linux-kernel - [PATCH] gpu: host1x: Allow loading tegra-drm without enabled engines

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Message-Id: <20250708-host1x-allow-no-subdevs-v1-1-93c66c251f03@nvidia.com>
Date: Tue, 08 Jul 2025 21:18:08 +0900
From: Mikko Perttunen <cyndis@...si.fi>
To: Thierry Reding <thierry.reding@...il.com>
Cc: dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Vamsee Vardhan Thummala <vthummala@...dia.com>, 
 Mikko Perttunen <mperttunen@...dia.com>
Subject: [PATCH] gpu: host1x: Allow loading tegra-drm without enabled
 engines

From: Vamsee Vardhan Thummala <vthummala@...dia.com>

Add support to register host1x devices without requiring subdevices.
This ensures syncpoint functionality remains available even when engine
subdevices are not present.

Add softdep for tegra-drm to make userspace interface available
without module autoloading through device tree entries.

Signed-off-by: Vamsee Vardhan Thummala <vthummala@...dia.com>
[mperttunen@...dia.com: some rewording]
Signed-off-by: Mikko Perttunen <mperttunen@...dia.com>
---
 drivers/gpu/host1x/bus.c | 12 ++++++++++++
 drivers/gpu/host1x/dev.c |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/gpu/host1x/bus.c b/drivers/gpu/host1x/bus.c
index 344cc9e741c1357420240467754fd151926b3bd5..723a80895cd43bde840adf8d26812e82d12385ab 100644
--- a/drivers/gpu/host1x/bus.c
+++ b/drivers/gpu/host1x/bus.c
@@ -471,6 +471,18 @@ static int host1x_device_add(struct host1x *host1x,
 
 	mutex_unlock(&clients_lock);
 
+	/*
+	 * Add device even if there are no subdevs to ensure syncpoint functionality
+	 * is available regardless of whether any engine subdevices are present
+	 */
+	if (list_empty(&device->subdevs)) {
+		err = device_add(&device->dev);
+		if (err < 0)
+			dev_err(&device->dev, "failed to add device: %d\n", err);
+		else
+			device->registered = true;
+	}
+
 	return 0;
 }
 
diff --git a/drivers/gpu/host1x/dev.c b/drivers/gpu/host1x/dev.c
index 1f93e5e276c0835eac2f713ffcd60a9db8db2c21..e1a9246d35f4b7685dd828c53eaf3910a5ef739e 100644
--- a/drivers/gpu/host1x/dev.c
+++ b/drivers/gpu/host1x/dev.c
@@ -821,6 +821,7 @@ u64 host1x_get_dma_mask(struct host1x *host1x)
 }
 EXPORT_SYMBOL(host1x_get_dma_mask);
 
+MODULE_SOFTDEP("post: tegra-drm");
 MODULE_AUTHOR("Thierry Reding <thierry.reding@...onic-design.de>");
 MODULE_AUTHOR("Terje Bergstrom <tbergstrom@...dia.com>");
 MODULE_DESCRIPTION("Host1x driver for Tegra products");

---
base-commit: 58ba80c4740212c29a1cf9b48f588e60a7612209
change-id: 20250708-host1x-allow-no-subdevs-f84bb7b48aa7