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SubjectRe: [PATCH v2] printk: Fix preferred console selection with multiple matches
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On Thu, 2019-12-19 at 14:50 +0100, Petr Mladek wrote:
>
> > NOTE: This may look convoluted because I'm trying to keep the existing
> > behaviour identical when it comes to things like Braille selection,
> > setup failures, on Braille consoles, or setup failures on normal consoles
> > which all have subtly different results in the current code.
> >
> > Some of those behaviour are a bit dubious and we might be able to simply
> > rely on CON_ENABLED and CON_BRL flags in newcon after the search but I
> > don't want to change those corner cases in this patch.
>
> Yes, it is dubious. IMHO, the 5 error codes make it even harder to
> see what happens in which case.

Agreed.

> The code really need simplification. I would prefer to take the risk
> and reduce the amount of added conditions as much as possible.
> I have an idea, see below.

I wanted you to say that :-) I'll rework along those lines. Just a nit
or two:
>
>
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2542,6 +2545,53 @@ static int __init keep_bootcon_setup(char *str)
> >
> > early_param("keep_bootcon", keep_bootcon_setup);
> >
> > +enum con_match {
> > + con_matched,
> > + con_matched_preferred,
> > + con_braille,
> > + con_failed,
> > + con_no_match,
> > +};
>
> Please, replace this with int, where:
>
> + con_matched -> 0
> + con_matched_preferred -> 0 and make "has_preferred" global variable
> + con_braile -> 0 later check for CON_BRL flag
> + con_failed -> -EFAULT
> + con_no_match -> -ENOENT

Not fan of using -EFAULT here, it's a detail since it's rather kernel
internal, but I'd rather use -ENXIO for no match and -EIO for failed
(or pass the original error code up if any). That said it's really bike
shed painting at this point :-)
>
> > @@ -2615,41 +2664,19 @@ void register_console(struct console *newcon)
> > + /* See if this console matches one we selected on the command line */
> > + match = try_match_new_console(newcon, true);
> > + /* If it didn't, try matching the platform ones */
> > + if (match == con_no_match)
> > + match = try_match_new_console(newcon, false);
> > + /* If we matched a Braille console, bail out */
> > + if (match == con_braille)
> > + return;
> > + /* Check if we found a preferred one */
> > + if (match == con_matched_preferred)
> > + has_preferred = true;
> >
> > + /* If we don't have an enabled console, bail out */
> > if (!(newcon->flags & CON_ENABLED))
> > return;
>
> Some of the comments describe what is obvious. I would simplify
> it the following way:
>
> /* Prefer command line over platform specific defaults. */
> err = try_match_new_console(newcon, true);
> if (err = -ENOENT)
> err = try_match_new_console(newcon, false);
>
> /* printk() messages are not printed to Braille consoles. */
> if (err || console->flags | CON_BRL)
> return;

So this changes the existing behaviour in one way that may or may not
matter, I don't know:

If setup() fails, the existing code will not exit. That means that if
the console has CON_ENABLED already set (some do set it statically or
set it from outside this function, I haven't looked into details the
various circumstances this can happen), the existing code will still
insert it. Your patch will make us not insert it.

> Finally, please split the change into two patches:
>
> 1st patch will "just" introduce try_match_new_console(console) and
> use it the following way:
>
> err = try_match_new_console(newcon);
>
> /* printk() messages are not printed to the Braille console. */
> if (err || console->flags | CON_BRL)
> return;
>
> 2nd patch will add the user_specified logic.
>
> This way bisection will distinguish regressions caused
> by the refactoring and by the changed search order.
>
> Best Regards,
> Petr
>
> PS: I have vacation between December 23 and January 1. I believe
> that v3 will be ready for merging. Anyway, I will not push it
> into linux-next before I am back from vacation. I would like
> to stay off the computer and do not want to eventually break
> linux-next for too long.

No worries. This isn't super urgent.

Cheers,
Ben.


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