On Thu, May 21, 2020 at 12:14 PM Russell King - ARM Linux admin
[off-list ref] wrote:

On Thu, May 21, 2020 at 11:06:23AM +0200, Arnd Bergmann wrote:

quoted

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

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