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I'm trying to 8192 bytes over SPI in a single transfer, but for some reason anything over 4096 bytes fails to send. How can I enable larger data transfers?

int ret;

uint8_t txn[] = { 0x00, ... };

struct spi_ioc_transfer tr = {
    .tx_buf = (unsigned long)txn,
    .len = (unsigned int)sizeof(txn),
    .delay_usecs = 0,
    .speed_hz = 10000000,
    .bits_per_word = 8,
};

ret = ioctl(fd, SPI_IOC_MESSAGE(1), &tr);
if (ret < 1)
    pabort("Can't send SPI message");

I'm working with the Raspberry Pi Zero W if it matters.

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1 Answer 1

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As you have noticed the default maximum SPI transfer is 4096 bytes with the standard Linux driver.

To change the default add spidev.bufsiz=65536 to /boot/cmdline.txt and reboot. Where 65536 is the maximum size you want to allow.

Note that /boot/cmdline.txt is a single line. After the above change mine looked like:

dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes spidev.bufsiz=65536 rootwait logo.nologo

As an aside my pigpio SPI driver defaults to a buffer size of 64k.

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  • I've done this but still get a the 4096 limit error.
    – Gilad
    Commented Aug 27, 2017 at 10:10
  • @Gilad I suggest you open a new question detailing exactly what you have done and any error messages you receive. Also try to explain why your set up might be different to the original questioner.
    – joan
    Commented Aug 27, 2017 at 11:20
  • What is this limitation good for? Except for making me lose 4 hours for troubleshooting, because one of the blocks was 4736 bytes long?
    – Pygmalion
    Commented Aug 7, 2023 at 12:47

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