Bug in the iMX8MP ECSPI module?
Background
I do have a system where I can swap between iMX8M Mini and iMX8M Plus CPU modules on the same carrier board.
I did write a a SPI driver for a device on the carrier board. The device is connected to the ECSPI1 (the CPU contains several ECSPI modules) and use the hardware chipselect 0 (SS0). The driver has been used with the iMX8MM CPU module for a while, but as soon I swapped to the iMX8MP it certainly stopped working.
Both iMX8MM and iMX8MP have the same ECSPI IP block that is managed by the spi-imx [1] Linux kernel driver, the application and root filesystem is the same as well.
Same driver, same application, different module. What is happening?
The driver layer also did not report anything suspicious, all SPI transactions contained the data I expected and was successfully sent out on the bus. After debugging the application, driver and devicetree for a while, I took a closer look on the actual SPI signals.
SPI signals
I'm not going to describe the SPI interface specifications, please see Wikipedia [2] or such for more details.
It turns out that the chip select goes inactive after each sent byte, which is a weird behavior. The chipselect should stay low during the whole data transaction.
Here is the signals of one transaction of two bytes:
The ECSPI modules supports dynamic burst size, so I was experimenting with that without any success.
Workaround
The best workaround I came up with was to MUX the chipselect pin to the GPIO function instead of SS0 and map that GPIO as chipselect to ECSPI1 by override the affected properties in the device tree file:
&ecspi1 {
cs-gpios =
<&gpio5 9 GPIO_ACTIVE_LOW>,
<&gpio2 8 GPIO_ACTIVE_LOW>;
};
&pinctrl_ecspi1_cs0 {
fsl,pins = <
MX8MP_IOMUXC_ECSPI1_SS0__GPIO5_IO09 0x40000
>;
};
Then the signals looks better:
Conclusion
I do not know if all ECSPI modules with all HW chipselects is affected or only SS0 @ ECSPI1. I could not find anything about it in the iMX8MP Errata.
The fact that the workaround did work makes me suspect a hardware bug in the iMX8MP processor. I guess we will see if it shows up in the errata later on.