[Intel-gfx] [PATCH v2 00/21] drm/dp: Various helper improvements and cleanups
Thierry Reding
thierry.reding at gmail.com
Mon Sep 23 14:52:02 UTC 2019
On Mon, Sep 23, 2019 at 04:52:50PM +0300, Jani Nikula wrote:
> On Fri, 20 Sep 2019, Thierry Reding <thierry.reding at gmail.com> wrote:
> > On Mon, Sep 02, 2019 at 01:31:00PM +0200, Thierry Reding wrote:
> >> From: Thierry Reding <treding at nvidia.com>
> >>
> >> Hi,
> >>
> >> this series of patches improves the DP helpers a bit and cleans up some
> >> inconsistencies along the way.
> >>
> >> v2 incorporates all review comments add collects Reviewed-bys from v1.
> >>
> >> Thierry
> >>
> >> Thierry Reding (21):
> >> drm/dp: Sort includes alphabetically
> >> drm/dp: Add missing kerneldoc for struct drm_dp_link
> >> drm/dp: Add drm_dp_link_reset() implementation
> >> drm/dp: Track link capabilities alongside settings
> >> drm/dp: Turn link capabilities into booleans
> >> drm/dp: Probe link using existing parsing helpers
> >> drm/dp: Read fast training capability from link
> >> drm/dp: Read TPS3 capability from sink
> >> drm/dp: Read channel coding capability from sink
> >> drm/dp: Read alternate scrambler reset capability from sink
> >> drm/dp: Read eDP version from DPCD
> >> drm/dp: Read AUX read interval from DPCD
> >> drm/dp: Do not busy-loop during link training
> >> drm/dp: Use drm_dp_aux_rd_interval()
> >> drm/dp: Add helper to get post-cursor adjustments
> >> drm/dp: Set channel coding on link configuration
> >> drm/dp: Enable alternate scrambler reset when supported
> >> drm/dp: Add drm_dp_link_choose() helper
> >> drm/dp: Add support for eDP link rates
> >> drm/dp: Remove a gratuituous blank line
> >> drm/bridge: tc358767: Use DP nomenclature
> >
> > Anyone interested in reviewing these?
>
> Thierry, I don't quite know how to put this nicely, but I also don't
> think it's nice to not reply at all. So I'll try to be fair but it'll be
> blunt. Fair enough?
Fair enough.
> I've glanced over the series, already before you pinged for reviews. It
> looks like you've put effort into it, and it all looks nice. However, it
> does not look like we could use this in i915, without significant effort
> both on top of this work and in i915. It does not feel like there's any
> incentive for us to review this in detail.
>
> It also feels like there's an increasing disconnect between "small" and
> "big" drivers (*) when it comes to handling DP link and training. It
> scares me a bit that this work is being done on the terms of the "small"
> drivers, and that later in time this might be considered the One True
> Way of handling DP.
I'm not sure I understand your concern here. The goal of the series is
primarily to extend the existing support for DP. It follows the pattern
established by existing helpers and then goes one step further and
provides some common way of actually storing the values that are being
read from the sink so that they can be used.
These are meant to be helpers and in no way should anyone feel obliged
to use them. If you've got this all figured out already, great! If you
do this already much better in i915, by all means stay away from this.
At the same time it seems counter-productive to write all of this code
as part of the Tegra DRM driver. In my opinion subsystems should provide
generic helpers that can help multiple drivers share code. This is
especially true for things that are defined in a specification because
there's not a lot of room for interpretation. The helpers in these
patches are meant to be that kind of helpers.
> One of the technical observations is that you fill the struct
> drm_dp_link and struct drm_dp_link_caps from the sink. It's not clear
> that the link caps really are an intersection of the source and sink
> caps. The eDP 1.4 link rates are the prime example. I think you should
> have sets of source and sink rates, and you should intersect those to
> find out the available link rates. The max rate is the highest number in
> that set. Similarly for many things, like training pattern support. I
> think it's only going to get more complicated with DP 2.0.
The idea here was to provide only helpers to collect the DPCD data
defined by the specification. Anything specific to the source is meant
to be handled by display driver. In case of eDP 1.4 link rates the code
will only add the rates read from DPCD. It's up to the driver to filter
out rates that it doesn't support from that list.
I think the fact that things will keep getting more complicated is an
argument in favour of sharing code rather than keep doing the same
(complicated) thing over and over again in every driver.
> Another pain point is the caching of the caps as bits in
> drm_dp_link_caps. How far are you going to take it? There's an insane
> and growing amount of things in the DPCD that describe the link in one
> way or another. Should they all be added to caps? Where do you draw the
> line? Do we add both the bit and the helper for getting that bit from
> the DPCD? And are you then going to add support for intersecting all
> those cap bits between the source and the sink?
Like I said the primary goal here is to have common code to read the
common values from DPCD. Once the link has been "probed" it is up to the
driver to do whatever it wants with that data.
Originally I had intended this shared code to do much more, but this was
shot down during review (I think by Daniel and yourself) for many of the
same reasons that you're pointing out. Initially there was code in this
series to standardize the link training sequence, for example. This was
all strictly according to the specification, so I thought that would
give us enough common ground for shared code. But you guys didn't agree,
so I've moved that out into Tegra specific patches since then.
As for how far to take this, I think the most sensible is to do what we
do everywhere else. We add to this whatever is needed on an on-demand
basis. The current series here adds what I found to be necessary to
support DP on Tegra. There's not a lot of fancy stuff here, I know, but
that doesn't mean this code is useless for everyone else.
So, can i915 use this? Probably yes. Would that be a good idea? Probably
not. And that's perfectly fine. But I could imagine that others may very
well want to use some shared code to avoid having to copy/paste code and
then later fix up cargo-culted bugs in every driver.
I'm also fully aware that this is not a lot and it may not be perfect.
But most helpers aren't initially. The point here is to start collecting
the common bits in one location and evolve them, just like we do for so
many other helpers.
Note also that I haven't made any attempt here to convert any drivers to
these helpers. That's because these are meant to be opt-in to simplify
drivers. If you want to do everything yourself, feel free to do that. It
is perfectly legit to do everything yourself if these helpers aren't
flexible enough to do what you want. The better option would be to help
improve the helpers to make them work for a wider range of drivers, but
if you don't want to, then don't.
> Overall I think there is value in unifying how we handle DP in drm. Even
> if just by providing helpers to simplify things in drivers. It's just
> that I feel this series isn't taking us closer to that goal, except for
> a subset of drivers. In the big picture, it may be increasing the
> divide.
So the bulk of this series is stuff that's purely parsing values from
DPCD, which is very much in line with existing helpers. I don't think
those are in any way contentious. There's also a bit of cleanup here
where new helpers are used to simplify existing ones. Maybe a handful
of these patches are what you claim might be "increasing the divide".
But I really don't understand where this is coming from. i915 doesn't
use a myriad of the other helpers (TTM, CMA, simple KMS, ...) and yet
these are not increasing any divide, are they? Why do you think these
helpers here are different?
Again, if you've got all of this implemented in i915 (or any of the
other "big" drivers), you probably want to stay away from these. But
does that mean everyone else has to go and figure all of this out from
scratch? Shouldn't we at least attempt to write common code? Or should
we all go and write our own DP stacks like the big drivers? I don't see
any advantage in that.
> If we get a confirmation from our drm overlords that drivers doing
> things the way they see fit in this regard is fine, then I'm okay with
> this. But I'm definitely not committing to switching to using the
> drm_dp_link structures and helpers in i915, quite the opposite actually.
I don't think anyone's going to force you to convert to the drm_dp_link
helpers if you don't want to. It's definitely not my intention to make
this "The One And Only Way To Do DP in DRM". The goal here is to create
helpers that can simplify adding DP support.
Now, if everyone else thinks the drm_dp_link helpers are a bad idea, I
will get over it and move this into Tegra code. But since we're being
blunt, I'd like to get a third (and ideally fourth) opinion on whether
we really want this stuff to be reinvented in every driver or whether
we want to try and come up with something that works for more than one
driver.
Thierry
> BR,
> Jani.
>
>
> (*) Please don't read too much into "small" and "big", just two groups
> of drivers handling things differently.
>
>
>
>
> >
> > Thierry
> >
> >>
> >> drivers/gpu/drm/bridge/tc358767.c | 22 +-
> >> drivers/gpu/drm/drm_dp_helper.c | 327 ++++++++++++++++++++++---
> >> drivers/gpu/drm/msm/edp/edp_ctrl.c | 12 +-
> >> drivers/gpu/drm/rockchip/cdn-dp-core.c | 8 +-
> >> drivers/gpu/drm/rockchip/cdn-dp-reg.c | 13 +-
> >> drivers/gpu/drm/tegra/dpaux.c | 8 +-
> >> drivers/gpu/drm/tegra/sor.c | 32 +--
> >> include/drm/drm_dp_helper.h | 124 +++++++++-
> >> 8 files changed, 459 insertions(+), 87 deletions(-)
> >>
> >> --
> >> 2.22.0
> >>
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Jani Nikula, Intel Open Source Graphics Center
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