linux-kernel - [PATCH RESEND] Revert "drm/bridge: ti-sn65dsi83: Fix enable error path"

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Message-Id: <20240426122259.46808-1-luca.ceresoli@bootlin.com>
Date: Fri, 26 Apr 2024 14:22:59 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Andrzej Hajda <andrzej.hajda@...el.com>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Robert Foss <rfoss@...nel.org>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Jonas Karlman <jonas@...boo.se>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Alexander Stein <alexander.stein@...tq-group.com>
Cc: Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH RESEND] Revert "drm/bridge: ti-sn65dsi83: Fix enable error path"

This reverts commit 8a91b29f1f50ce7742cdbe5cf11d17f128511f3f.

The regulator_disable() added by the original commit solves one kind of
regulator imbalance but adds another one as it allows the regulator to be
disabled one more time than it is enabled in the following scenario:

 1. Start video pipeline -> sn65dsi83_atomic_pre_enable -> regulator_enable
 2. PLL lock fails -> regulator_disable
 3. Stop video pipeline -> sn65dsi83_atomic_disable -> regulator_disable

The reason is clear from the code flow, which looks like this (after
removing unrelated code):

  static void sn65dsi83_atomic_pre_enable()
  {
      regulator_enable(ctx->vcc);

      if (PLL failed locking) {
          regulator_disable(ctx->vcc);  <---- added by patch being reverted
          return;
      }
  }

  static void sn65dsi83_atomic_disable()
  {
      regulator_disable(ctx->vcc);
  }

The use case for introducing the additional regulator_disable() was
removing the module for debugging (see link below for the discussion). If
the module is removed after a .atomic_pre_enable, i.e. with an active
pipeline from the DRM point of view, .atomic_disable is not called and thus
the regulator would not be disabled.

According to the discussion however there is no actual use case for
removing the module with an active pipeline, except for
debugging/development.

On the other hand, the occurrence of a PLL lock failure is possible due to
any physical reason (e.g. a temporary hardware failure for electrical
reasons) so handling it gracefully should be supported. As there is no way
for .atomic[_pre]_enable to report an error to the core, the only clean way
to support it is calling regulator_disabled() only in .atomic_disable,
unconditionally, as it was before.

Link: https://lore.kernel.org/all/15244220.uLZWGnKmhe@steina-w/
Fixes: 8a91b29f1f50 ("drm/bridge: ti-sn65dsi83: Fix enable error path")
Reviewed-by: Alexander Stein <alexander.stein@...tq-group.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---

Many thanks to Alexander for the discussion.
---
 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index 4814b7b6d1fd..57a7ed13f996 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -478,7 +478,6 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 		dev_err(ctx->dev, "failed to lock PLL, ret=%i\n", ret);
 		/* On failure, disable PLL again and exit. */
 		regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00);
-		regulator_disable(ctx->vcc);
 		return;
 	}
 
-- 
2.34.1