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

[Lyude Paul]

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