spi_mcux_ecspi does not handle gpio chipselect correctly

[voltamund]

Describe the bug

Target platform: MIMX8MM6 Cortex M4
If I use more than 4 gpios in the spi node's cs-gpios the driver it will not handle gpio with index > 3.

zephyr/drivers/spi/spi_mcux_ecspi.c

Line 166 in 6dce233

if (spi_cfg->slave > kECSPI_Channel3) {

will fail. If the spi node uses cs-gpios it should just use kECSPI_Channel0. I am not even sure that using something like kECSPI_Channel3 will work for the built-in cs.

Expected behavior
SPI transfer with cs-gpios index > 3 succeeds

Logs and console output
Slave 4 is greater than 3

Environment (please complete the following information):

• OS: Linux
• Toolchain: zephyr-sdk-0.16.8
• Zephyr Version 3.7.0

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[pdgendt]

You are correct, moreover this the case in many driver implementations. An index is needed and used to select the corresponding "hard-wired" chip-select OR the corresponding GPIO.
Attempting to support both cases in the same driver has led to this limitation, sadly.