linux-kernel - Re: [PATCH v6 3/7] kunit: Add kunit wrappers for (root) device creation

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Message-ID: <e027fc0c-83e0-be6f-d62b-dac00ce9b761@gmail.com>
Date:   Mon, 27 Mar 2023 15:20:06 +0300
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        David Gow <davidgow@...gle.com>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        Stephen Boyd <sboyd@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org
Subject: Re: [PATCH v6 3/7] kunit: Add kunit wrappers for (root) device
 creation

On 3/27/23 15:01, Greg Kroah-Hartman wrote:
> On Mon, Mar 27, 2023 at 02:34:02PM +0300, Matti Vaittinen wrote:
>> A few tests need to have a valid struct device. One such example is
>> tests which want to be testing devm-managed interfaces.
>>
>> Add kunit wrapper for root_device_[un]register(), which create a root
>> device and also add a kunit managed clean-up routine for the device
>> destruction upon test exit.
> 
> I really do not like this as a "root device" is a horrible hack and
> should only be used if you have to hang other devices off of it and you
> don't have a real device to tie those devices to.
> 
> Here you are abusing it and attempting to treat it as a real device,
> which it is not at all, because:
> 
>> Special note: In some cases the device reference-count does not reach
>> zero and devm-unwinding is not done if device is not sitting on a bus.
>> The root_device_[un]register() are dealing with such devices and thus
>> this interface may not be usable by all in its current form. More
>> information can be found from:
>> https://lore.kernel.org/dri-devel/20221117165311.vovrc7usy4efiytl@houat/
> 
> See, not a real device, doesn't follow normal "struct device" rules and
> lifetimes, don't try to use it for a test as it will only cause problems
> and you will be forced to work around that in a test.

Ok. I understood using the root-device has been a work-around in some 
other tests. Thus continuing use it for tests where we don't need the 
bus until we have a proper alternative was suggested by David.

> Do the right thing here, create a fake bus and add devices to it.
> 
> Heck, I'll even write that code if you want it, what's the requirement,
> something like:
> 	struct device *kunit_device_create(struct kunit *test, const char *name);
> 	void kunit_device_destroy(struct device *dev);

Thanks for the offer Greg. This, however, is being already worked on by 
David. I don't want to step on his toes by writing the same thing, nor 
do I think I should be pushing him to rush on his work.

> Why do you want a "match" function?  You don't provide documentation
> here for it so I have no idea.
> 
> Anything else needed?
> 
>> The use of root-devices in the kunit helpers is intended to be an
>> intermediate solution to allow tests which do not require device to sit
>> on a bus avoid directly abusing the root_device_[un]register() while
>> proper kunit device solution is being worked on. Related discussion can be
>> found from:
>> https://lore.kernel.org/lkml/CABVgOSmx3A4Vwos2_8xO-XQrQAw5gvY0nc5zLpLmcJ7FtA-dTQ@mail.gmail.com/
> 
> Again, no, please let's not get this wrong now and say "we will fix this
> later" as that's not how kernel development should work...

Ok. In that case I need to drop the tests from the series until we get 
the new APIs in place. It really sucks but I guess I understand the 
rationale for not wanting to "intermediate" solutions merged. Yes, I 
hoped it'd be Ok as David is already working on it - but I was still 
kind of expecting your response. This is why I made it very clear in the 
cover-letter and this commit message what is suggested here.

Jonathan, should I re-spin the series without patches 3/7 and 5/7 or can 
you please review this and I'll just drop those for the next version?

Thanks for the review Greg, I think this case is now "closed".

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~