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

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Date:   Mon, 27 Mar 2023 14:38:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
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 Mon, Mar 27, 2023 at 03:20:06PM +0300, Matti Vaittinen wrote:
> 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.

Ok, David, my offer stands, if you want me to write this I will be glad
to do so.

thanks,

greg k-h