[PATCH v3 1/2] drm: bridge: Allow daisy chaining of bridges

[Daniel Vetter]

On Tue, May 19, 2015 at 04:37:44PM +0530, Archit Taneja wrote:
> On 05/19/2015 03:04 PM, Daniel Vetter wrote:
> >On Tue, May 19, 2015 at 02:05:04PM +0530, Archit Taneja wrote:
> >>+void drm_bridge_post_disable(struct drm_bridge *bridge)
> >>+{
> >>+	if (!bridge)
> >>+		return;
> >>+
> >>+	drm_bridge_post_disable(bridge->next);
> >>+
> >>+	bridge->funcs->post_disable(bridge);
> >
> >For symmetry I'd call the post_disable hook for the next brigde _after_
> >this one. Otherwise we break abstraction. E.g. with 1 bridge we'd have
> >
> >bridgeA_disable();
> >encoder_disable();
> >bridgeA_post_disable();
> >
> >But if on some other part bridge A is connected to bridge B (i.e. we
> >replace the encoder with a combo of other_encoder+bridgeA) with your code
> >the post_disable is suddenly interleaved with the preceeding stages in the
> >pipeline:
> >
> >
> >bridgeA_disable();
> >bridgeB_disable();
> >other_encoder_disable();
> >bridgeA_post_disable();
> >bridgeB_post_disable();
> >
> >Which means from the pov of bridgeA there's a difference between whether
> >it's connected to "encoder" or "other_encoder+bridgeB". But if we reorder
> >the post_disable sequence like I suggest we'll get:
> >
> >bridgeA_disable();
> >bridgeB_disable();
> >other_encoder_disable();
> >bridgeB_post_disable();
> >bridgeA_post_disable();
> >
> >Which means that "encoder" and "other_encoder+bridgeB" look the same for
> >bridgeA. To avoid confusion the two pipelines in hw are:
> >
> >encoder ---> bridgeA
> >
> >other_encoder ---> bridgeB ---> bridgeA
> 
> I agree with this, thanks for the explanation. Although, I'm not sure about
> the pre_enable part below.
> 
> <snip>
> 
> >>+void drm_bridge_pre_enable(struct drm_bridge *bridge)
> >>+{
> >>+	if (!bridge)
> >>+		return;
> >>+
> >>+	bridge->funcs->pre_enable(bridge);
> >>+
> >>+	drm_bridge_pre_enable(bridge->next);
> >
> >Same as with post_disable, pre_enable for bridge->next should be called
> >_before_ the pre_enable for this bridge.
> 
> 
> The order of nesting suggested by you gives the sequence:
> 
> bridgeA_pre_enable();
> bridgeB_pre_enable();
> other_encoder_enable();
> bridgeB_enable();
> bridgeA_enable();
> 
> This won't work if the bridge A relies on bridge B to be enabled first. This
> happens in the encoder path I'm working on:
> 
> msm mdp5 encoder -> dsi bridge -> adv7511 dsi to hdmi bridge chip
> 
> 
> The adv chip relies on dsi's clock for it to function. If dsi's pre_enable()
> isn't called first, then adv's pre_enable would fail.

If you need the clock, then imo that's code which should be in the enable
hook, and not in the pre-enable hook. At least thus far the rules have
roughly been:
- pre_enable: anything that needs to be done _before_ clock+timings are
  enabled by the source for this bridge.
- enable: anything that needs clock+timings to be on from the source for
  this bridge.

Similar for the disable side:
- disable still has clocks+timing on.
- post_disable should be called after clocks are off.

In i915 we once had a callback in between clock enabling and timing
enabling, but we've removed that again after some restructuring. Maybe we
need such a hook bridges? But that also means we need to split up
encoder/crtc callbacks, and that will get nasty real fast.

> We could modify the call order in drm_bridge_enable() instead to achieve:
> 
> bridgeB_pre_enable();
> bridgeA_pre_enable();
> other_encoder_enable();
> bridgeA_enable();
> bridgeB_enable();
> 
> This fixes the sequence for pre_enable() calls, but assumes that the
> configurations in the enable() don't need to follow a specific sequence to
> ensure proper behavior.
> 
> pre_enable() should ideally represent things to be done before we enable the
> next encoder in the chain. So the sequence right above seems to be better.

Nah, that's even more backwards imo. From bridgeA's pov it really should
make no difference at all whether the input is directly from an encoder or
whether it's "other_encoder+bridgeB". If we leak this detail down the
bridge chain, then the abstraction is leaky and bridge drivers will be
impossible to be reused.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch