linux-kernel - Re: [PATCH 29/49] vcs: add poll/fasync support

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Message-ID: <AANLkTinVp4C9bRW6=EMHwh1UyeadFbOXKe1aDsxknJf8@mail.gmail.com>
Date:	Wed, 10 Nov 2010 01:12:55 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Greg Kroah-Hartman <gregkh@...e.de>
Cc:	linux-kernel@...r.kernel.org, Nicolas Pitre <nico@...xnic.net>,
	Nicolas Pitre <nicolas.pitre@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 29/49] vcs: add poll/fasync support

On Fri, Oct 22, 2010 at 20:21, Greg Kroah-Hartman <gregkh@...e.de> wrote:
> From: Nicolas Pitre <nico@...xnic.net>
>
> The /dev/vcs* devices are used, amongst other things, by accessibility
> applications such as BRLTTY to display the screen content onto refreshable
> braille displays.  Currently this is performed by constantly reading from
> /dev/vcsa0 whether or not the screen content has changed.  Given the
> default braille refresh rate of 25 times per second, this easily qualifies
> as the biggest source of wake-up events preventing laptops from entering
> deeper power saving states.
>
> To avoid this periodic polling, let's add support for select()/poll() and
> SIGIO with the /dev/vcs* devices.  The implemented semantic is to report
> data availability whenever the corresponding vt has seen some update after
> the last read() operation.  The application still has to lseek() back
> as usual in order to read() the new data.

Shouldn't it raise POLLPRI/POLLERR then, when it's not about new data
to read? We do this for several files in the kernel where we just want
to wakup someone, but the pretty well-defined semantics of poll()
don't apply.

Kay
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