From: Simon Ser <contact@emersion.fr>
To: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Benjamin Berg <bberg@redhat.com>, David Airlie <airlied@linux.ie>,
Thomas Zimmermann <tzimmermann@suse.de>,
Christian Kellner <ckellner@redhat.com>,
Javier Martinez Canillas <javierm@redhat.com>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
Hans de Goede <hdegoede@redhat.com>,
Mark Pearson <mpearson@lenovo.com>,
Rajat Jain <rajatja@google.com>,
Nitin Joshi1 <njoshi1@lenovo.com>
Subject: Re: Operating KMS UAPI (Re: RFC: Drm-connector properties managed by another driver / privacy screen support)
Date: Mon, 20 Apr 2020 12:33:24 +0000 [thread overview]
Message-ID: <uJ44RSX2HiIaTgyv5Ja8mUntCnl4rtbjBapdV4IqubIHEauj_CWYMjDHztQdus6U75hUwdUd5AdxEV0HnZSdQboV0ai5mIlO_wmdxwcDfnU=@emersion.fr> (raw)
In-Reply-To: <20200420152235.3473851f@eldfell.localdomain>
On Monday, April 20, 2020 2:22 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> On Mon, 20 Apr 2020 10:15:39 +0000
> Simon Ser contact@emersion.fr wrote:
>
> > On Monday, April 20, 2020 10:27 AM, Pekka Paalanen ppaalanen@gmail.com wrote:
> >
> > > The only "random" KMS state is the properties the userspace KMS
> > > program does not know that are set on start-up. I have been assuming
> > > that as long as you had fbdev active before the KMS program started,
> > > the unknown properties have "harmless" default values. And maybe even at
> > > driver device init if fbdev does not exist?
>
> I meant fbcon, not fbdev above.
>
> > Note, this is not the case when using e.g. a display manager. In the
> > past there have been cases of a display manager setting a hw cursor
> > and launching a compositor not supporting hw cursors. This results in
> > a stuck hw cursor.
>
> Indeed. So the display manager might get sensible defaults, but the
> session compositor might not. Or maybe boot splash uses KMS already, so
> even display manager doesn't get all-defaults state.
>
> It seems we really do need "sane defaults" from the kernel explicitly.
> Writing a userspace tool to save it at boot time before any KMS program
> runs would be awkward.
Agreed.
> > > Btw. I searched for all occurrences of link_status in
> > > https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html and it seems it
> > > only has two possible values, good and bad, and no mention whether it
> > > is writable. Looks like it's writable. There does not seem to be a) an
> > > explanation how exactly it needs to the handled (writing it does
> > > something? what can you write?) or b) any way discern between kernel
> > > and userspace set values like HDCP "Content Protection" has.
> >
> > User-space needs to reset the value to GOOD when recovering from a BAD
> > value.
>
> What if userspace writes BAD?
>
> BAD cannot be default state, so getting default state from somewhere
> would solve this property's restoring as well. Reading back the true
> current value could accidentally return BAD.
Interestingly, setting it to BAD is a no-op (BAD is silently
discarded):
/* Never downgrade from GOOD to BAD on userspace's request here,
* only hw issues can do that.
*
* For an atomic property the userspace doesn't need to be able
* to understand all the properties, but needs to be able to
* restore the state it wants on VT switch. So if the userspace
* tries to change the link_status from GOOD to BAD, driver
* silently rejects it and returns a 0. This prevents userspace
* from accidently breaking the display when it restores the
* state.
*/
if (state->link_status != DRM_LINK_STATUS_GOOD)
state->link_status = val;
So restoring the "sane default" would be work, even if the link happens
to be BAD when saving said "sane defaults".
> Just to reiterate for everyone, the important thing here is to figure
> out how userspace is supposed to reset unknown properties to sensible
> defaults. Once we know how that should work, we can review whether new
> properties support or break that.
Yes, that's a good description of the problem.
I see two main solutions here: either the kernel provides the default
values in its property descriptions (e.g. drmModeGetProperty), either
user-space can ask the kernel to reset properties to their default
values.
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prev parent reply other threads:[~2020-04-20 12:33 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 9:42 RFC: Drm-connector properties managed by another driver / privacy screen support Hans de Goede
2020-04-15 9:52 ` Daniel Vetter
2020-04-15 10:11 ` Hans de Goede
2020-04-15 10:22 ` Daniel Vetter
2020-04-15 11:39 ` Hans de Goede
2020-04-15 11:56 ` Hans de Goede
2020-04-15 12:01 ` Daniel Vetter
2020-04-15 13:02 ` Hans de Goede
2020-04-15 17:54 ` Daniel Vetter
2020-04-15 18:19 ` Hans de Goede
2020-04-15 18:29 ` Daniel Vetter
2020-04-15 19:50 ` Hans de Goede
2020-04-16 6:46 ` Daniel Vetter
2020-04-15 15:28 ` Jani Nikula
2020-04-15 15:40 ` Hans de Goede
2020-04-15 17:14 ` [External] " Mark Pearson
2020-04-15 18:06 ` Hans de Goede
2020-04-15 19:20 ` Rajat Jain
2020-04-15 21:10 ` Jani Nikula
2020-04-15 21:21 ` Hans de Goede
2020-04-15 21:51 ` [External] " Mark Pearson
2020-04-17 9:05 ` Pekka Paalanen
2020-04-17 9:02 ` Pekka Paalanen
2020-04-17 11:55 ` Jani Nikula
2020-04-17 14:18 ` Daniel Vetter
2020-04-17 14:54 ` Benjamin Berg
2020-04-21 12:37 ` Hans de Goede
2020-04-21 12:40 ` Daniel Vetter
2020-04-21 14:46 ` Pekka Paalanen
2020-04-23 18:21 ` Rajat Jain
2020-04-24 7:40 ` Pekka Paalanen
2020-04-24 8:24 ` Hans de Goede
2020-04-24 9:08 ` Pekka Paalanen
2020-04-24 10:32 ` Hans de Goede
2020-04-17 14:17 ` Daniel Vetter
2020-04-20 8:27 ` Operating KMS UAPI (Re: RFC: Drm-connector properties managed by another driver / privacy screen support) Pekka Paalanen
2020-04-20 10:04 ` Pekka Paalanen
2020-04-20 10:18 ` Simon Ser
2020-04-21 12:15 ` Daniel Vetter
2020-04-21 14:33 ` Pekka Paalanen
2020-04-21 14:39 ` Simon Ser
2020-04-23 15:01 ` Daniel Vetter
2020-04-24 8:32 ` Pekka Paalanen
2020-04-28 14:51 ` Daniel Vetter
2020-04-29 10:07 ` Pekka Paalanen
2020-04-30 13:53 ` Daniel Vetter
2020-05-04 9:49 ` Pekka Paalanen
2020-05-04 11:00 ` Daniel Vetter
2020-05-04 12:22 ` Pekka Paalanen
2020-05-05 8:48 ` Daniel Vetter
2020-05-07 9:03 ` Pekka Paalanen
2020-04-20 10:15 ` Simon Ser
2020-04-20 12:22 ` Pekka Paalanen
2020-04-20 12:33 ` Simon Ser [this message]
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