On Tue, Dec 03, 2024 at 11:39:05AM -0500, Zack Rusin wrote: > On Tue, Dec 3, 2024 at 11:32 AM Jonas Ådahl <jadahl@xxxxxxxxx> wrote: > > > > On Tue, Dec 03, 2024 at 11:27:52AM -0500, Zack Rusin wrote: > > > On Tue, Dec 3, 2024 at 10:57 AM Jonas Ådahl <jadahl@xxxxxxxxx> wrote: > > > > > > > > On Wed, Nov 20, 2024 at 07:52:18AM -0500, Zack Rusin wrote: > > > > > On Wed, Nov 20, 2024 at 5:22 AM Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On Wed, 20 Nov 2024, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote: > > > > > > > Hi > > > > > > > > > > > > > > > > > > > > > Am 19.11.24 um 20:40 schrieb Ian Forbes: > > > > > > >> Most compositors are using a change in EDID as an indicator to > > > > > > >> refresh their connector information on hotplug regardless of whether the > > > > > > >> connector was previously connected. Originally the hotplug_mode_update > > > > > > >> property was supposed to provide a hint to userspace to always refresh > > > > > > >> connector info on hotplug as virtual devices such as vmwgfx and QXL > > > > > > >> changed the connector without disconnecting it first. This was done to > > > > > > >> implement Autofit. Unfortunately hotplug_mode_update was not widely > > > > > > >> adopted and compositors used other heuristics to determine whether to > > > > > > >> refresh the connector info. > > > > > > >> > > > > > > >> Currently a change in EDID is the one heuristic that seems to be universal. > > > > > > >> No compositors currently implement hotplug_mode_update correctly or at all. > > > > > > >> By implementing a fake EDID blob we can ensure that our EDID changes on > > > > > > >> hotplug and therefore userspace will refresh the connector info so that > > > > > > >> Autofit will work. This is the approach that virtio takes. > > > > > > >> > > > > > > >> This also removes the need to add hotplug_mode_update support for all > > > > > > >> compositors as traditionally this niche feature has fallen on > > > > > > >> virtualized driver developers to implement. > > > > > > > > > > > > > > Why don't you fix the compositors instead? > > > > > > > > > > > > > > I feel like NAK'ing this patch. The code itself is not so much a > > > > > > > problem, but the commit message. > > > > > > > > > > > > Oh, I think the code is problematic too. > > > > > > > > > > > > Please avoid all struct edid based interfaces, in this case > > > > > > drm_connector_update_edid_property(). They will be removed in the > > > > > > future, and adding more is counter-productive. Everything should be > > > > > > struct drm_edid based going forward. > > > > > > > > > > > > Of course, actually grafting the EDID needs struct edid. And that's kind > > > > > > of annoying too. Do we really want to spread the EDID details all over > > > > > > the place? This one combines drm_edid.h structs and magic numbers in a > > > > > > jumble. I'm kind of hoping we'd get rid of driver usage of struct edid, > > > > > > though that's a long road. But we've made a lot of progress towards it, > > > > > > there aren't that many places left that directly look at the guts of > > > > > > EDID, and most of it is centralized in drm_edid.c. > > > > > > > > > > > > Of course, not using the standard drm_edid_read* interfaces also lacks > > > > > > on features such as providing the EDID via the firmware loader or > > > > > > debugfs, which can be handy for testing and debugging, but that's a > > > > > > minor issue. > > > > > > > > > > > > > Maybe it resolves problems with > > > > > > > compositors, but it is a step backwards for the overall ecosystem. If > > > > > > > the connector changes, your driver should increment the epoch counter. > > > > > > > [1] That will send a hotplug event to userspace. The EDID alone does not > > > > > > > say anything about connector status. > > > > > > > > > > > > Yeah, unplugging and replugging the same display with the same EDID > > > > > > isn't a problem for other drivers, and they don't have to do this kind > > > > > > of stuff to trick userspace. Maybe vmwgfx should handle (or simulate) > > > > > > hotplugs better? > > > > > > > > > > I don't think that's what Ian is trying to fix. There's two different issues: > > > > > 1) The code using struct edid which is frowned upon. > > > > > 2) The virtualized drivers not behaving like real GPU's and thus > > > > > breaking userspace. > > > > > > > > > > vmwgfx and qxl do not provide edid at all. It's null. But every time > > > > > someone resizes a host side window in which the para-virtualized > > > > > driver is displaying, the preferred mode changes. Userspace kept > > > > > checking whether the edid changes on each hotplug event to figure out > > > > > if it got new modes and refresh if it noticed that edid changed. > > > > > Because on qxl and vmwgfx the edid never changes (it's always null) > > > > > Dave added hotplug_mode_update property which only qxl and vmwgfx send > > > > > and its presence indicates that the userspace should refresh modes > > > > > even if edid didn't change. > > > > > > > > > > Because that property is only used by qxl and vmwgfx everyone gets it > > > > > wrong. The property was specifically added to fix gnome and Ian > > > > > noticed that currently even gnome is broken: > > > > > https://gitlab.gnome.org/GNOME/mutter/-/blob/main/src/backends/native/meta-kms-connector.c#L940 > > > > > hotplug_mode_update doesn't change, it's just a flag that indicates > > > > > that userspace needs a full mode rescan. > > > > > > > > The linked line just means the property value itself not changing > > > > doesn't result in a full compositor side monitor reconfiguration. > > > > > > Right, that's exactly the point I'm making :) The property isn't used > > > correctly because the full-rescan is expected when that property is > > > present, not if it changed. > > > > Well, a full rescan did happen, and the linked code only determines if > > anything actually did change, including currently advertised modes, that > > will have any potential effect on the final monitor configuration. > > The point I'm making is that no one is using this property correctly. > Mutter triggering a full-rescan as a result of other changes doesn't > change the fact that its usage of that property is broken. > I think > you're interpreting my comment that usage of that property is broken > (or not used at all) everywhere as "Mutter is not refreshing > correctly" which is not the case. Mutter does resize correctly despite > the fact that the property check is broken. Ah, yes, I interpret it as something was broken, but I suppose it is working as expected. We're indeed apparently using it incorrectly by reading the property value. After having seen this thread, I went and checked the commit history, and it seems the implementation has flip flopped between reading the property, and checking its existence, for more than a decade. I suspect a reason for this might have been that the property itself doesn't seem to be documented anywhere. Or is it? I have not yet found that it is. The following would probably make the implementation conform to the expectations of the property: ``` diff --git a/src/backends/native/meta-kms-connector.c b/src/backends/native/meta-kms-connector.c index cc6cd89f56..a4b9deb85e 100644 --- a/src/backends/native/meta-kms-connector.c +++ b/src/backends/native/meta-kms-connector.c @@ -369,7 +369,7 @@ state_set_properties (MetaKmsConnectorState *state, prop = &props[META_KMS_CONNECTOR_PROP_HOTPLUG_MODE_UPDATE]; if (prop->prop_id) - state->hotplug_mode_update = prop->value; + state->hotplug_mode_update = TRUE; prop = &props[META_KMS_CONNECTOR_PROP_SCALING_MODE]; if (prop->prop_id) ``` In mutter, it has primarily been interpreted as a hint when implementing policy to decide when to avoid look up monitor configurations from persistent storage. Jonas > > z
