Re: New sysfs interface for privacy screens

[Daniel Thompson <daniel.thompson@linaro.org>]

On Wed, Oct 02, 2019 at 12:30:05PM +0300, Jani Nikula wrote:
> On Tue, 01 Oct 2019, Mat King <mathewk@google.com> wrote:
> > Resending in plain text mode
> >
> > I have been looking into adding Linux support for electronic privacy
> > screens which is a feature on some new laptops which is built into the
> > display and allows users to turn it on instead of needing to use a
> > physical privacy filter. In discussions with my colleagues the idea of
> > using either /sys/class/backlight or /sys/class/leds but this new
> > feature does not seem to quite fit into either of those classes.
> >
> > I am proposing adding a class called "privacy_screen" to interface
> > with these devices. The initial API would be simple just a single
> > property called "privacy_state" which when set to 1 would mean that
> > privacy is enabled and 0 when privacy is disabled.
> >
> > Current known use cases will use ACPI _DSM in order to interface with
> > the privacy screens, but this class would allow device driver authors
> > to use other interfaces as well.
> >
> > Example:
> >
> > # get privacy screen state
> > cat /sys/class/privacy_screen/cros_privacy/privacy_state # 1: privacy
> > enabled 0: privacy disabled
> >
> > # set privacy enabled
> > echo 1 > /sys/class/privacy_screen/cros_privacy/privacy_state
> >
> >  Does this approach seem to be reasonable?
> 
> What part of the userspace would be managing the privacy screen? Should
> there be a connection between the display and the privacy screen that
> covers the display? How would the userspace make that connection if it's
> a sysfs interface?
> 
> I don't know how the privacy screen operates, but if it draws any power,
> you'll want to disable it when you switch off the display it covers.
> 
> If the privacy screen control was part of the graphics subsystem (say, a
> DRM connector property, which feels somewhat natural), I think it would
> make it easier for userspace to have policies such as enabling the
> privacy screen automatically depending on the content you're viewing,
> but only if the content is on the display that has a privacy screen.

Connectors versus sysfs came up on a backlight thread recently.

Daniel Vetter wrote an excellent summary on why it has been (and still
is) difficult to migrate backlight controls towards the DRM connector
interface:
https://lkml.org/lkml/2019/8/20/752

Many of the backlight legacy problems do not apply to privacy screens
but I do suggest reading this post and some of the neighbouring parts
of the thread. In particular the ACPI driver versus real driver issues
Daniel mentioned could occur again. Hopefully not though, I mean how
wrong can a 1-bit control go? (actually no... don't answer that).

It would definitely be a shame to build up an unnecessary sysfs legacy
for privacy screens so definitely worth seeing if this can use DRM
connector properties.


Daniel.