Re: [PATCH] usb: ehci-orion: Handle errors of clk_prepare_enable() in probe

[Evgeny Novikov]

On 26.08.2021 18:24, Alan Stern wrote:
> On Thu, Aug 26, 2021 at 04:30:22PM +0300, Evgeny Novikov wrote:
> > Hi Alan,
> >
> > On 25.08.2021 20:29, Alan Stern wrote:
> > > On Wed, Aug 25, 2021 at 08:09:02PM +0300, Evgeny Novikov wrote:
> > > > ehci_orion_drv_probe() did not account for possible errors of
> > > > clk_prepare_enable() that in particular could cause invocation of
> > > > clk_disable_unprepare() on clocks that were not prepared/enabled yet,
> > > > e.g. in remove or on handling errors of usb_add_hcd() in probe. Though,
> > > > there were several patches fixing different issues with clocks in this
> > > > driver, they did not solve this problem.
> > > >
> > > > Add handling of errors of clk_prepare_enable() in ehci_orion_drv_probe()
> > > > to avoid calls of clk_disable_unprepare() without previous successful
> > > > invocation of clk_prepare_enable().
> > > >
> > > > Found by Linux Driver Verification project (linuxtesting.org).
> > > >
> > > > Fixes: 8c869edaee07 ("ARM: Orion: EHCI: Add support for enabling clocks")
> > > > Signed-off-by: Evgeny Novikov <novikov@xxxxxxxxx>
> > > > Co-developed-by: Kirill Shilimanov <kirill.shilimanov@xxxxxxxxxx>
> > > > Signed-off-by: Kirill Shilimanov <kirill.shilimanov@xxxxxxxxxx>
> > > > ---
> > > Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > >
> > > Do you intend to submit patches for the other EHCI drivers that don't
> > > check for errors in clk_prepare_enable()?  It looks like
> > > ehci-atmel.c, ehci-mv.c, and ehci-spear.c all need some attention.
> > >
> > > The same is true for a bunch of the OHCI drivers: ohci-at91.c,
> > > ohci-exynos.c, ohci-s3c2410.c, and ohci-spear.c.
> > >
> > > Didn't the Linux Driver Verification project identify this problem in
> > > all of these drivers?
> > Our verification framework report numerous issues like the one for which I
> > sent the given patch. There are many warnings for different USB drivers and
> > other types of device drivers as well. We sent several patches that were
> > acknowledged by the developers already, though, after the Andrew's reply [1]
> > I have doubts that we need to treat these warnings as potential bugs and fix
> > them. The verification framework performs static analysis in a way that I
> > described before [2]. Regarding the clock API it uses such models of
> > clk_prepare() and clk_enable() that can fail independently on underlying
> > hardware since is not easy to either model all such hardware or try to
> > relate and consider corresponding drivers in addition to drivers using
> > clocks at verification. Thus, potentially the verification framework can
> > produce warnings for all drivers that invoke clk_prepare(), clk_enable() or
> > clk_prepare_enable(), but do not check for their return values.
> >
> > I look forward whether you will confirm that it makes sense to handle errors
> > from mentioned functions anyway or it would be better not to sent such bug
> > reports unless we will be strictly sure that they can happen. In the former
> > case it would be better if somebody will teach built-in Linux kernel static
> > analyzers to detect these issues on a regular basis.
> I don't know whether these errors can occur or not.  To find out, you need to
> ask someone who knows more about the clock framework.
>
> On the other hand, the fact that the functions do return an error code means
> that they expect callers to check its value.  In fact, whoever changed the API
> should have gone through all the callers to make sure they did so.
>
> (Let's put it this way:  If those functions can return an error, we should
> check the return code.  If they can't return an error then they shouldn't be
> defined to return an int, so the API should be changed.)
>
> So on the whole, I think making these changes would be a good thing.  At the
> very least, it will help make all the different EHCI and OHCI drivers
> consistent with each other.
I agree with your reasoning. Even though today these bugs can not happen 
since appropriate hardware and its drivers do not fail in a "necessary" 
way ever, this can suddenly change in the future. Maybe it will not be 
so easy to track the root causes especially taking into account that 
failures happen very seldom.

I am unready to make any considerable changes in the API. Also, as I 
mentioned before, it would be better if a bunch of appropriate fixes 
will be prepared by somebody who involved in development of the Linux 
kernel much more than me and who can automatize the process of finding 
such issues using static analysis tools. Most likely those tools are 
able to find all such places very quickly even for drivers on very 
specific architectures. I added Dan to the discussion since he is a 
developer of one of such tools.

Best regards,
Evgeny Novikov
> Alan Stern
>
> > [1] https://lkml.org/lkml/2021/8/25/794
> > [2] https://lkml.org/lkml/2021/8/17/239
> >
> > Best regards,
> > Evgeny Novikov