Message-ID: <20210706100942.3754a8e3@eldfell>
Date:   Tue, 6 Jul 2021 10:09:42 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Werner Sembach <wse@...edocomputers.com>
Cc:     sunpeng.li@....com, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        airlied@...ux.ie, dri-devel@...ts.freedesktop.org,
        tzimmermann@...e.de, rodrigo.vivi@...el.com,
        alexander.deucher@....com, christian.koenig@....com
Subject: Re: [PATCH v4 12/17] drm/uAPI: Add "preferred color format" drm
 property as setting for userspace

On Mon, 5 Jul 2021 17:49:42 +0200
Werner Sembach <wse@...edocomputers.com> wrote:

> Am 01.07.21 um 15:24 schrieb Pekka Paalanen:
> > On Thu, 1 Jul 2021 14:50:13 +0200
> > Werner Sembach <wse@...edocomputers.com> wrote:
> >  
> >> Am 01.07.21 um 10:07 schrieb Pekka Paalanen:
> >>  
> >>> On Wed, 30 Jun 2021 11:20:18 +0200
> >>> Werner Sembach <wse@...edocomputers.com> wrote:
> >>>    
> >>>> Am 30.06.21 um 10:41 schrieb Pekka Paalanen:
> >>>>    
> >>>>> On Tue, 29 Jun 2021 13:39:18 +0200
> >>>>> Werner Sembach <wse@...edocomputers.com> wrote:
> >>>>>      
> >>>>>> Am 29.06.21 um 13:17 schrieb Pekka Paalanen:      
> >>>>>>> On Tue, 29 Jun 2021 08:12:54 +0000
> >>>>>>> Simon Ser <contact@...rsion.fr> wrote:
> >>>>>>>         
> >>>>>>>> On Tuesday, June 22nd, 2021 at 09:15, Pekka Paalanen <ppaalanen@...il.com> wrote:
> >>>>>>>>         
> >>>>>>>>> yes, I think this makes sense, even if it is a property that one can't
> >>>>>>>>> tell for sure what it does before hand.
> >>>>>>>>>
> >>>>>>>>> Using a pair of properties, preference and active, to ask for something
> >>>>>>>>> and then check what actually worked is good for reducing the
> >>>>>>>>> combinatorial explosion caused by needing to "atomic TEST_ONLY commit"
> >>>>>>>>> test different KMS configurations. Userspace has a better chance of
> >>>>>>>>> finding a configuration that is possible.
> >>>>>>>>>
> >>>>>>>>> OTOH, this has the problem than in UI one cannot tell the user in
> >>>>>>>>> advance which options are truly possible. Given that KMS properties are
> >>>>>>>>> rarely completely independent, and in this case known to depend on
> >>>>>>>>> several other KMS properties, I think it is good enough to know after
> >>>>>>>>> the fact.
> >>>>>>>>>
> >>>>>>>>> If a driver does not use what userspace prefers, there is no way to
> >>>>>>>>> understand why, or what else to change to make it happen. That problem
> >>>>>>>>> exists anyway, because TEST_ONLY commits do not give useful feedback
> >>>>>>>>> but only a yes/no.      
> >>>>>>>> By submitting incremental atomic reqs with TEST_ONLY (i.e. only changing one
> >>>>>>>> property at a time), user-space can discover which property makes the atomic
> >>>>>>>> commit fail.      
> >>>>>>> That works if the properties are independent of each other. Color
> >>>>>>> range, color format, bpc and more may all be interconnected,
> >>>>>>> allowing only certain combinations to work.
> >>>>>>>
> >>>>>>> If all these properties have "auto" setting too, then it would be
> >>>>>>> possible to probe each property individually, but that still does not
> >>>>>>> tell which combinations are valid.
> >>>>>>>
> >>>>>>> If you probe towards a certain configuration by setting the properties
> >>>>>>> one by one, then depending on the order you pick the properties, you
> >>>>>>> may come to a different conclusion on which property breaks the
> >>>>>>> configuration.      
> >>>>>> My mind crossed another point that must be considered: When plugin in
> >>>>>> a Monitor a list of possible Resolutions+Framerate combinations is
> >>>>>> created for xrandr and other userspace (I guess by atomic checks? but
> >>>>>> I don't know).      
> >>>>> Hi,
> >>>>>
> >>>>> I would not think so, but I hope to be corrected if I'm wrong.
> >>>>>
> >>>>> My belief is that the driver collects a list of modes from EDID, some
> >>>>> standard modes, and maybe some other hardcoded modes, and then
> >>>>> validates each entry against all the known limitations like vertical
> >>>>> and horizontal frequency limits, discarding modes that do not fit.
> >>>>>
> >>>>> Not all limitations are known during that phase, which is why KMS
> >>>>> property "link-status" exists. When userspace actually programs a mode
> >>>>> (not a TEST_ONLY commit), the link training may fail. The kernel prunes
> >>>>> the mode from the list and sets the link status property to signal
> >>>>> failure, and sends a hotplug uevent. Userspace needs to re-check the
> >>>>> mode list and try again.
> >>>>>
> >>>>> That is a generic escape hatch for when TEST_ONLY commit succeeds, but
> >>>>> in reality the hardware cannot do it, you just cannot know until you
> >>>>> actually try for real. It causes end user visible flicker if it happens
> >>>>> on an already running connector, but since it usually happens when
> >>>>> turning a connector on to begin with, there is no flicker to be seen,
> >>>>> just a small delay in finding a mode that works.
> >>>>>      
> >>>>>> During this drm
> >>>>>> properties are already considered, which is no problem atm because as
> >>>>>> far as i can tell there is currently no drm property that would make
> >>>>>> a certain Resolutions+Framerate combination unreachable that would be
> >>>>>> possible with everything on default.      
> >>>>> I would not expect KMS properties to be considered at all. It would
> >>>>> reject modes that are actually possible if the some KMS properties were
> >>>>> changed. So at least going forward, current KMS property values cannot
> >>>>> factor in.      
> >>>> At least the debugfs variable "force_yuv420_output" did change the 
> >>>> available modes here: 
> >>>> https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c#L5165 
> >>>> before my patch 
> >>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=68eb3ae3c63708f823aeeb63bb15197c727bd9bf    
> >>> Hi,
> >>>
> >>> debugfs is not proper UAPI, so we can just ignore it. Display servers
> >>> cannot be expected to poke in debugfs. Debugfs is not even supposed to
> >>> exist in production systems, but I'm sure people use it for hacking
> >>> stuff. But that's all it is for: developer testing and hacking.    
> >> e.g. Ubuntu has it active by default, but only read (and writable) by root.  
> > Hi,
> >
> > that's normal, yes. Root can do damage anyway, and it's useful for
> > debugging. KMS clients OTOH often do not run as root.
> >  
> >>>    
> >>>> Forcing a color format via a DRM property in this function would 
> >>>> reintroduce the problem.    
> >>> The property would need to be defined differently because its presence
> >>> could otherwise break existing userspace. Well, I suppose it could
> >>> break existing userspace no matter what, so we would need the generic
> >>> "reset to sane defaults" mechanism first IMO.
> >>>
> >>> DRM has client caps for exposing video modes that old userspace might
> >>> not expect, to avoid breaking old userspace. Care needs to be taken
> >>> with all new UAPI, because if a kernel upgrade makes something wrong,
> >>> it's the kernel's fault no matter what userspace is doing, in principle.    
> >> Can you give me a link describing how I define this caps?  
> > I don't have any, but you can find all the existing ones by grepping
> > for DRM_CLIENT_CAP_.
> >
> > I'm not saying that we need it, but mentioning them as a possible
> > workaround if userspace breakage seems imminent or is proven.
> >  
> >>> Debugfs has no problem breaking userspace AFAIU, since it's not proper
> >>> UAPI.
> >>>    
> >>>> And I think i915 driver works similar in this regard.
> >>>>    
> >>>>>      
> >>>>>> However for example forcing YCbCr420 encoding would limit the
> >>>>>> available resolutions (my screen for example only supports YCbCr420
> >>>>>> on 4k@60 and @50Hz and on no other resolution or frequency (native is
> >>>>>> 2560x1440@...Hz).
> >>>>>>
> >>>>>> So would a "force color format" that does not get resetted on
> >>>>>> repluging/reenabling a monitor break the output, for example, of an
> >>>>>> not updated xrandr, unaware of this new property?      
> >>>>> Yes, not because the mode list would be missing the mode, but because
> >>>>> actually setting the mode would fail.      
> >>>> Well, like described above, I think the mode would actually be missing, 
> >>>> which is also an unexpected behavior from a user perspective.    
> >>> I think that is not how the property should work.
> >>>
> >>> If KMS properties would affect the list of modes, then userspace would
> >>> need to set the properties for real (TEST_ONLY cannot change anything)
> >>> and re-fetch the mode lists (maybe there is a hotplug event, maybe
> >>> not). That workflow just doesn't work.    
> >> The properties are set before the list is created in the first place.
> >> Because, in my example, the properties get set before the monitor is
> >> plugged in and the list can only be created as soon as the monitor is
> >> plugged in.  
> > That's just an accident, it's not what I mean.
> >
> > What I mean is, we cannot have the KMS properties affect the list of
> > modes, because then userspace that want to use specific values on those
> > properties would have to program those properties first, and then get
> > the list of modes. KMS UAPI does not work that way AFAIK.
> >
> > If the initial mode list is created on hotplug like you say, then the
> > initial list could already be missing some modes that would be valid if
> > some KMS properties had different values.  
> 
> Depends if the mode list is created by TEST_ONLY:

Hi,

I'm pretty sure it's not created by any kind of atomic test probing,
exactly because some properties might affect the result. Also because
of legacy: the mode lists predate atomic by far. It just doesn't make
sense to prune the mode list based on current arbitrary property values.

The function drm_helper_probe_single_connector_modes() looks relevant
to me. It has a big comment that seems to point towards more things to
look at.


Thanks,
pq

> - The force properties should return false on TEST_ONLY
> 
> - The force properties should not prevent the mode from showing up in the list
> 
> If the list is created by TEST_ONLY both things can't be fulfilled at the same time obviously.
> 
> I hope some can give more insights or has an idea how the properties could work best.
> 
> >  
> >>> A very interesting question is when should link-status failure not drop
> >>> the current mode from the mode list, if other KMS properties affect the
> >>> bandwidth etc. requirements of the mode. That mechanism is engineered
> >>> for old userspace that doesn't actually handle link-status but does
> >>> handle hotplug, so the hotplug triggers re-fetching the mode list and
> >>> userspace maybe trying again with a better luck since the offending
> >>> mode is gone. How to keep that working when introducing KMS properties
> >>> forcing the cable format, I don't know.
> >>>
> >>> As long as the other affecting KMS properties are all "auto", the
> >>> driver will probably exhaust all possibilities to make the mode work
> >>> before signalling link-status failure and pruning the mode.
> >>> Theoretically, as I have no idea what drivers actually do.    
> >> Isn't that exactly how the "preferred color format" property works in
> >> my patchset now?  
> > There was an argument that "preferred" with no guarantees is not
> > useful enough. So I'm considering the force property instead.
> > The problem is, "auto" is not the only possible value.
> >
> > When the value is not "auto", should link failure drop the mode or not?
> > Userspace might change the value back to "auto" next time. If you
> > dropped the mode, it would be gone. If you didn't drop the mode,
> > userspace might be stuck picking the same non-working mode again and
> > again if it doesn't know about the force mode property.
> >
> > You could argue that changing the value back to "auto" needs to reset
> > the mode list, but that only gets us back to the "need to set
> > properties before getting mode list".
> >
> > Maybe there needs to be an assumption that if "force color format" is
> > not "auto", then link failure does not drop modes and userspace knows
> > to handle this. Messy.
> >
> > I'm afraid I just don't know to give any clear answer. It's also
> > possible that, as I'm not a kernel dev, I have some false assumptions
> > here.
> >
> >
> > Thanks,
> > pq
> > _______________________________________________
> > amd-gfx mailing list
> > amd-gfx@...ts.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/amd-gfx  


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