SPI transfer fails with buffer size greater than 4096

Question

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.

Answer 1

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.