[PATCH RFC PoC 2/2] misc: implement a dummy early platform driver

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Implement a very simple early platform driver. Its purpose is to show
how such drivers can be registered and to emit a message when probed.

It can be then added to the device tree or machine code to verify that
the early platform devices work as expected.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
---
 drivers/misc/Kconfig       |  8 +++++++
 drivers/misc/Makefile      |  1 +
 drivers/misc/dummy-early.c | 46 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 55 insertions(+)
 create mode 100644 drivers/misc/dummy-early.c

diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
index 5d713008749b..99cde8aefdb0 100644
--- a/drivers/misc/Kconfig
+++ b/drivers/misc/Kconfig
@@ -91,6 +91,14 @@ config DUMMY_IRQ
 	  The sole purpose of this module is to help with debugging of systems on
 	  which spurious IRQs would happen on disabled IRQ vector.
 
+config DUMMY_EARLY
+	bool "Dummy early platform driver"
+	select EARLYDEV
+	default n
+	help
+	  This module's only function is to register itself with the early
+	  platform device framework and be probed early in the boot process.
+
 config IBM_ASM
 	tristate "Device driver for IBM RSA service processor"
 	depends on X86 && PCI && INPUT
diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index 20be70c3f118..84ad0225eb14 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -11,6 +11,7 @@ obj-$(CONFIG_INTEL_MID_PTI)	+= pti.o
 obj-$(CONFIG_ATMEL_SSC)		+= atmel-ssc.o
 obj-$(CONFIG_ATMEL_TCLIB)	+= atmel_tclib.o
 obj-$(CONFIG_DUMMY_IRQ)		+= dummy-irq.o
+obj-$(CONFIG_DUMMY_EARLY)	+= dummy-early.o
 obj-$(CONFIG_ICS932S401)	+= ics932s401.o
 obj-$(CONFIG_LKDTM)		+= lkdtm/
 obj-$(CONFIG_TIFM_CORE)       	+= tifm_core.o
diff --git a/drivers/misc/dummy-early.c b/drivers/misc/dummy-early.c
new file mode 100644
index 000000000000..f00fb1fbd5fa
--- /dev/null
+++ b/drivers/misc/dummy-early.c
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018 Texas Instruments
+ *
+ * Author:
+ *   Bartosz Golaszewski <bgolaszewski@baylibre.com>
+ *
+ * Dummy testing driver whose only purpose is to be registered and probed
+ * using the early platform device mechanism.
+ */
+
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/of_device.h>
+#include <linux/platform_device.h>
+#include <linux/earlydev.h>
+
+static int dummy_early_probe(struct platform_device *pdev)
+{
+	if (earlydev_probing_early())
+		dev_notice(&pdev->dev, "dummy-early driver probed early!\n");
+	else
+		dev_notice(&pdev->dev, "dummy-early driver probed late!\n");
+
+	return 0;
+}
+
+static const struct of_device_id dummy_early_of_match[] = {
+	{ .compatible = "none,dummy-early", },
+	{ },
+};
+
+static struct earlydev_driver dummy_early_driver = {
+	.plat_drv = {
+		.probe = dummy_early_probe,
+		.driver = {
+			.name = "dummy-early",
+			.of_match_table = dummy_early_of_match,
+		},
+	}
+};
+earlydev_platform_driver(dummy_early_driver);
+
+MODULE_AUTHOR("Bartosz Golaszewski <bgolaszewski@baylibre.com>");
+MODULE_DESCRIPTION("Dummy early platform device driver");
+MODULE_LICENSE("GPL v2");
-- 
2.17.0