On Thu, Feb 20, 2020 at 02:11:51PM +0000, Oleksandr Suvorov wrote:
> SPI_IOC_WR_MAX_SPEED_HZ command always sets spi->max_speed_hz
> to the initial value come from DT.

This is intentional.  It's doing a call to spi_setup() then restoring
the original value, the goal being just to run spi_setup() with the new
value - it's not really a good idea to change the maximum speed in the
first place.

> It leads to set a wrong max speed with IOCTL call.

In what way does it lead to the wrong speed being set?

> Fix the logic of a max speed assignment.

If the expectation is that the default speed should be changed for the
device this should be handled at the spidev level rather than in the
core.