Message-ID: <20170418215717.4381dd6e@bbrezillon>
Date:   Tue, 18 Apr 2017 21:57:17 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Brian Starkey <brian.starkey@....com>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     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

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.

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).

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.
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().

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 :-).

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