linux-kernel - Re: [PATCH v4 0/7] firmware: smccc: Add basic SMCCC v1.2 + ARCH_SOC

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Message-ID: <20200521114607.GP1551@shell.armlinux.org.uk>
Date:   Thu, 21 May 2020 12:46:07 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steven Price <steven.price@....com>, harb@...erecomputing.com,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 0/7] firmware: smccc: Add basic SMCCC v1.2 +
 ARCH_SOC_ID support

On Thu, May 21, 2020 at 12:31:32PM +0200, Arnd Bergmann wrote:
> On Thu, May 21, 2020 at 12:14 PM Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> > On Thu, May 21, 2020 at 11:06:23AM +0200, Arnd Bergmann wrote:
> > > Note that the warning should come up for either W=1 or C=1, and I also
> > > think that
> > > new code should generally be written sparse-clean and have no warnings with
> > > 'make C=1' as a rule.
> >
> > No, absolutely not, that's a stupid idea, there are corner cases
> > where hiding a sparse warning is the wrong thing to do.  Look at
> > many of the cases in fs/ for example.
> >
> > See https://lkml.org/lkml/2004/9/12/249 which should make anyone
> > who sees a use of __force in some random code stop and question
> > why it is there, and whether it is actually correct, or just there
> > to hide a sparse warning.
> >
> > Remember, sparse is there to warn that something isn't quite right,
> > and the view taken is, if it isn't right, then we don't "cast the
> > warning away" with __force, even if we intend not to fix the code
> > immediately.
> >
> > So, going for "sparse-clean" is actually not correct. Going for
> > "no unnecessary warnings" is.
> >
> > And don't think what I've said above doesn't happen; I've rejected
> > patches from people who've gone around trying to fix every sparse
> > warning that they see by throwing __force incorrectly at it.
> >
> > The thing is, if you hide all the warnings, even for incorrect code,
> > then sparse becomes completely useless to identify where things in
> > the code are not quite correct.
> 
> Adding __force is almost always the wrong solution, and I explictly
> was not talking about existing code here where changing it would
> risk introducing bugs or require bad hacks.

I'm using existing code to illustrate the problem with your idea of
"sparse-clean" new code, trying to show you that it is not about
being sparse clean, but about being correct.

> However, when writing a new driver, sparse warnings usually
> indicate that you are doing something wrong that is better addressed
> by doing something different that does not involve adding __force.

Right, but if you lay down a rule that says "new submissions must be
sparse clean" you will get people using __force to shut sparse up.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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