linux-kernel - Re: [systemd-devel] [PATCH] tty: Set correct tty name in 'active' sysfs attribute

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Date:	Wed, 05 Feb 2014 09:19:04 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Herrmann <dh.herrmann@...il.com>
CC:	Hannes Reinecke <hare@...e.de>,
	systemd Mailing List <systemd-devel@...ts.freedesktop.org>,
	Kay Sievers <kay@...y.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [systemd-devel] [PATCH] tty: Set correct tty name in 'active'
 sysfs attribute

This patch won't get very far if not addressed to the actual maintainers

[ +cc Greg Kroah-Hartman, Jiri Slaby]

On 02/05/2014 09:05 AM, David Herrmann wrote:
> Hi
>
> On Wed, Feb 5, 2014 at 2:53 PM, Peter Hurley <peter@...leysoftware.com> wrote:
>> On 02/05/2014 07:53 AM, David Herrmann wrote:
>>>
>>> Hi
>>>
>>> On Wed, Feb 5, 2014 at 11:11 AM, Hannes Reinecke <hare@...e.de> wrote:
>>>>
>>>> The 'active' sysfs attribute should refer to the currently
>>>> active tty devices the console is running on, not the currently
>>>> active console.
>>>> The console structure doesn't refer to any device in sysfs,
>>>> only the tty the console is running on has.
>>>> So we need to print out the tty names in 'active', not
>>>> the console names.
>>>>
>>>> Cc: Lennart Poettering <lennart@...ttering.net>
>>>> Cc: Kay Sievers <kay@...y.org>
>>>> Signed-off-by: Werner Fink <werner@...e.de>
>>>> Signed-off-by: Hannes Reinecke <hare@...e.de>
>>>> ---
>>>>    drivers/tty/tty_io.c | 14 ++++++++++++--
>>>>    1 file changed, 12 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>>>> index c74a00a..17db8ca 100644
>>>> --- a/drivers/tty/tty_io.c
>>>> +++ b/drivers/tty/tty_io.c
>>>> @@ -3545,9 +3545,19 @@ static ssize_t show_cons_active(struct device
>>>> *dev,
>>>>                   if (i >= ARRAY_SIZE(cs))
>>>>                           break;
>>>>           }
>>>> -       while (i--)
>>>> +       while (i--) {
>>>> +               const struct tty_driver *driver;
>>>> +               const char *name = cs[i]->name;
>>>> +               int index = cs[i]->index;
>>>> +
>>>> +               driver = cs[i]->device(cs[i], &index);
>>>> +               if (driver) {
>>>> +                       index += driver->name_base;
>>>> +                       name = driver->name;
>>>> +               }
>>>>                   count += sprintf(buf + count, "%s%d%c",
>>>> -                                cs[i]->name, cs[i]->index, i ? '
>>>> ':'\n');
>>>> +                                name, index, i ? ' ':'\n');
>>>> +       }
>>>
>>>
>>> Nice catch and indeed, systemd already relies on these names to be
>>> identical to their char-dev name. Fortunately, VTs and most serial
>>> devices register the console with the same name as the TTY, so we're
>>> fine.
>>
>>
>> What device did this trip over?
>
> I haven't seen one so far, but to me it's a coincident, not something
> we should rely on.
>
>> Also, this file is not private to systemd. Maybe these changes should
>> be forked into a different sysfs attribute, "active_devices"?
>
> What's the use-case to return the name of the console-driver? There is
> no way for user-space to read active console-drivers anywhere so I
> think returning the TTY makes more sense. We already have working
> user-space that can spawn gettys on active consoles via this file. I
> am open to change this to "active_devices" as the existing interface
> was clearly not designed to return the device-names.
>
> However, given the fact that both matched so far, I think changing the
> existing interface to the only user I am aware of is better than
> adding a new interface just to keep this unused attribute. But
> obviously it's the maintainer's/your decision and you might know
> user-space which requires the console-names instead of the tty-names.
> So please let us know which way to go as we would like to see a
> reliable way to match active consoles to TTY devices for automated
> getty-startup.

Sure, I get it. Just wanted to point out the obvious right up front
so that if it turns out there is another userspace dependency and
this gets rewound, possibly across future -stable kernels, the
fallout could get ugly.

Regards,
Peter Hurley

PS - The "active" attribute won't be unused; old systemd's will
still depend on it, yes?

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