on previous versions of raspbian (stretch is the latest i tried), i changed the kernel to support spidev module with larger chunk sizes (64kB).
this was don by editing the spidev.c file in linux/drivers/spi/spidev.c as follow:
static unsigned bufsiz = 65536;
this used to work well untill rpi4 with Buster.
currently im getting an error stating that the bufsiz is 4096 which doesnt match my changes.
does Buster/RPI changed the SPI module usage / drivers?