Re: [PATCH v2] watchdog: sp5100_tco: add reversible legacy MMIO relocation

[Guenter Roeck]

On 8/20/26 16:14, Christoph Berliner wrote:
> Older SP5100/SB7x0 systems may provide a watchdog MMIO base that
> overlaps another reserved resource. On the tested system firmware
> programs the watchdog at 0xfec000f0, which falls inside the IOAPIC
> resource and causes sp5100_tco to fail probing with -EBUSY.
>
> A relocation mechanism for this class of conflict was introduced by
> commit 740fbddf5c3f ("watchdog: sp5100_tco: Add SB8x0 chipset support")
> for Linux 3.8.
>
> It was deliberately removed shortly afterwards by commit 18e4321276fc
> ("watchdog: sp5100_tco: Remove code that may cause a boot failure")
> after an SB700 machine failed to load BIOS after running a kernel
> containing the relocation path until power was completely removed.
>
> The exact cause of that failure is not documented. One relevant
> difference is that the old relocation path reprogrammed the watchdog
> base but did not restore the firmware-programmed PM register state when
> the driver was removed.
>
> Add a reversible relocation path for the legacy SP5100 register layout.
>
> When the firmware-provided watchdog address cannot be reserved, search a
> bounded 4 KiB range above it for a free, naturally aligned 8-byte MMIO
> window. Use devm_request_mem_region() for each candidate so resource-tree
> traversal, conflict checking and IORESOURCE_BUSY registration are handled
> by the resource core under its lock.
>
> Before reprogramming the watchdog, save the original PM control and base
> registers as well as the PCI watchdog decode register. Disable PCI MMIO
> decode and the watchdog timer before changing the byte-wise base
> registers, avoiding transient decoded addresses while BASE0..3 are being
> updated.
>
> Restore the original firmware PM and PCI state on probe failure or device
> removal and release the relocated MMIO resource through devres.
>
> On the tested system the complete load/unload lifecycle is:
>
>    firmware:  0xfec000f0, PM control 0x03
>    relocated: 0xfec00400, PM control 0x06
>    restored:  0xfec000f0, PM control 0x03
>
> The relocated range appears as an exclusive SP5100 TCO resource in
> /proc/iomem, /dev/watchdog0 registers successfully, and unloading the
> module restores the original firmware state and removes the resource.
>
> A warm reboot with the relocated watchdog loaded was also tested
> successfully; the machine booted normally and the firmware watchdog state
> after reboot was again 0xfec000f0 with PM control 0x03.
>
> Tested on Linux 7.1.8 with an AMD SBx00 SMBus controller
> (PCI 1002:4385, revision 0x3c). The sp5100_tco.c driver in current
> mainline is identical to the tested 7.1.8 version.
>
> Signed-off-by: Christoph Berliner <caberliner@xxxxxxxxx>

Your solution is way too risky. Try to find one that lets the driver use the
region without reserving it, specifically suited for the system affected
by the problem. You should be able to identify that system using DMI.

Thanks,
Guenter