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SubjectRe: [PATCH v3 5/5] console: Introduce ->exit() callback
On Thu, Jan 30, 2020 at 02:22:46PM +0100, Petr Mladek wrote:
> On Wed 2020-01-29 16:25:58, Andy Shevchenko wrote:
> > On Wed, Jan 29, 2020 at 10:41:41PM +0900, Sergey Senozhatsky wrote:
> > > On (20/01/28 11:44), Andy Shevchenko wrote:

...

> > > > > If the console was not registered (hence not enabled) is it still required
> > > > > to call ->exit()? Is there a requirement that ->exit() should handle such
> > > > > cases?
> > > >
> > > > This is a good point. The ->exit() purpose is to keep balance for whatever
> > > > happened at ->setup().
> > > >
> > > > But ->setup() is being called either when we have has_preferred == false or
> > > > when we got no matching we call it for all such consoles, till it returns an
> > > > error (can you elaborate the logic behind it?).
> > >
> > > ->match() does alias matching and ->setup(). If alias matching failed,
> > > exact name match takes place. We don't call ->setup() for all consoles,
> > > but only for those that have exact name match:
> > >
> > > if (strcmp(c->name, newcon->name) != 0)
> > > continue;
> > >
> > > As to why we don't stop sooner in that loop - I need to to do some
> > > archaeology. We need to have CON_CONSDEV at proper place, which is
> > > IIRC the last matching console.
> > >
> > > Pretty much every time we tried to change the logic we ended up
> > > reverting the changes.
> >
> > I understand. Seems the ->setup() has to be idempotent. We can tell the same
> > for ->exit() in some comment.
>
> I believe that ->setup() can succeesfully be called only once.
> It is tricky like hell:

Indeed. I think this code is highly starving for comments.

> 1st piece:
>
> if (!has_preferred || bcon || !console_drivers)
> has_preferred = preferred_console >= 0;
>
> note:
>
> + "has_preferred" is updated here only when it was not "true" before.
> + "has_preferred" is set to "true" here only when "preferred_console"
> is set in __add_preferred_console()
>
> 2nd piece:
>
> + __add_preferred_console() is called for console defined on
> the command line. "preferred_console" points to the console
> defined by the last "console=" parameter.
>
> 3rd piece:
>
> + "has_preferred" is set to "true" later in register_console() when
> a console with tty binding gets enabled.
>
> 4th piece:
>
> + The code:
>
> /*
> * See if we want to use this console driver. If we
> * didn't select a console we take the first one
> * that registers here.
> */
> if (!has_preferred)
> ... try to enable the given console
>
> The comment is a bit unclear. The code is used as a fallback
> when no console was defined on the command line.
>
> Note that "has_preferred" is always true when "preferred_console"
> was defined via command line, see 2nd piece above.
>
>
> By other words:
>
> + The fallback code (4th piece) is called only when
> "preferred_console" was not defined on the command line.
>
> + The cycle below matches the given console only when
> it was defined on the command line.
>
>
> As a result, I believe that ->setup() could never be called
> in both paths for the same console. Especially I think that
> fallback code should not be used when the console was defined on
> the command line.
>
> I am not 100% sure but I am ready to risk this. Anyway, I think
> that many ->setup() callbacks are not ready to be successfully
> called twice.
>
> (Sigh, I have started to clean up this code two years ago.
> But I have never found time to finish the patchset. It is
> such a huge mess.)

Thanks for the elaboration in such details!

> > Can you describe, btw, struct console in kernel doc format?
> > It will be very helpful!
> >
> > > > In both cases we will get the console to have CON_ENABLED flag set.
> > >
> > > And there are sneaky consoles that have CON_ENABLED before we even
> > > register them.
> >
> > So, taking into consideration my comment to the previous patch, what would be
> > suggested guard here?
> >
> > For a starter something like this?
> >
> > if ((console->flags & CON_ENABLED) && console->exit)
> > console->exit(console);
>
> I would do:
>
> if (!res && console->exit)
> console->exit(console);
>
> I mean. I would call ->exit() only when console->setup() succeeded in
> register_console(). In this case, the console was later added to
> the console_drivers list.

Yes, that is exactly what I meant in previous mails to you.

--
With Best Regards,
Andy Shevchenko


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