Re: [RFC PATCH 0/2] Add a test to verify device probing on ACPI platforms

[Laura Nao]

On 11/23/23 16:14, Dan Carpenter wrote:
> On Thu, Nov 23, 2023 at 01:09:42PM +0100, Laura Nao wrote:
>>> Your talk was interesting at Linux Plumbers.
>>>
>>> https://www.youtube.com/watch?v=oE73eVSyFXQ [time +2:35]
>>>
>>> This is probably a stupid question, but why not just add something to
>>> call_driver_probe() which creates a sysfs directory tree with all the
>>> driver information?
>>>
>>
>> Thanks for the feedback!
>>
>> Improving the device driver model to publish driver and devices info
>> was indeed another option we considered. We could have a debugfs entry
>> storing this kind of information, similar to what devices_deferred
>> does and in a standardized format. This would provide an interface
>> that is easier to query at runtime for getting a list of devices that
>> were probed correctly.
>> This would cover devices with a driver that's built into the kernel or
>> as a module; in view of catching also those cases where a device is
>> not probed because the relevant config is not enabled, I think we'd
>> still need another way of building a list of devices present on the
>> platform to be used as reference.
> 
> Yeah.  So we'd still need patch #1 as-is and but patch #2 would probably
> be simpler if we had this information in sysfs.  Or a different solution
> would be to do what someone said in the LPC talk and just save the
> output of the previous boot and complain if there was a regression where
> something didn't probe.
> 

Right. The main drawback of using the status of a known good boot as
reference is to keep it up to date over time. If support for a
peripheral gets added at a later stage, the reference needs to be updated
as well.

>>
>> The solution proposed in this RFC follows the same approach used for
>> dt based platforms for simplicity. But if adding a new sysfs entry
>> storing devices and driver info proves to be a viable option for
>> upstream, we can surely explore it and improve the probe test to
>> leverage that.
> 
> You're saying "simplicity" but I think you mean easiest from a political
> point of view.  It's not the most simple format at all.  It's like
> massive detective work to find the information and then you'll have to
> redo it for DT and for USB.  Are there other kinds of devices which can
> be probed?
> 

Yeah, that's what I meant. The ACPI use case is in a way simpler to
handle than the dt one, as we can get information on non removable
devices on enumerable buses such as PCI from the ACPI
tables (leveraging the _ADR objects). But it still requires quite a lot
digging in sysfs to get info on what was actually probed.
So having a list of probed devices would help both use cases.

> I feel like you're not valuing your stuff at the right level.  This
> shouldn't be in debugfs.  It should be a first class citizen in sysfs.
> 
> The exact format for this information is slightly tricky and people will
> probably debate that.  But I think most people will agree that it's
> super useful.
>

Right, agreeing on a format will be tricky. Judging by the response here
and in LPC it's still worth a shot though. I'll put some thought into
this and experiment a bit to come up with a proposal to submit in
another RFC.

Again, thanks for the helpful feedback!

Best,
Laura