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

[Hans de Goede <johannes.goede@oss.qualcomm.com>]

Hi,

On 9-Jul-26 16:21, Sudeep Holla wrote:
> On Thu, Jul 09, 2026 at 04:07:17PM +0200, Hans de Goede wrote:
>> 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@oss.qualcomm.com>
>>>>>>
>>>>>> 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 need to recall why we moved from static list of devices to dynamic.
> One reason I can think right now is the vendor protocols and their drivers
> But in general it was an attempt to help multiple drivers bind to different
> scmi_devices that have same protocol ID. E.g. the performance protocol
> can be used by cpufreq and devfreq/performance genpd drivers.

Note it is ok to have multiple drivers bind to the same modalias,
depending on the reason why there are multiple drivers either one
should detect that it is not compatible and exit probe() with -ENODEV
or there should be some other mechanism to make sure only one driver
loads.

E.g. duplicate USB device-ids happen (they shouldn't but they do) and
then the drivers typically figure out if they are talking to the device
which they were written for, or the other device with the same USB-ids
and then one of the 2 drivers exits with -ENODEV.

>> 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.
>>
> 
> I started with that few years ago and we then moved to this dynamic
> device creation. But I agree if it is deviation from the norms(which I
> wasn't aware of at the time), we can remove it.

Looking at the issue this is causing for automatic module loading if we
can get back to the bus enumeration code always creating a device without
waiting for the driver kmod to load then that would be good IMHO.

Regards,

Hans