Raspberry Pi Forums

Posted: Tue Nov 11, 2025 11:17 am

by whatever213

Hello,
for a project i need the Pi 5 to function as a SPI slave.

According to the datasheet (https://datasheets.raspberrypi.com/rp1/ ... herals.pdf) SPI4 should be the only available Instance via the GPIOs.
I wasn't able to activate it via dtoverlay and in the overlay map spi4 seems to be missing for the Pi 5 (https://github.com/raspberrypi/linux/bl ... ay_map.dts). At least i couldn't find one with the bcm2712.

What am i missing here? And if this solution is not possible am i able to control SPI4 via the RPI1 peripherals directly?

Thank you very much!

Posted: Tue Nov 11, 2025 11:45 am

by scotty101

Every post I have seen in the past on this topic has suggested that the Pi can't be used as an SPI slave.

Things may have changed however.

Posted: Tue Nov 11, 2025 1:18 pm

by PhilE

Pi 5/RP1 does have an accessible SPI slave, SPI4 on GPIOs 8-11, but the Linux driver for the DW SPI interfaces in RP1 does not support target/slave mode, hence the lack of overlay support. However, I see that the "next" branch of the kernel has just gained a patch titled "spi: dw: add target mode support". If you were to mention this in an issue on our kernel GitHub repo (https://github.com/raspberrypi/linux/issues), there's a chance it could be backported - it depends on how many supporting patches are required in order for it to be made to work.

Posted: Tue Nov 11, 2025 3:15 pm

by PhilE

I was curious, so I took a look at the backport. Going back to 6.12 would be painful, but 6.18 is much simpler, and likely to be the next LTS kernel. https://github.com/raspberrypi/linux/pull/7132 is a backport to rpi-6.18.y. It lacks overlay support, but it is a first step.

Posted: Tue Nov 11, 2025 5:04 pm

by whatever213

First of all thank you very much! That sounds really good.

Now my next dumb (sorry, i haven't done this before) question:
How do i proceed from here? I guess it will take some time before the next kernel arrives so do i use the rpi-6.18.y version?
Can i just update the SPI kernel modules?

Posted: Tue Nov 11, 2025 5:19 pm

by robertojguerra

I have only seen dedicated chips being SPI slaves: LCD controllers, DACs... things that can guarantee to respond in less than a microsecond, without any other control wires, like ACK, CTS, DTR.

With Linux orchestrating every function, I don't see it possible.

Maybe your Raspberry Pi and its master should be communicating over ethernet.

Posted: Tue Nov 11, 2025 5:21 pm

by PhilE

You can install a 6.18 kernel now, provided you aren't using initramfs in order to boot (which you can check by setting "auto_initramfs=0" in config.txt and confirming that your Pi still boots).

All you need to do is: