[Intel-gfx] [RFC DP-typeC 0/2] Support USB typeC based DP on BXT

[Daniel Vetter]

On Mon, Sep 28, 2015 at 04:48:32PM +0000, R, Durgadoss wrote:
> Hi Daniel,
> 
> >-----Original Message-----
> >From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel Vetter
> >Sent: Monday, September 28, 2015 1:50 PM
> >To: R, Durgadoss
> >Cc: Daniel Vetter; Jani Nikula; intel-gfx at lists.freedesktop.org
> >Subject: Re: [Intel-gfx] [RFC DP-typeC 0/2] Support USB typeC based DP on BXT
> >
> >On Fri, Sep 25, 2015 at 12:22:42PM +0000, R, Durgadoss wrote:
> >> Hi Daniel,
> >>
> >> Thanks for having a look at it..
> >>
> >> >-----Original Message-----
> >> >From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel Vetter
> >> >Sent: Wednesday, September 23, 2015 3:14 PM
> >> >To: R, Durgadoss
> >> >Cc: Jani Nikula; intel-gfx at lists.freedesktop.org
> >> >Subject: Re: [Intel-gfx] [RFC DP-typeC 0/2] Support USB typeC based DP on BXT
> >> >
> >> >On Wed, Sep 16, 2015 at 10:57:45AM +0000, R, Durgadoss wrote:
> >> >> Hi Jani,
> >> >>
> >> >> >-----Original Message-----
> >> >> >From: Jani Nikula [mailto:jani.nikula at linux.intel.com]
> >> >> >Sent: Wednesday, September 16, 2015 3:18 PM
> >> >> >To: R, Durgadoss; intel-gfx at lists.freedesktop.org
> >> >> >Cc: R, Durgadoss
> >> >> >Subject: Re: [RFC DP-typeC 0/2] Support USB typeC based DP on BXT
> >> >> >
> >> >> >On Tue, 15 Sep 2015, Durgadoss R <durgadoss.r at intel.com> wrote:
> >> >> >> This is an RFC series to start the review/discussion on approach
> >> >> >> to support USB type C based DP panel.
> >> >> >>
> >> >> >> To support USB type C alternate DP mode, the display driver needs to
> >> >> >> know the number of lanes required by the DP panel as well as number
> >> >> >> of lanes that can be supported by the type-C cable. Sometimes, the
> >> >> >> type-C cable may limit the bandwidth even if Panel can support
> >> >> >> more lanes.
> >> >> >>
> >> >> >> The goal is to find out the number of lanes which can be supported
> >> >> >> using a particular cable so that we can cap 'max_available_lanes'
> >> >> >> to that number during modeset.
> >> >> >>
> >> >> >> These two patches are based on 4.2-rc2 and tested only on
> >> >> >> a BXT A1 platform for now.
> >> >> >>
> >> >> >> Brief summary of the approach taken:
> >> >> >> -----------------------------------
> >> >> >> 1.As soon as DP-hotplug is detected, driver starts link training
> >> >> >>   with highest number of lanes/bandwidth possible. If it fails,
> >> >> >>   driver retries link training with lane/2 for same bandwidth.
> >> >> >>   We continue this procedure until we find a working configuration
> >> >> >>   of lane/bandwidth values. This 'number of lanes' is then
> >> >> >>   set as the 'max_available_lanes', so that the following
> >> >> >>   intel_dp_compute_config() during modeset picks it up as
> >> >> >>   max_lane_count (instead of 4 always, from DPCD).
> >> >> >
> >> >> >Would all of this work automatically if our link training sequence
> >> >> >followed the DP spec to the letter wrt degrading the link on failures?
> >> >>
> >> >> That is one part of it.
> >> >>
> >> >> Our intention is also to filter out the modes that cannot be set
> >> >> with 'max_available_lanes' through connector->mode_valid
> >> >> callback, which uses these variables. Otherwise, we risk failing
> >> >> a modeset that uses higher resolutions than possible.
> >> >>
> >> >> Sorry, I should have also added this as part of the commit message.
> >> >
> >> >One approach to implement DP link training to the spec is that if things
> >> >fail we enable the pipe anyway (since anything else would seriously
> >> >surprise userspace, especially for async modesets, and lead to hangs in
> >> >userspace if vblank interrupts don't happen). And then we generate a
> >> >hotplug event to inform userspace that something change with the monitor
> >> >configuration, to give userspace a chance to look at the filtered mode
> >> >list and select a new config it likes.
> >> >
> >> >That approach would fit rather well into the overall framework of how
> >> >detection/mode-config changes are done currently by keeping all the policy
> >> >for selecting the precise mode config in userspace. Downside is that for
> >> >usb type-C it would cause flicker since if we only have 2 lanes we'll
> >> >always first try the high-res mode and fail. So I think in the end we need
> >>
> >> Yes, agreed.
> >>
> >> >both approaches. Wrt the rfc it would be great if we can make it at least
> >> >somewhat platform-agnostic - anything on big core since hsw+ supports
> >>
> >> By platform-agnostic,
> >>
> >> do you mean to try and implement _upfront_link_train()
> >> for few more platforms since HSW+ to see how we can re-use common code
> >> if any ?
> >>
> >> If it is something else, please elaborate a bit more..
> >>
> >> >enabling the DP port without enabling a pipe (because dp mst needs that),
> >> >so could support your approach here too.
> >>
> >> We have this kind of implementation tested in CHV and BXT.
> >> Can I consider at least the BXT part as a sample for HSW+ platforms ?
> >
> >Yeah, bxt works like hsw+ for ddi ports. So the idea is that extract the
> >link training for all of those with a high-level intel_* function used by
> >the upfront link training code. I did not know that we can enable the port
> >without a pipe on CHV though, do you have the code for that already?
> 
> Yes, we have a similar implementation working on CHV also.
> I chose to post BXT code since it is on a recent kernel (4.2-rc2) whereas
> our CHV implementation was on 3.14..

Would be interesting to post them all. Just to understand what the overall
requirements are.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch