Re: [PATCH v4 0/7] firmware: smccc: Add basic SMCCC v1.2 + ARCH_SOC_ID support

[Arnd Bergmann <arnd@arndb.de>]

On Thu, May 21, 2020 at 12:14 PM Russell King - ARM Linux admin
<linux@armlinux.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.

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.

      Arnd