Nouveau - Sep 2020 - [PATCH] drm/nouveau/kms/nv50-: Fix clock checking algorithm in nv50_dp_mode_valid()

On Tue, 2020-09-22 at 17:22 -0400, Ilia Mirkin wrote:
> On Tue, Sep 22, 2020 at 5:14 PM Lyude Paul <lyude at redhat.com>
wrote:
> > On Tue, 2020-09-22 at 17:10 -0400, Ilia Mirkin wrote:
> > > Can we use 6bpc on arbitrary DP monitors, or is there a
capability for
> > > it? Maybe only use 6bpc if display_info.bpc == 6 and otherwise
use 8?
> > 
> > I don't think that display_info.bpc actually implies a minimum
bpc, only a
> > maximum bpc iirc (Ville would know the answer to this one). The other
thing
> > to
> > note here is that we want to assume the lowest possible bpc here since
we're
> > only concerned if the mode passed to ->mode_valid can be set under
-any-
> > conditions (including those that require lowering the bpc beyond
it's
> > maximum
> > value), so we definitely do want to always use 6bpc here even once we
get
> > support for optimizing the bpc based on the available display
bandwidth.
> 
> Yeah, display_info is the max bpc. But would an average monitor
> support 6bpc? And if it does, does the current link training code even
> try that when display_info.bpc != 6?
So I did confirm that 6bpc support is mandatory for DP, so yes-6 bpc will always
work.

But also, your second comment doesn't really apply here. So: to be clear,
we're
not really concerned here about whether nouveau will actually use 6bpc or not. 
In truth I'm not actually sure either if we have any code that uses 6bpc
(iirc
we don't), since we don't current optimize bpc. I think it's very
possible for
us to use 6bpc for eDP displays if I recall though, but I'm not sure on
that.

But that's also not the point of this code. ->mode_valid() is only used
in two
situations in DRM modesetting: when probing connector modes, and when checking
if a mode is valid or not during the atomic check for atomic modesetting. Its
purpose is only to reject display modes that are physically impossible to set in
hardware due to static hardware constraints. Put another way, we only check the
given mode against constraints which will always remain constant regardless of
the rest of the display state. An example of a static constraint would be the
max pixel clock supported by the hardware, since on sensible hardware this never
changes. A dynamic constraint would be something like how much bandwidth is
currently unused on an MST topology, since that value is entirely dependent on
the rest of the display state.

So - with that said, bpc is technically a dynamic constraint because while a
sink and source both likely have their own bpc limits, any bpc which is equal or
below that limit can be used depending on what the driver decides - which will
be based on the max_bpc property, and additionally for MST displays it will also
depend on the available bandwidth on the topology. The only non-dynamic thing
about bpc is that at a minimum, it will be 6 - so any mode that doesn't fit
on
the link with a bpc of 6 is guaranteed to be a mode that we'll never be able
to
set and therefore want to prune.

So, even if we're not using 6 in the majority of situations, I'm fairly
confident it's the right value here. It's also what i915 does as well
(and they
previously had to fix a bug that was the result of assuming a minimum of 8bpc
instead of 6).
> 
>   -ilia
> 
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat

On Fri, Sep 25, 2020 at 6:08 PM Lyude Paul <lyude at redhat.com>
wrote:
>
> On Tue, 2020-09-22 at 17:22 -0400, Ilia Mirkin wrote:
> > On Tue, Sep 22, 2020 at 5:14 PM Lyude Paul <lyude at redhat.com>
wrote:
> > > On Tue, 2020-09-22 at 17:10 -0400, Ilia Mirkin wrote:
> > > > Can we use 6bpc on arbitrary DP monitors, or is there a
capability for
> > > > it? Maybe only use 6bpc if display_info.bpc == 6 and
otherwise use 8?
> > >
> > > I don't think that display_info.bpc actually implies a
minimum bpc, only a
> > > maximum bpc iirc (Ville would know the answer to this one). The
other thing
> > > to
> > > note here is that we want to assume the lowest possible bpc here
since we're
> > > only concerned if the mode passed to ->mode_valid can be set
under -any-
> > > conditions (including those that require lowering the bpc beyond
it's
> > > maximum
> > > value), so we definitely do want to always use 6bpc here even
once we get
> > > support for optimizing the bpc based on the available display
bandwidth.
> >
> > Yeah, display_info is the max bpc. But would an average monitor
> > support 6bpc? And if it does, does the current link training code even
> > try that when display_info.bpc != 6?
>
> So I did confirm that 6bpc support is mandatory for DP, so yes-6 bpc will
always
> work.
>
> But also, your second comment doesn't really apply here. So: to be
clear, we're
> not really concerned here about whether nouveau will actually use 6bpc or
not.
> In truth I'm not actually sure either if we have any code that uses
6bpc (iirc
> we don't), since we don't current optimize bpc. I think it's
very possible for
> us to use 6bpc for eDP displays if I recall though, but I'm not sure on
that.
>
> But that's also not the point of this code. ->mode_valid() is only
used in two
> situations in DRM modesetting: when probing connector modes, and when
checking
> if a mode is valid or not during the atomic check for atomic modesetting.
Its
> purpose is only to reject display modes that are physically impossible to
set in
> hardware due to static hardware constraints. Put another way, we only check
the
> given mode against constraints which will always remain constant regardless
of
> the rest of the display state. An example of a static constraint would be
the
> max pixel clock supported by the hardware, since on sensible hardware this
never
> changes. A dynamic constraint would be something like how much bandwidth is
> currently unused on an MST topology, since that value is entirely dependent
on
> the rest of the display state.
>
> So - with that said, bpc is technically a dynamic constraint because while
a
> sink and source both likely have their own bpc limits, any bpc which is
equal or
> below that limit can be used depending on what the driver decides - which
will
> be based on the max_bpc property, and additionally for MST displays it will
also
> depend on the available bandwidth on the topology. The only non-dynamic
thing
> about bpc is that at a minimum, it will be 6 - so any mode that doesn't
fit on
> the link with a bpc of 6 is guaranteed to be a mode that we'll never be
able to
> set and therefore want to prune.
>
> So, even if we're not using 6 in the majority of situations, I'm
fairly
> confident it's the right value here. It's also what i915 does as
well (and they
> previously had to fix a bug that was the result of assuming a minimum of
8bpc
> instead of 6).
Here's the situation I'm trying to avoid:

1. Mode is considered "OK" from a bandwidth perspective @6bpc
2. Modesetting logic never tries 6bpc and the modeset fails

As long as the two bits of logic agree on what is settable, I'm happy.

Cheers,

  -ilia

On Fri, 2020-09-25 at 19:53 -0400, Ilia Mirkin wrote:
> On Fri, Sep 25, 2020 at 6:08 PM Lyude Paul <lyude at redhat.com>
wrote:
> > On Tue, 2020-09-22 at 17:22 -0400, Ilia Mirkin wrote:
> > > On Tue, Sep 22, 2020 at 5:14 PM Lyude Paul <lyude at
redhat.com> wrote:
> > > > On Tue, 2020-09-22 at 17:10 -0400, Ilia Mirkin wrote:
> > > > > Can we use 6bpc on arbitrary DP monitors, or is there a
capability
> > > > > for
> > > > > it? Maybe only use 6bpc if display_info.bpc == 6 and
otherwise use
> > > > > 8?
> > > > 
> > > > I don't think that display_info.bpc actually implies a
minimum bpc,
> > > > only a
> > > > maximum bpc iirc (Ville would know the answer to this one).
The other
> > > > thing
> > > > to
> > > > note here is that we want to assume the lowest possible bpc
here since
> > > > we're
> > > > only concerned if the mode passed to ->mode_valid can be
set under
> > > > -any-
> > > > conditions (including those that require lowering the bpc
beyond it's
> > > > maximum
> > > > value), so we definitely do want to always use 6bpc here
even once we
> > > > get
> > > > support for optimizing the bpc based on the available
display
> > > > bandwidth.
> > > 
> > > Yeah, display_info is the max bpc. But would an average monitor
> > > support 6bpc? And if it does, does the current link training code
even
> > > try that when display_info.bpc != 6?
> > 
> > So I did confirm that 6bpc support is mandatory for DP, so yes-6 bpc
will
> > always
> > work.
> > 
> > But also, your second comment doesn't really apply here. So: to be
clear,
> > we're
> > not really concerned here about whether nouveau will actually use 6bpc
or
> > not.
> > In truth I'm not actually sure either if we have any code that
uses 6bpc
> > (iirc
> > we don't), since we don't current optimize bpc. I think
it's very possible
> > for
> > us to use 6bpc for eDP displays if I recall though, but I'm not
sure on
> > that.
> > 
> > But that's also not the point of this code. ->mode_valid() is
only used in
> > two
> > situations in DRM modesetting: when probing connector modes, and when
> > checking
> > if a mode is valid or not during the atomic check for atomic
modesetting.
> > Its
> > purpose is only to reject display modes that are physically impossible
to
> > set in
> > hardware due to static hardware constraints. Put another way, we only
> > check the
> > given mode against constraints which will always remain constant
> > regardless of
> > the rest of the display state. An example of a static constraint would
be
> > the
> > max pixel clock supported by the hardware, since on sensible hardware
this
> > never
> > changes. A dynamic constraint would be something like how much
bandwidth
> > is
> > currently unused on an MST topology, since that value is entirely
> > dependent on
> > the rest of the display state.
> > 
> > So - with that said, bpc is technically a dynamic constraint because
while
> > a
> > sink and source both likely have their own bpc limits, any bpc which
is
> > equal or
> > below that limit can be used depending on what the driver decides -
which
> > will
> > be based on the max_bpc property, and additionally for MST displays it
> > will also
> > depend on the available bandwidth on the topology. The only
non-dynamic
> > thing
> > about bpc is that at a minimum, it will be 6 - so any mode that
doesn't
> > fit on
> > the link with a bpc of 6 is guaranteed to be a mode that we'll
never be
> > able to
> > set and therefore want to prune.
> > 
> > So, even if we're not using 6 in the majority of situations,
I'm fairly
> > confident it's the right value here. It's also what i915 does
as well (and
> > they
> > previously had to fix a bug that was the result of assuming a minimum
of
> > 8bpc
> > instead of 6).
> 
> Here's the situation I'm trying to avoid:
> 
> 1. Mode is considered "OK" from a bandwidth perspective @6bpc
> 2. Modesetting logic never tries 6bpc and the modeset fails
> 
> As long as the two bits of logic agree on what is settable, I'm happy.
I gotcha-I guess I was just confused because this is already possible with the
current code we have (and it was also possible before we started adding these
checks into ->mode_valid, which is why I need to get to the max_bpc
thing soon). I guess I'll just use the connector's reported bpc for now
until
we add support for that
> 
> Cheers,
> 
>   -ilia
> 
-- 
Cheers,
	Lyude Paul (she/her)
	Software Engineer at Red Hat