linux-kernel - [PATCH] component: Support masters with no subcomponents

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Date: Wed, 17 Apr 2024 11:12:09 +0200
From: Herman van Hazendonk <github.com@...rie.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 "Rafael J. Wysocki" <rafael@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>, 
 Herman van Hazendonk <github.com@...rie.org>
Subject: [PATCH] component: Support masters with no subcomponents

This happens in the MSM DRM driver when it is used
without any subcomponents, which is a special corner
case.

If the MDP4 is used with nothing but the LVDS display,
we get this problem that no components are found since
the LVDS is part of the MDP4 itself.

We cannot use a NULL match, so create a dummy match
with no components for this case so the driver will
still probe nicely without adding a secondary
complicated probe() path to the MSM DRM driver.

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
Signed-off-by: Herman van Hazendonk <github.com@...rie.org>
---
This happens in the MSM DRM driver when it is used
without any subcomponents, which is a special corner
case.

If the MDP4 is used with nothing but the LVDS display,
we get this problem that no components are found since
the LVDS is part of the MDP4 itself.

We cannot use a NULL match, so create a dummy match
with no components for this case so the driver will
still probe nicely without adding a secondary
complicated probe() path to the MSM DRM driver.
---
 drivers/base/component.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/base/component.c b/drivers/base/component.c
index 741497324d78..bb79e7a77bb0 100644
--- a/drivers/base/component.c
+++ b/drivers/base/component.c
@@ -497,6 +497,10 @@ static void free_aggregate_device(struct aggregate_device *adev)
 	kfree(adev);
 }

+static struct component_match dummy_match = {
+	.num = 0,
+};
+
 /**
  * component_master_add_with_match - register an aggregate driver
  * @parent: parent device of the aggregate driver
@@ -516,6 +520,9 @@ int component_master_add_with_match(struct device *parent,
 	struct aggregate_device *adev;
 	int ret;

+	if (!match)
+		match = &dummy_match;
+
 	/* Reallocate the match array for its true size */
 	ret = component_match_realloc(match, match->num);
 	if (ret)

---
base-commit: 96fca68c4fbf77a8185eb10f7557e23352732ea2
change-id: 20240417-component-dummy-a9aae5ac7234

Best regards,
-- 
Herman van Hazendonk <github.com@...rie.org>