hardware FIFO threshold */
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.rx_threshold = 8, /* SSP hardware FIFO threshold */
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.dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
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.timeout = 235, /* See Intel documentation */
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};
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static struct spi_board_info streetracer_spi_board_info[] __initdata = {
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{
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.modalias = "cs8415a", /* Name of spi_driver for this device */
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.max_speed_hz = 3686400, /* Run SSP as fast a possible */
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.bus_num = 2, /* Framework bus number */
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.chip_select = 0, /* Framework chip select */
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.platform_data = NULL; /* No spi_driver specific config */
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.controller_data = &cs8415a_chip_info, /* Master chip config */
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.irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
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},
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{
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.modalias = "cs8405a", /* Name of spi_driver for this device */
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.max_speed_hz = 3686400, /* Run SSP as fast a possible */
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.bus_num = 2, /* Framework bus number */
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.chip_select = 1, /* Framework chip select */
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.controller_data = &cs8405a_chip_info, /* Master chip config */
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.irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
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},
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};
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static void __init streetracer_init(void)
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{
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spi_register_board_info(streetracer_spi_board_info,
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ARRAY_SIZE(streetracer_spi_board_info));
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}
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DMA and PIO I/O Support
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-----------------------
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The pxa2xx_spi driver supports both DMA and interrupt driven PIO message
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transfers. The driver defaults to PIO mode and DMA transfers must be enabled
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by setting the "enable_dma" flag in the "pxa2xx_spi_controller" structure.
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For the newer platforms, that are known to support DMA, the driver will enable
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it automatically and try it first with a possible fallback to PIO. The DMA
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mode supports both coherent and stream based DMA mappings.
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The following logic is used to determine the type of I/O to be used on
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a per "spi_transfer" basis::
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if !enable_dma then
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always use PIO transfers
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if spi_message.len > 8191 then
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print "rate limited" warning
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use PIO transfers
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if spi_message.is_dma_mapped and rx_dma_buf != 0 and tx_dma_buf != 0 then
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use coherent DMA mode
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if rx_buf and tx_buf are aligned on 8 byte boundary then
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use streaming DMA mode
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otherwise
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use PIO transfer
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THANKS TO
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---------
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David Brownell and others for mentoring the development of this driver.
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