Re: [PATCH v2 0/2] firmware: arm_scmi: Ensure automatic module loading

[Hans de Goede]

Hi Brian,

On 9-Jul-26 15:28, Brian Masney wrote:
> Hi Hans,
> 
> On Thu, Jul 09, 2026 at 03:22:29PM +0200, Hans de Goede wrote:
>> On 9-Jul-26 12:10, Sudeep Holla wrote:
>>> On Thu, Jun 18, 2026 at 10:31:12PM +0200, Hans de Goede wrote:
>>>> On 18-Jun-26 17:56, Bjorn Andersson wrote:
>>>>> SCMI drivers such as the Arm SCMI CPUfreq driver are allowed to built as
>>>>> modules, but they are then not automatically loaded. Rework the SCMI
>>>>> device table alias support to make modpost consume the information from
>>>>> MODULE_DEVICE_TABLE(scmi, ...) and allow drivers to be loaded based on
>>>>> this information, if known. Also add a protocol-based alias to also
>>>>> trigger driver loading when only the SCMI protocol id is known.
>>>>>
>>>>> Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxxxxxxxx>
>>>>
>>>> So I just gave this a test spin and unfortunately it does not work.
>>>>
>>>> The problem with Fedora's kernel-config / setup is that the
>>>> request_module() from patch 2/2 runs from the initramfs, but
>>>> the scmi_cpufreq module is only available in the rootfs.
>>>>
>>>> It does work if I explictly add the scmi_cpufreq module to
>>>> the initramfs, then it does get autoloaded.
>>>>
>>>> We really need some place to put a uevent sysfs attr which then
>>>> gets replayed when udev is restarted from the rootfs and then
>>>> re-reads all the uevent files as part of its coldplug
>>>> enumeration.
>>>>
>>>
>>> I don't have much knowledge on uevent to provide any suggestions/help.
>>> But isn't this a generic requirement ? I mean you could have modules
>>> install on the rootfs and not all of them are packed in initramfs ?
>>> Just wondering if that works for other modules, we can examine how
>>> do they work and what are we missing ?
>>
>> scmi is special because the actual devices under /sys/bus/scmi/devices
>> only get created when the module with the driver is loaded because
>> of some funtion/id mapping requiring info from the driver.
>>
>> Patch 2/2 tries to work around this by loading all scmi drivers matching
>> the scmi protocol which is known at bus enumeration time, but this only
>> works if the actual scmi driver is in the initramfs because this done
>> through directly calling modprobe() from the kernel which does not
>> get "replayed" when switching to the real rootfs.
> 
> Should the SCMI drivers be added to the dracut module here?
> 
> https://github.com/dracut-ng/dracut/blob/main/modules.d/70kernel-modules/module-setup.sh#L73
> 
> A few years ago we had to add the interconnect drivers to the list for
> Fedora.

That would be one solution. I first want to understand the problem better
though. The scmi bus not creating the devices until the kmod with the driver
has loaded is weird. I wonder if we can just move a small part of the drivers
(some mapping table) into the bus code and then just have this work as it
does on regular busses. I hope to be able to make some time to look into
this soonish.

Regards,

Hans