linux-kernel - Re: [RFC/PATCH] printk: Fix preferred console selection with multiple matches

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Date:   Wed, 11 Dec 2019 13:53:03 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/PATCH] printk: Fix preferred console selection with
 multiple matches

On Wed 2019-12-11 15:02:02, Benjamin Herrenschmidt wrote:
> On Wed, 2019-12-11 at 11:01 +0900, Sergey Senozhatsky wrote:
> > On (19/12/11 09:26), Benjamin Herrenschmidt wrote:
> > 
> 
> > As far as I know, ->match() does not only match but also does ->setup().
> > If we have two console list entries that match (one via aliasing and one
> > via exact match) then the console driver is setup twice. Do all console
> > drivers handle it? [double setup]
> 
> I don't think it's an issue but I may be wrong. I had a quick look
> at some of the drivers and I don't really see why they would break but
> I couldn't look at them all and I might be mistaken.
> 
> We could skip setup if the console is already enabled but I would
> advise against that since the two calls might have different options
> (the firmware baud rate could be different from the command line one
> for example) and we want the options for the last one.

For example:

  + ip22zilog_console_setup() calls  __ip22zilog_startup() that does
    some initialization of the device

  + pl011_console_setup() does some non-trivial things as well.

Honestly, I am not familiar with these devices. I am not sure if it is
dangerous or safe to call these functions twice.

I am not sure if anybody would know this for sure. Therefore I suggest a
conservative approach and avoid calling setup() twice().

I think that it is dangerous and error-prone design. The best solution
would be to split setup() and match() functionality.

Best Regards,
Petr