Date:   Mon, 25 Oct 2021 22:11:47 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     Sam Ravnborg <sam@...nborg.org>,
        Douglas Anderson <dianders@...omium.org>,
        <dri-devel@...ts.freedesktop.org>,
        Philip Chen <philipchen@...omium.org>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        "Neil Armstrong" <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/bridge: Fix the bridge chain order for pre_enable /
 post_disable

Hi,

On 25.10.2021 13:21, Laurent Pinchart wrote:
> Hello,
>
> On Mon, Oct 25, 2021 at 01:00:10PM +0200, Andrzej Hajda wrote:
>> On 21.10.2021 22:21, Sam Ravnborg wrote:
>>> On Thu, Oct 21, 2021 at 12:29:01PM -0700, Douglas Anderson wrote:
>>>> Right now, the chaining order of
>>>> pre_enable/enable/disable/post_disable looks like this:
>>>>
>>>> pre_enable:   start from connector and move to encoder
>>>> enable:       start from encoder and move to connector
>>>> disable:      start from connector and move to encoder
>>>> post_disable: start from encoder and move to connector
>>>>
>>>> In the above, it can be seen that at least pre_enable() and
>>>> post_disable() are opposites of each other and enable() and disable()
>>>> are opposites. However, it seems broken that pre_enable() and enable()
>>>> would not move in the same direction. In other parts of Linux you can
>>>> see that various stages move in the same order. For instance, during
>>>> system suspend the "early" calls run in the same order as the normal
>>>> calls run in the same order as the "late" calls run in the same order
>>>> as the "noirq" calls.
>>>>
>>>> Let fix the above so that it makes more sense. Now we'll have:
>>>>
>>>> pre_enable:   start from encoder and move to connector
>>>> enable:       start from encoder and move to connector
>>>> disable:      start from connector and move to encoder
>>>> post_disable: start from connector and move to encoder
>>>>
>>>> This order is chosen because if there are parent-child relationships
>>>> anywhere I would expect that the encoder would be a parent and the
>>>> connector a child--not the other way around.
>>> This makes good sense as you describe it. I hope others can add more
>>> useful feedback.
>>> Added Andrzej Hajda <andrzej.hajda@...el.com> to the mail, as he have
>>> expressed concerns with the chain of bridges before.
>> Thanks Sam, but I am not sure about useful feedback - when I see bridge
>> chain issues it automatically triggers "whining mode" in my head :)
>>
>>>> This can be important when using the DP AUX bus to instantiate a
>>>> panel. The DP AUX bus is likely part of a bridge driver and is a
>>>> parent of the panel. We'd like the bridge to be pre_enabled before the
>>>> panel and the panel to be post_disabled before the
>>>> bridge. Specifically, this allows pm_runtime_put_sync_suspend() in a
>>>> bridge driver's post_suspend to work properly even a panel is under
>>>> it.
>>>>
>>>> NOTE: it's entirely possible that this change could break someone who
>>>> was relying on the old order. Hopefully this isn't the case, but if
>>>> this does break someone it seems like it's better to do it sonner
>>>> rather than later so we can fix everyone to handle the order that
>>>> makes the most sense.
>> It will break for sure. So the question is: if it is worth changing?
>>
>> New order seems good for eDP, DSI sinks [1], probably other as well.
>>
>> Old order is better for example for THC63LVD1024 [2 p. 20], I guess for
>> many other sinks as well.
>>
>> I am not even sure if it is protocol specific (LVDS, RGB, HDMI,...), or
>> it depends on specific hw pairs (source->sink).
>>
>> This is why I complain about the bridge chain - assumption that one
>> fixed call order will work for all setups seems to me ridiculous.
>>
>> Going back to the question - changing the order from fixed one to
>> another fixed one will not solve general issue.
>>
>> What can we do then?
>>
>> 1. Configurable call order? Probably doable: every chain element should
>> expose info if it's call should be before or after source, then some
>> core helper will create queue of callbacks. Seems quite complicated,
>> hides the logic from implementer and not fully flexible (for example,
>> there are protocols which require to perform sth on source, then on
>> sink, then again on the source).
>>
>> 2. Stop using bridge chain and call sink ops directly from the source
>> (this is what Exynos and VC4 do): is flexible and straightforward, gives
>> full control to the source.
> And breaks interoperability, because different sources end up calling
> operations in different orders. We end up having different sinks that
> expect calls in different ways, and divide the world in sink/source
> groups that don't interoperate :-(


I have an impression you describe current status :) More seriously, it 
is matter of proper specification/documentation/implementations of the 
operations. If we really need strict constraints we could try to 
implement them on protocol level.


>
>> 3. Use different abstractions to enforce proper initialization order
>> (like extending mipi_dsi_host_ops): requires existence of transport bus
>> abstraction (only DSI at the moment(?)).
> A real bus seems overkill, but we could have drm_bridge operations
> specific to particular hardware interfaces.
>
>> ... other ideas?
> I don't like it because of the amount of work it would require to switch
> to such a model, but I'm really starting to think that a variation of
> the second option would be best, where the sink controls the source
> instead of having the source controlling the sink. It's the sink that
> knows about its enabling/disabling sequence, and about how the source
> needs to be controlled to match it.


I am afraid it depends on the protocol and cross-calls (source->sink, 
sink->source) can be hard to avoid in case of some protocols.


>
>> Another idea, connected to the subject - some protocols require some
>> negotiations between source and sink bus format, or more steps than
>> pre_enable, enable ops to establish link. I wonder if encapsulating
>> drm_bridge in some protocol specific struct wouldn't be a solution, it
>> can be helpful as well in case of the subject.
>>
>> For example:
>>
>> struct drm_bridge_edp {
>>
>>       const struct drm_bridge_edp_funcs *funcs;
>>
>>       struct drm_bridge base;
>>
>>       ...
>>
>> };
>>
>> Then source could promote bridge pointer to bridge_edp pointer (if
>> applicable) and perform edp specific stuff. To make it working well
>> pre-enable order should be as proposed in this patchsets (encoder ->
>> connector), as the source should initiate negotiations.
>>
>> Btw this encapsulation stuff above asks to rename drm_bridge to
>> drm_sink, otherwise it would be confusing as bridges have two ends.
> drm_sink would be equally confusing when used for devices that have a
> sink and a source :-) I'm not against a rename though, if we can find a
> better name.


But in this case we are interested only in sink part of the bridge (OR 
panel). If source is looking for specific bridge or panel 
(drm_of_find_panel_or_bridge), it is in fact looking for sink.


Regards

Andrzej


>
>> Regards
>>
>> Andrzej
>>
>>
>> [1]: I use term sink as short equivalent for 'bridges AND panels'
>> (another issue in DRMs).
>>
>> [2]: https://www.mouser.com/datasheet/2/286/THC63LVD1024-1396205.pdf
>>
>>>> A FURTHER NOTE: Looking closer at commit 4e5763f03e10 ("drm/bridge:
>>>> ti-sn65dsi86: Wrap panel with panel-bridge") you can see that patch
>>>> inadvertently changed the order of things. The order used to be
>>>> correct (panel prepare was at the tail of the bridge enable) but it
>>>> became backwards. We'll restore the original order with this patch.
>>>>
>>>> Fixes: 4e5763f03e10 ("drm/bridge: ti-sn65dsi86: Wrap panel with panel-bridge")
>>>> Fixes: 05193dc38197 ("drm/bridge: Make the bridge chain a double-linked list")
>>>> Signed-off-by: Douglas Anderson <dianders@...omium.org>
>>> To make the patch complete the descriptions in drm_bridge_funcs
>>> need to be updated to reflect the new reality.
>>>
>>>> ---
>>>>
>>>>    drivers/gpu/drm/drm_bridge.c | 28 ++++++++++++++--------------
>>>>    1 file changed, 14 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>>>> index c96847fc0ebc..98808af59afd 100644
>>>> --- a/drivers/gpu/drm/drm_bridge.c
>>>> +++ b/drivers/gpu/drm/drm_bridge.c
>>>> @@ -583,18 +583,14 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_set);
>>>>    void drm_bridge_chain_pre_enable(struct drm_bridge *bridge)
>>> If you, or someone else, could r-b or ack the pending patches to remove
>>> this function, this part of the patch would no longer be needed.
>>>
>>>>    {
>>>>    	struct drm_encoder *encoder;
>>>> -	struct drm_bridge *iter;
>>>>    
>>>>    	if (!bridge)
>>>>    		return;
>>>>    
>>>>    	encoder = bridge->encoder;
>>>> -	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
>>>> -		if (iter->funcs->pre_enable)
>>>> -			iter->funcs->pre_enable(iter);
>>>> -
>>>> -		if (iter == bridge)
>>>> -			break;
>>>> +	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
>>>> +		if (bridge->funcs->pre_enable)
>>>> +			bridge->funcs->pre_enable(bridge);
>>>>    	}
>>>>    }
>>>>    EXPORT_SYMBOL(drm_bridge_chain_pre_enable);
>>>> @@ -684,26 +680,30 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
>>>>    					  struct drm_atomic_state *old_state)
>>>>    {
>>>>    	struct drm_encoder *encoder;
>>>> +	struct drm_bridge *iter;
>>> s/iter/bridge/ would make the patch simpler
>>> And then the bridge argument could be last_bridge or something.
>>> This would IMO increase readability of the code and make the patch smaller.
>>>>    
>>>>    	if (!bridge)
>>>>    		return;
>>>>    
>>>>    	encoder = bridge->encoder;
>>>> -	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
>>>> -		if (bridge->funcs->atomic_post_disable) {
>>>> +	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
>>>> +		if (iter->funcs->atomic_post_disable) {
>>>>    			struct drm_bridge_state *old_bridge_state;
>>>>    
>>>>    			old_bridge_state =
>>>>    				drm_atomic_get_old_bridge_state(old_state,
>>>> -								bridge);
>>>> +								iter);
>>>>    			if (WARN_ON(!old_bridge_state))
>>>>    				return;
>>>>    
>>>> -			bridge->funcs->atomic_post_disable(bridge,
>>>> -							   old_bridge_state);
>>>> -		} else if (bridge->funcs->post_disable) {
>>>> -			bridge->funcs->post_disable(bridge);
>>>> +			iter->funcs->atomic_post_disable(iter,
>>>> +							 old_bridge_state);
>>>> +		} else if (iter->funcs->post_disable) {
>>>> +			iter->funcs->post_disable(iter);
>>>>    		}
>>>> +
>>>> +		if (iter == bridge)
>>>> +			break;
>>> I cannot see why this is needed, we are at the end of the list here
>>> anyway.
>>>
>>>>    	}
>>>>    }
>>>>    EXPORT_SYMBOL(drm_atomic_bridge_chain_post_disable);

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