mcp23s08: invalid spi-present-mask in 6.18.39

[jsnnerve]

Describe the bug

After upgrading from 6.18.34 to 6.18.39, my 2 mcp23s08 gpio expanders are not working anymore.

In dmesg I get this:

[ 6.025048] mcp23s08 spi0.0: invalid spi-present-mask

/proc/device-tree/soc/spi@7e204000/mcp23s08@0 $ xxd < microchip,spi-present-mask
00000000: 0000 0003

The mcp23s08 part of my dts file, looks like this:

        mcp23s08@0 {
            compatible = "microchip,mcp23s08";
            microchip,spi-present-mask = <3>;
            gpio-controller;
            #gpio-cells = <2>;
            interrupt-parent = <&gpio>;
            interrupts = <23 2>;
            interrupt-controller;
            #interrupt-cells = <2>;
            reg = <0>;
            spi-max-frequency = <10000000>;
        };

If I change spi-present-mask, to this:
microchip,spi-present-mask = <0x03000000>;

/proc/device-tree/soc/spi@7e204000/mcp23s08@0 $ xxd < microchip,spi-present-mask
00000000: 0300 0000

It works again.

The problem seems to be this commit:
3fdcca838f971

Steps to reproduce the behaviour

Upgrade from 6.18.34 to 6.18.39

Device (s)

Raspberry Pi 4 Mod. B

System

$ cat /etc/rpi-issue
Raspberry Pi reference 2025-12-04
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 4997bf4e4e49bc3305eb182a4a08bd023529da04, stage2

$ vcgencmd version
May 21 2026 11:20:25
Copyright (c) 2012 Broadcom
version 288930ab4712b99596f32732664aaaeb881ef1e0 (clean) (release) (start)

$ uname -a
Linux prod-test 6.18.39+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.18.39-1+rpt1 (2026-07-29) aarch64 GNU/Linux

Logs

[ 6.025048] mcp23s08 spi0.0: invalid spi-present-mask

Additional context

https://lore.kernel.org/all/CAD++jLkmOHF8sPa2OMW9QF3+MHaFWtmpsZje0UU2q0EaN4OExA@mail.gmail.com/

[6by9]

Your device tree file is incorrect according to the bindings.

https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/pinctrl/microchip%2Cmcp23s08.yaml#L159

microchip,spi-present-mask = /bits/ 8 <0x01>;

I will accept that this backport is pretty horrid, and it would have made more sense to me for the binding to be amended to declare it as 32bit instead of changing the driver behaviour. However the DT maintainers take the opposite view that the binding is generally considered gospel (in theory other kernel implementations use them), so the driver gets updated to match.

I am slightly amazed that there appear to be no users of either mcp23s08 or mcp23s17 within the mainline kernel, which may be why they considered changing the driver behaviour reasonable.

[popcornmix]

Can you wait about 40 minutes for CI to complete, then run sudo rpi-update pulls/7562 and test if that fix works.
Read the rpi-update caveats first.

[jsnnerve]

I get the same error as before.

$ uname -a
Linux prod-test 6.18.44-v8+ #1 SMP PREEMPT Tue Aug 18 11:57:11 UTC 2026 aarch64 GNU/Linux

[ 6.041914] mcp23s08 spi0.0: invalid spi-present-mask

/proc/device-tree/soc/spi@7e204000/mcp23s08@0 $ xxd < microchip,spi-present-mask
00000000: 0000 0003

Am I missing something? Do I need to rebuild the dtb file?

This is what I did:

$ sudo rpi-update pulls/7562
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
FW_REV:
BOOTLOADER_REV:842800056e3eec923e33f50897d6e6a464c398a5
*** We're running for the first time
*** Backing up files (this will take a few minutes)
*** Remove old firmware backup
*** Backing up firmware
*** Remove old modules backup
*** Backing up modules 6.18.39+rpt-rpi-v8
WANT_32BIT:0 WANT_64BIT:1 WANT_64BIT_RT:0 WANT_PI4:1 WANT_PI5:1

Updating a system with initramfs configured is not supported by rpi-update.
If your system relies on drivers provided by the initramfs (e.g. custom filesystem options)
it may not boot without regenerating the initramfs.
If you are unsure, test if your system boots with initramfs options disabled from config.txt

Would you like to proceed? (y/N)
Downloading bootloader tools
Downloading bootloader images
*** Using builds: bcm2711 bcm2712
*** Downloading specific artifact revision (this will take a few minutes)
curl -L https://builds.raspberrypi.com/github/linux/21df9974548e9c7b88c74938fb47799a61d78a69/bcm2711 | zcat | tar xf - -C //root/.rpi-firmware --strip-components=2
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- 0:00:01 --:--:-- 0
100 31.6M 100 31.6M 0 0 815k 0 0:00:39 0:00:39 --:--:-- 825k
curl -L https://builds.raspberrypi.com/github/linux/21df9974548e9c7b88c74938fb47799a61d78a69/bcm2712 | zcat | tar xf - -C //root/.rpi-firmware --strip-components=2
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
100 31.6M 100 31.6M 0 0 773k 0 0:00:41 0:00:41 --:--:-- 803k
BOOTLOADER: up to date
CURRENT: Tue 4 Aug 12:38:13 UTC 2026 (1785847093)
LATEST: Tue 4 Aug 12:38:13 UTC 2026 (1785847093)
RELEASE: latest (/usr/lib/firmware/raspberrypi/bootloader-2711/latest)
Use raspi-config to change the release.

VL805_FW: Using bootloader EEPROM
VL805: up to date
CURRENT: 000138c0
LATEST: 000138c0
*** Updating kernel modules
*** depmod 6.18.44-v8-16k+
*** depmod 6.18.44-v8+
*** As requested, not updating VideoCore libraries
*** As requested, not updating SDK
*** Running ldconfig
*** Storing current firmware revision
*** Deleting downloaded files
*** Syncing changes to disk
*** If no errors appeared, your firmware was successfully updated to 21df997
*** A reboot is needed to activate the new firmware
$ sudo reboot

[6by9]

Are you using our mcp23s17 overlay or your own? That PR fixes up our overlay to now correctly follow the binding doc.

If you're using your own overlay, then it needs fixing.

[jsnnerve]

I am using my own overlay.

Do I understand this correctly, the fix is for me to change my overlay from:
microchip,spi-present-mask = <3>;
to :
microchip,spi-present-mask = /bits/ 8 <3>;

and then it will work with 6.18.39?

[6by9]

I am using my own overlay.

Do I understand this correctly, the fix is for me to change my overlay from: microchip,spi-present-mask = <3>; to : microchip,spi-present-mask = /bits/ 8 <3>;

and then it will work with 6.18.39?

Correct.

[jsnnerve]

Hmm .. This will break backwards compatibility.

We just need to be careful going forward and make sure kernel version and overlay match.

Thanks @popcornmix for the fast response and help.

[6by9]

This is why I acknowledge that it's a little grim, but DT bindings are considered ABI. The bindings say the value is 8 bit, therefore it should have always been 8 bit.

The switch from text files to YAML is all so that usage can be validated against the bindings.

The original text file didn't mention the size of the field, so one could have hoped the author of the conversion would have checked what the driver did, but it appears it wasn't even considered
V1 - https://lore.kernel.org/all/20241022060157.36372-1-himanshu.bhavani@siliconsignals.io/
V2 - https://lore.kernel.org/all/20241024124654.26775-1-himanshu.bhavani@siliconsignals.io/
V3 - https://lore.kernel.org/all/20241028063939.6276-1-himanshu.bhavani@siliconsignals.io/
If there had been any users in mainline then they would probably have screamed as their DT wouldn't validate against the binding, but there aren't.

We now have a fixup to make the existing driver match the bindings. Oddly it claims Fixes: https://github.com/raspberrypi/linux/commit/3ad8d3ec6d87b16cdd336aa065d8e150096a192b ("dt-bindings: pinctrl: convert pinctrl-mcp23s08.txt to yaml format"), but it fixes the driver instead.

However it's been merged and backported now, so that is how it will be with mainline Linux from now on.

We could revert it on the 6.18 branch, but we generally try to stay as close to mainline as possible. That means it wouldn't be reverted on later kernels, so you'd hit the issue of needing to update the overlay come the next major kernel version bump.