Date:   Sat, 10 Jun 2017 22:57:28 +0800
From:   icenowy@...c.io
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     devicetree@...r.kernel.org,
        Jernej Škrabec <jernej.skrabec@...l.net>,
        linux-sunxi@...glegroups.com, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, Chen-Yu Tsai <wens@...e.org>,
        Rob Herring <robh+dt@...nel.org>, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 03/11] drm: sun4i: ignore swapped mixer<->tcon
 connection for DE2

在 2017-06-09 22:46，Maxime Ripard 写道：
> On Thu, Jun 08, 2017 at 01:01:53PM +0800, icenowy@...c.io wrote:
>> 在 2017-06-07 22:38，Maxime Ripard 写道：
>> > On Wed, Jun 07, 2017 at 06:01:02PM +0800, Icenowy Zheng wrote:
>> > > >I have no idea what this is supposed to be doing either.
>> > > >
>> > > >I might be wrong, but I really feel like there's a big mismatch
>> > > >between your commit log, and what you actually implement.
>> > > >
>> > > >In your commit log, you should state:
>> > > >
>> > > >A) What is the current behaviour
>> > > >B) Why that is a problem
>> > > >C) How do you address it
>> > > >
>> > > >And you don't.
>> > > >
>> > > >However, after discussing it with Chen-Yu, it seems like you're trying
>> > > >to have all the mixers probed before the TCONs. If that is so, there's
>> > > >nothing specific to the H3 here, and we also have the same issue on
>> > > >dual-pipeline DE1 (A10, A20, A31). Chen-Yu worked on that a bit, but
>> > > >the easiest solution would be to move from a DFS algorithm to walk
>> > > >down the graph to a BFS one.
>> > > >
>> > > >That way, we would add all mixers first, then the TCONs, then the
>> > > >encoders, and the component framework will probe them in order.
>> > >
>> > > No. I said that they're swappable, however, I don't want to
>> > > implement the swap now, but hardcode 0-0 1-1 connection.
>> >
>> > We're on the same page, it's definitely not what I was mentionning
>> > here. This would require a significant rework, and the usecase is
>> > still unclear for now.
>> >
>> > > However, as you and Chen-Yu said, device tree should reflect the
>> > > real hardware, there will be bonus endpoints for the swapped
>> > > connection.
>> >
>> > If by bonus you mean connections from mixer 0 to tcon 1 and mixer 1 to
>> > tcon 0, then yes, we're going to need it.
>> >
>> > > What I want to do is to ignore the bonus connection, in order to
>> > > prevent them from confusing the code.
>> > >
>> > > If you just change the bind sequence, I think it cannot be
>> > > prevented that wrong connections will be bound.
>> >
>> > This is where I don't follow you anymore. The component framework
>> > doesn't list connections but devices. The swapped connections do not
>> > matter here, we have the same set of devices: mixer0, mixer1, tcon0
>> > and tcon1.
>> >
>> > The thing that does change with your patch is that before, the binding
>> > sequence would have been mixer0, tcon0, tcon1, mixer1. With your
>> > patch, it's mixer0, tcon0, mixer1, tcon1.
>> >
>> > So, again, stating what issue you were seeing before making this patch
>> > would be very helpful to see what you're trying to do / fix.
>> 
>> So maybe I can drop the forward search (searching output) code, and 
>> keep
>> only the backward search (search input) code in TCON?
>> 
>> Forward search code is only used when binding, but backward search is 
>> used
>> for TCON to find connected mixer.
> 
> It is hard to talk about a solution, when it's not clear what the
> issue is.
> 
> So please state
>> > > >A) What is the current behaviour
>> > > >B) Why that is a problem
>> > > >C) How do you address it
> 
> We'll talk about a solution once this is done.

(All those things are based on the assumption that mixer0, mixer1, tcon0
and tcon1 are all enabled in DT. If one group of mixer-tcon pair is 
fully
disabled in DT it will behave properly.)

For the backward search:

A) The current behaviour is to take the first engine found, which will 
be
wrong in the situation of tcon1 if mixer0 and mixer1 are both enabled:
mixer0 is taken for tcon1 instead of mixer1.

B) It takes mixer0 as it matches the first endpoint of tcon0's input.

C) It's a logic failure in the backward search, as it only considered
the DE1 situation, in which TCONs will only have one engine as input.

For the bind process:

A) The current behaviour is to try to bind all output endpoints of the
engine, during binding all outputs of mixer0, these will happen:
   1. tcon1 is bound with mixer0 as its engine if backward searching
   is not fixed.
   2. tcon1 fails to be bound as its engine is not yet bound when
   backward searching works properly, then sun4i_drv will refuse
   to continue as a component is not properly bound.
B) The binding process in sun4i_drv will bind a component that is not
really an working output of the forward component, but only exists in
the endpoint list as a theortically possible output (in fact not an
real output).
C) I tested with this patch's sun4i_drv_node_is_swappable_de2_mixer
function masked (always return false), and then the multiple
mixer+tcon situations don't work properly.

P.S. I think the BFS solution is really a dirty hack -- although we
bind components, not connections, we should decide the next component
to bind according to the connections -- not really connected
components shouldn't be bound.

For example, if we enabled mixer0, tcon0 and tcon1, tcon1 shouldn't
be bound at all. However in BFS situation tcon1 will also be bound
and then fail to be bound if the backward engine searching is fixed.

> 
> Maxime

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