Date:   Wed, 19 Apr 2017 10:51:23 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        liviu.dudau@....com, laurent.pinchart@...asonboard.com,
        linux-media@...r.kernel.org
Subject: Re: [PATCH 1/6] drm: Add writeback connector type

On Tue, Apr 18, 2017 at 09:57:17PM +0200, Boris Brezillon wrote:
>Hi Brian,
>
>On Tue, 18 Apr 2017 18:34:43 +0100
>Brian Starkey <brian.starkey@....com> wrote:
>
>> >> @@ -214,6 +214,19 @@ struct drm_connector_state {
>> >>  	struct drm_encoder *best_encoder;
>> >>
>> >>  	struct drm_atomic_state *state;
>> >> +
>> >> +	/**
>> >> +	 * @writeback_job: Writeback job for writeback connectors
>> >> +	 *
>> >> +	 * Holds the framebuffer for a writeback connector. As the writeback
>> >> +	 * completion may be asynchronous to the normal commit cycle, the
>> >> +	 * writeback job lifetime is managed separately from the normal atomic
>> >> +	 * state by this object.
>> >> +	 *
>> >> +	 * See also: drm_writeback_queue_job() and
>> >> +	 * drm_writeback_signal_completion()
>> >> +	 */
>> >> +	struct drm_writeback_job *writeback_job;
>> >
>> >Maybe I'm wrong, but is feels weird to have the writeback_job field
>> >directly embedded in drm_connector_state, while drm_writeback_connector
>> >inherits from drm_connector.
>> >
>> >IMO, either you decide to directly put the drm_writeback_connector's
>> >job_xxx fields in drm_connector and keep the drm_connector_state as is,
>> >or you create a drm_writeback_connector_state which inherits from
>> >drm_connector_state and embeds the writeback_job field.
>>
>> I did spend a decent amount of time looking at tracking the writeback
>> state along with the normal connector state. I couldn't come up with
>> anything I liked.
>>
>> As the comment mentions, one of the problems is that you have to make
>> sure the relevant parts of the connector_state stay around until the
>> writeback is finished. That means you've got to block before
>> "swap_state()" until the previous writeback is done, and that
>> effectively limits your frame rate to refresh/2.
>>
>> The Mali-DP HW doesn't have that limitation - we can queue up a new
>> commit while the current writeback is ongoing. For that reason I
>> didn't want to impose such a limitation in the framework.
>>
>> In v1 I allowed that by making the Mali-DP driver hold its own
>> references to the relevant bits of the state for as long as it needed
>> them. In v3 I moved most of that code back to the core (in part
>> because Gustavo didn't like me signalling the DRM-"owned" fence from
>> my driver code directly). I think the new approach of "queue_job()"
>> and "signal_job()" reduces the amount of tricky code in drivers, and
>> is generally more clear (also familiar, when compared to vsync
>> events).
>>
>> I'm certain there's other ways to do it (refcount atomic states?), but
>> it seemed like a biggish overhaul to achieve what would basically be
>> the same thing.
>>
>> I was expecting each driver supporting writeback to have its own
>> different requirements around writeback lifetime/duration. For example
>> I think VC4 specifically came up, in that its writeback could take
>> several frames, whereas on Mali-DP we either finish within the frame
>> or we fail.
>>
>> Letting the driver manage its writeback_job lifetime seemed like a
>> reasonable way to handle all that, with the documentation stating the
>> only behaviour which is guaranteed to work on all drivers:
>>
>>    *     Userspace should wait for this fence to signal before making another
>>    *     commit affecting any of the same CRTCs, Planes or Connectors.
>>    *     **Failure to do so will result in undefined behaviour.**
>>    *     For this reason it is strongly recommended that all userspace
>>    *     applications making use of writeback connectors *always* retrieve an
>>    *     out-fence for the commit and use it appropriately.
>>
>>
>>
>> ... so all of that is why the _job fields don't live in a *_state
>> structure directly, and instead have to live in the separately-managed
>> structure pointed to by ->writeback_job.
>>
>> Now, I did look at creating drm_writeback_connector_state, but as it
>> would only be holding the job pointer (see above) it didn't seem worth
>> scattering around the
>>
>>     if (conn_state->connector->connector_type ==
>>         DRM_MODE_CONNECTOR_WRITEBACK)
>>
>> checks everywhere before up-casting - {clear,reset,duplicate}_state(),
>> prepare_signalling(), complete_signalling(), etc. It just touched a
>> lot of code for the sake of an extra pointer field in each connector
>> state.
>>
>> I can easily revisit that part if you like.
>
>I think there's a misunderstanding. I was just suggesting to be
>consistent in the inheritance vs 'one object to handle everything'
>approach.

doh.. right yeah I misread. Sorry for the tangent. :-)

>
>I'm perfectly fine with embedding the writeback_job pointer directly in
>drm_connector_state, but then it would IMO make more sense to do the
>same for the drm_connector object (embed drm_writeback_connector fields
>into drm_connector instead of making drm_writeback_connector inherit
>from drm_connector).
>

I agree that it's inconsistent. I guess I did it out of pragmatism -
there's quite a lot of new fields in drm_writeback_connector, and the
code needed to support it was comparatively small. On the other hand
there's only one additional field in drm_connector_state and the code
required to subclass it looked comparatively large.

>Anyway, that's just a detail.
>
>>
>> >
>> >Anyway, wait for Daniel's feedback before doing this change.
>> >
>>
>> Am I expecting some more feedback from Daniel?
>
>No, I was just saying that before doing the changes I was suggesting,
>you should wait for Daniel's opinion, because I might be wrong.
>
>>
>> >>  };
>> >>
>> >>  /**
>> >> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
>> >> index bf9991b2..3d3d07f 100644
>> >> --- a/include/drm/drm_mode_config.h
>> >> +++ b/include/drm/drm_mode_config.h
>> >> @@ -634,6 +634,20 @@ struct drm_mode_config {
>> >>  	 */
>> >>  	struct drm_property *suggested_y_property;
>> >>
>> >> +	/**
>> >> +	 * @writeback_fb_id_property: Property for writeback connectors, storing
>> >> +	 * the ID of the output framebuffer.
>> >> +	 * See also: drm_writeback_connector_init()
>> >> +	 */
>> >> +	struct drm_property *writeback_fb_id_property;
>> >> +	/**
>> >> +	 * @writeback_pixel_formats_property: Property for writeback connectors,
>> >> +	 * storing an array of the supported pixel formats for the writeback
>> >> +	 * engine (read-only).
>> >> +	 * See also: drm_writeback_connector_init()
>> >> +	 */
>> >> +	struct drm_property *writeback_pixel_formats_property;
>> >> +
>> >>  	/* dumb ioctl parameters */
>> >>  	uint32_t preferred_depth, prefer_shadow;
>> >>
>> >> diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
>> >> new file mode 100644
>> >> index 0000000..6b2ac45
>> >> --- /dev/null
>> >> +++ b/include/drm/drm_writeback.h
>> >> @@ -0,0 +1,78 @@
>> >> +/*
>> >> + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
>> >> + * Author: Brian Starkey <brian.starkey@....com>
>> >> + *
>> >> + * This program is free software and is provided to you under the terms of the
>> >> + * GNU General Public License version 2 as published by the Free Software
>> >> + * Foundation, and any use by you of this program is subject to the terms
>> >> + * of such GNU licence.
>> >> + */
>> >> +
>> >> +#ifndef __DRM_WRITEBACK_H__
>> >> +#define __DRM_WRITEBACK_H__
>> >> +#include <drm/drm_connector.h>
>> >> +#include <linux/workqueue.h>
>> >> +
>> >> +struct drm_writeback_connector {
>> >> +	struct drm_connector base;
>> >
>> >AFAIU, a writeback connector will always require an 'dummy' encoder to
>> >make the DRM framework happy (AFAIK, a connector is always connected to
>> >a CRTC through an encoder).
>> >
>> >Wouldn't it make more sense to have a drm_encoder object embedded in
>> >drm_writeback_connector so that people don't have to declare an extra
>> >structure containing both the drm_writeback_connector connector and a
>> >drm_encoder? Is there a good reason to keep them separate?
>> >
>>
>> Yeah that's not a bad idea. The encoder funcs could be passed in to
>> drm_writeback_connector_init() (in which case adding a writeback
>> encoder type would also make sense).
>
>Well, the more I look at it the more I find it weird to represent this
>writeback feature as a connector. To me, it seems to be closer to a
>plane object (DRM_PLANE_TYPE_WRITEBACK?) than a real connector.
>Actually, the Atmel HLCDC IP use the same register interface to expose
>the overlay planes and writeback features.
>Of course, representing it as a plane requires patching the core to
>allow enabling a CRTC that has no active connectors connected to it if
>at least one writeback plane is enabled and connected to the CRTC, but
>it should be doable.

Could you expand a bit on how you think planes fit better? It is
something we've previously talked about internally, but so far I'm not
convinced :-)

>By doing that, we would also get rid of these fake connector/encoder
>objects as well as the fake modes we are returning in
>connector->get_modes().

What makes the connector/encoder fake? They represent a real piece of
hardware just the same as a drm_plane would.

I don't mind dropping the mode list and letting userspace just make
up whatever timing it wants - it'd need to do the same if writeback
was a plane - but in some respects giving it a list of modes the same
way we normally do seems nicer for userspace.

>
>As far as I can tell, the VC4 and Atmel HLCDC IP should fit pretty well
>in this model, not sure about the mali-dp though.
>
>Brian, did you consider this approach, and if you did, can you detail
>why you decided to expose this feature as a connector?
>
>Daniel (or anyone else), please step in if you think this is a stupid
>idea :-).

Ville originally suggested using a connector, which Eric followed up
by saying that's what he was thinking of for VC4 writeback too[1].
That was my initial reason for focussing on a connector-based
approach.

I prefer connector over plane conceptually because it keeps with the
established data flow: planes are sources, connectors are sinks.

In some respects the plane _object_ looks like it would fit - it has a
pixel format list and an FB_ID. But everything else would be acting
the exact opposite to a normal plane, and I think there's a bunch of
baked-in assumptions in the kernel and userspace around CRTCs always
having at least one connector.

On the other hand, a writeback connector gains a few extra properties
over a normal connector, but most stuff stays the same. The pipeline
setup looks the same as normal to userspace, you don't need a CRTC to
be active with no connectors, output cloning is the same etc.

Thanks,
-Brian

[1] https://lists.freedesktop.org/archives/dri-devel/2016-July/113329.html

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