Re: [PATCH 1/2] drm: Introduce DRM_BRIDGE_OP_UPSTREAM_FIRST to alter bridge init order

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On 22.02.2022 09:43, Dave Stevenson wrote:

Hi Laurent.

Thanks for the review.

On Tue, 22 Feb 2022 at 06:34, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:

Hi Dave,

Thank you for the patch.

On Wed, Feb 16, 2022 at 04:59:43PM +0000, Dave Stevenson wrote:

DSI sink devices typically want the DSI host powered up and configured
before they are powered up. pre_enable is the place this would normally
happen, but they are called in reverse order from panel/connector towards
the encoder, which is the "wrong" order.

Add a new flag DRM_BRIDGE_OP_UPSTREAM_FIRST that any bridge can set
to swap the order of pre_enable (and post_disable) so that any upstream
bridges are called first to create the desired state.

eg:
- Panel
- Bridge 1
- Bridge 2 DRM_BRIDGE_OP_UPSTREAM_FIRST
- Bridge 3
- Encoder
Would result in pre_enable's being called as Panel, Bridge 1, Bridge 3,
Bridge 2.

If there was a Bridge 4 between Bridge 3 and Encoder, would it be

Panel, Bridge 1, Bridge 3, Bridge 4, Bridge 2

? I'd capture that here, to be explicit.

No.
  - Panel
  - Bridge 1
  - Bridge 2 DRM_BRIDGE_OP_UPSTREAM_FIRST
  - Bridge 3
  - Bridge 4
   - Encoder
Would result in pre_enable's being called as Panel, Bridge 1, Bridge
3, Bridge 2, Bridge 4, Encoder.
ie it only swaps the order of bridges 2 & 3.

  - Panel
  - Bridge 1
  - Bridge 2 DRM_BRIDGE_OP_UPSTREAM_FIRST
  - Bridge 3 DRM_BRIDGE_OP_UPSTREAM_FIRST
  - Bridge 4
  - Encoder
Would result in pre_enable's being called as Panel, Bridge 1, Bridge
4, Bridge 3, Bridge 2, Encoder.
(Bridge 2&3 have asked for upstream to be enabled first, which means
bridge 4. Bridge 2 wants upstream enabled first, which means bridge
3).

  - Panel
  - Bridge 1
  - Bridge 2 DRM_BRIDGE_OP_UPSTREAM_FIRST
  - Bridge 3
  - Bridge 4 DRM_BRIDGE_OP_UPSTREAM_FIRST
  - Bridge 5
  - Encoder
Would result in Panel, Bridge 1, Bridge 3, Bridge 2, Bridge 5, Bridge
4, Encoder.

So we only reverse the order whilst the bridges request that they want
upstream enabled first, but we can do that multiple times within the
chain. I hope that makes sense.


Signed-off-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/drm_bridge.c | 197 +++++++++++++++++++++++++++++++++++++------
  include/drm/drm_bridge.h     |   8 ++
  2 files changed, 180 insertions(+), 25 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index c96847fc0ebc..7c24e8340efa 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -522,21 +522,58 @@ EXPORT_SYMBOL(drm_bridge_chain_disable);
   * Calls &drm_bridge_funcs.post_disable op for all the bridges in the
   * encoder chain, starting from the first bridge to the last. These are called
   * after completing the encoder's prepare op.

Missing blank line, as well as in three locations below.


+ * If a bridge sets the DRM_BRIDGE_OP_UPSTREAM_FIRST, then the post_disable for
+ * that bridge will be called before the previous one to reverse the pre_enable
+ * calling direction.
   *
   * Note: the bridge passed should be the one closest to the encoder
   */
  void drm_bridge_chain_post_disable(struct drm_bridge *bridge)
  {
       struct drm_encoder *encoder;
+     struct drm_bridge *next, *limit;

       if (!bridge)
               return;

       encoder = bridge->encoder;
       list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+             limit = NULL;
+
+             if (!list_is_last(&bridge->chain_node, &encoder->bridge_chain)) {
+                     next = list_next_entry(bridge, chain_node);
+
+                     if (next->ops & DRM_BRIDGE_OP_UPSTREAM_FIRST) {
+                             limit = next;
+
+                             list_for_each_entry_from(next, &encoder->bridge_chain,
+                                                      chain_node) {
+                                     if (!(next->ops &
+                                             DRM_BRIDGE_OP_UPSTREAM_FIRST)) {
+                                             next = list_prev_entry(next, chain_node);
+                                             limit = next;
+                                             break;
+                                     }
+                             }
+
+                             list_for_each_entry_from_reverse(next, &encoder->bridge_chain,
+                                                              chain_node) {
+                                     if (next == bridge)
+                                             break;
+
+                                     if (next->funcs->post_disable)
+                                             next->funcs->post_disable(next);
+                             }
+                     }
+             }
+
               if (bridge->funcs->post_disable)
                       bridge->funcs->post_disable(bridge);
+
+             if (limit)
+                     bridge = limit;
       }
+
  }
  EXPORT_SYMBOL(drm_bridge_chain_post_disable);

@@ -577,22 +614,53 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_set);
   * Calls &drm_bridge_funcs.pre_enable op for all the bridges in the encoder
   * chain, starting from the last bridge to the first. These are called
   * before calling the encoder's commit op.
+ * If a bridge sets the DRM_BRIDGE_OP_UPSTREAM_FIRST, then the pre_enable for
+ * the previous bridge will be called before pre_enable of this bridge.
   *
   * Note: the bridge passed should be the one closest to the encoder
   */
  void drm_bridge_chain_pre_enable(struct drm_bridge *bridge)
  {
       struct drm_encoder *encoder;
-     struct drm_bridge *iter;
+     struct drm_bridge *iter, *next, *limit;

       if (!bridge)
               return;

       encoder = bridge->encoder;
+
       list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
+             if (iter->ops & DRM_BRIDGE_OP_UPSTREAM_FIRST) {
+                     next = iter;
+                     limit = bridge;
+                     list_for_each_entry_from_reverse(next,
+                                                      &encoder->bridge_chain,
+                                                      chain_node) {
+                             if (next == bridge)
+                                     break;
+
+                             if (!(next->ops &
+                                     DRM_BRIDGE_OP_UPSTREAM_FIRST)) {
+                                     limit = list_prev_entry(next, chain_node);
+                                     break;
+                             }
+                     }
+
+                     list_for_each_entry_from(next, &encoder->bridge_chain, chain_node) {
+                             if (next == iter)
+                                     break;
+
+                             if (next->funcs->pre_enable)
+                                     next->funcs->pre_enable(next);
+                     }
+             }
+
               if (iter->funcs->pre_enable)
                       iter->funcs->pre_enable(iter);

+             if (iter->ops & DRM_BRIDGE_OP_UPSTREAM_FIRST)
+                     iter = limit;
+
               if (iter == bridge)
                       break;
       }
@@ -667,6 +735,25 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge,
  }
  EXPORT_SYMBOL(drm_atomic_bridge_chain_disable);

+static void drm_atomic_bridge_call_post_disable(struct drm_bridge *bridge,
+                                             struct drm_atomic_state *old_state)
+{
+     if (bridge->funcs->atomic_post_disable) {
+             struct drm_bridge_state *old_bridge_state;
+
+             old_bridge_state =
+                     drm_atomic_get_old_bridge_state(old_state,
+                                                     bridge);
+             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);
+     }
+}
+
  /**
   * drm_atomic_bridge_chain_post_disable - cleans up after disabling all bridges
   *                                     in the encoder chain
@@ -677,6 +764,9 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_disable);
   * &drm_bridge_funcs.post_disable) op for all the bridges in the encoder chain,
   * starting from the first bridge to the last. These are called after completing
   * &drm_encoder_helper_funcs.atomic_disable
+ * If a bridge sets the DRM_BRIDGE_OP_UPSTREAM_FIRST, then the post_disable for
+ * that bridge will be called before the previous one to reverse the pre_enable
+ * calling direction.
   *
   * Note: the bridge passed should be the one closest to the encoder
   */
@@ -684,30 +774,69 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
                                         struct drm_atomic_state *old_state)
  {
       struct drm_encoder *encoder;
+     struct drm_bridge *next, *limit;

       if (!bridge)
               return;

       encoder = bridge->encoder;
+
       list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
-             if (bridge->funcs->atomic_post_disable) {
-                     struct drm_bridge_state *old_bridge_state;
+             limit = NULL;
+
+             if (!list_is_last(&bridge->chain_node, &encoder->bridge_chain)) {
+                     next = list_next_entry(bridge, chain_node);
+
+                     if (next->ops & DRM_BRIDGE_OP_UPSTREAM_FIRST) {
+                             limit = next;
+
+                             list_for_each_entry_from(next, &encoder->bridge_chain,
+                                                      chain_node) {
+                                     if (!(next->ops &
+                                             DRM_BRIDGE_OP_UPSTREAM_FIRST)) {
+                                             next = list_prev_entry(next, chain_node);
+                                             limit = next;
+                                             break;
+                                     }
+                             }
+
+                             list_for_each_entry_from_reverse(next, &encoder->bridge_chain,
+                                                              chain_node) {
+                                     if (next == bridge)
+                                             break;
+
+                                     drm_atomic_bridge_call_post_disable(next,
+                                                                         old_state);
+                             }
+                     }
+             }

-                     old_bridge_state =
-                             drm_atomic_get_old_bridge_state(old_state,
-                                                             bridge);
-                     if (WARN_ON(!old_bridge_state))
-                             return;
+             drm_atomic_bridge_call_post_disable(bridge, old_state);

-                     bridge->funcs->atomic_post_disable(bridge,
-                                                        old_bridge_state);
-             } else if (bridge->funcs->post_disable) {
-                     bridge->funcs->post_disable(bridge);
-             }
+             if (limit)
+                     bridge = limit;
       }
  }
  EXPORT_SYMBOL(drm_atomic_bridge_chain_post_disable);

+static void drm_atomic_bridge_call_pre_enable(struct drm_bridge *bridge,
+                                           struct drm_atomic_state *old_state)
+{
+     if (bridge->funcs->atomic_pre_enable) {
+             struct drm_bridge_state *old_bridge_state;
+
+             old_bridge_state =
+                     drm_atomic_get_old_bridge_state(old_state,
+                                                     bridge);
+             if (WARN_ON(!old_bridge_state))
+                     return;
+
+             bridge->funcs->atomic_pre_enable(bridge, old_bridge_state);
+     } else if (bridge->funcs->pre_enable) {
+             bridge->funcs->pre_enable(bridge);
+     }
+}
+
  /**
   * drm_atomic_bridge_chain_pre_enable - prepares for enabling all bridges in
   *                                   the encoder chain
@@ -718,6 +847,8 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_post_disable);
   * &drm_bridge_funcs.pre_enable) op for all the bridges in the encoder chain,
   * starting from the last bridge to the first. These are called before calling
   * &drm_encoder_helper_funcs.atomic_enable
+ * If a bridge sets the DRM_BRIDGE_OP_UPSTREAM_FIRST, then the pre_enable for
+ * the previous bridge will be called before pre_enable of this bridge.
   *
   * Note: the bridge passed should be the one closest to the encoder
   */
@@ -725,26 +856,42 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
                                       struct drm_atomic_state *old_state)
  {
       struct drm_encoder *encoder;
-     struct drm_bridge *iter;
+     struct drm_bridge *iter, *next, *limit;

       if (!bridge)
               return;

       encoder = bridge->encoder;
+
       list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
-             if (iter->funcs->atomic_pre_enable) {
-                     struct drm_bridge_state *old_bridge_state;
+             if (iter->ops & DRM_BRIDGE_OP_UPSTREAM_FIRST) {
+                     next = iter;
+                     limit = bridge;
+                     list_for_each_entry_from_reverse(next,
+                                                      &encoder->bridge_chain,
+                                                      chain_node) {
+                             if (next == bridge)
+                                     break;
+
+                             if (!(next->ops &
+                                     DRM_BRIDGE_OP_UPSTREAM_FIRST)) {
+                                     limit = list_prev_entry(next, chain_node);
+                                     break;
+                             }
+                     }
+
+                     list_for_each_entry_from(next, &encoder->bridge_chain, chain_node) {
+                             if (next == iter)
+                                     break;
+
+                             drm_atomic_bridge_call_pre_enable(next, old_state);
+                     }
+             }

This is hard to understand, I have trouble figuring out if it does the
right thing when multiple bridges set the DRM_BRIDGE_OP_UPSTREAM_FIRST
flag (or actually even when a single bridge does so). Comments would
help, but I wonder if it wouldn't be simpler to switch to a recursive
implementation.

Recursive - such joys!
With the explanation above, I'm not sure that recursive helps, but
certainly can add comments.


It could be recursive, or just proper iteration, for example pseudocode:

list_for_each_entry_reverse(...) {
	if (iter->upstream_first && iter->prev)
		continue;
	tmp = iter;
	do {
		call_op
		tmp = tmp->next;
	} while (tmp && tmp->upstream_first);
}


Regards
Andrzej
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