[RFC PATCH] clk: OMAP: introduce device tree binding to kernel clockdata

From: Nishanth Menon
Date: Tue Apr 09 2013 - 20:26:40 EST

OMAP clock data is located in arch/arm/mach-omap2/cclockXYZ_data.c.
However, this presents an obstacle for using these clock nodes in
Device Tree definitions. There are many possible approaches to this
problem as discussed in the following thread:
http://marc.info/?t=136370325600009&r=1&w=2
Highlights of the options:
a) device specific clk_add_alias:
cons: driver handling required
b) using an generic clk node and indexing to reach the clock required.
This is similar in approach taken by tegra and few other platforms.
example clock = <&clk 5>;
cons: potential to have mismatches in indexed table and associated
dtb data. In addition, managing continued documentation in bindings
as clock indexing increases. Even though readability angle could be
improved by using preprocessing of DT using macros, indexed approach
is inherently risky from cases like the following:
clk indexes in kernel:
1 - mpu_dpll
2 - aux_clk1
3 - core_clk
DT entry for peripheral x uses <&clk 2>, kernel updates to:
1 - mpu_dpll
2 - per_dpll
3 - aux_clk1
4 - core_clk
using the old dtb(or dts missing an update), on new kernel which has
updated indices will result in per_dpll now controlled for peripheral
X without warning or any potential error detection and warning.

Even though we can claim this is user error, such errors are hard to
track down and fix.

An alternate approach introduced here is to introduce device tree bindings
corresponding to the clock nodes required in DT definition for SoC which
automatically maps back to the definitions in cclockXYZ_data.c.

The driver introduced here to do this mapping will eventually be the
place where the clock handling will migrate to. We need to consider this
angle as well so that the solution will be an valid transition point for
moving the clock data out of kernel image (into device tree or firmware load
etc..).

Overall strategy introduced here is simple: an clock node described in
device tree blob is used to identify the exact clock provided in the SoC
specific data. This is then linked back using of_clk_add_provider to the
device node to be accessible by of_clk_get.

Based on discussion contributions from Roger Quadros, Grygorii Strashko
and others.

[tony@xxxxxxxxxxx: co-developed]
Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
Signed-off-by: Nishanth Menon <nm@xxxxxx>
---
.../devicetree/bindings/clock/omap-clock.txt | 40 +++++++++
drivers/clk/Makefile | 1 +
drivers/clk/omap/Makefile | 1 +
drivers/clk/omap/clk.c | 94 ++++++++++++++++++++
4 files changed, 136 insertions(+)
create mode 100644 Documentation/devicetree/bindings/clock/omap-clock.txt
create mode 100644 drivers/clk/omap/Makefile
create mode 100644 drivers/clk/omap/clk.c

diff --git a/Documentation/devicetree/bindings/clock/omap-clock.txt b/Documentation/devicetree/bindings/clock/omap-clock.txt
new file mode 100644
index 0000000..07e3ff8
--- /dev/null
+++ b/Documentation/devicetree/bindings/clock/omap-clock.txt
@@ -0,0 +1,40 @@
+Device Tree Clock bindings for Texas Instrument's OMAP compatible platforms
+
+This binding is a work-in-progress, and meant to be stage #1 of transitioning
+OMAP clock data out of kernel image.
+
+This binding uses the common clock binding[1].
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+Required properties:
+- compatible : shall be "ti,omap-clock"
+- #clock-cells : from common clock binding; shall be set to 0.
+NOTE:
+node name should map to clock database in arch/arm/mach-omap2/cclock<SoC>_data.c
+Since all clocks are described with _ck, the node name is optimized to drop the
+usage of _ck. For example, a clock called dpll1_ck will be defined as dpll1.
+
+Example #1: describing clock node for CPU on OMAP34xx platform:
+Ref: arch/arm/mach-omap2/cclock3xxx_data.c
+describes the CPU clock to be as follows
+ CLK(NULL, "dpll1_ck", &dpll1_ck, CK_3XXX),
+Corresponding binding will be:
+ dpll1: dpll1 {
+ #clock-cells = <0>;
+ compatible = "ti,omap-clock";
+ };
+And it's usage will be:
+ clocks = <&dpll1>;
+
+Example #2: describing clock node for auxilary clock #3 on OMAP443x platform:
+Ref: arch/arm/mach-omap2/cclock44xx_data.c
+describes the auxclk3 clock to be as follows:
+ CLK(NULL, "auxclk3_ck", &auxclk3_ck, CK_443X),
+Corresponding binding will be:
+ auxclk3: auxclk3 {
+ #clock-cells = <1>;
+ compatible = "ti,omap-clock";
+ };
+And it's usage will be:
+ clocks = <&auxclk3>;
diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
index 300d477..a0209d7 100644
--- a/drivers/clk/Makefile
+++ b/drivers/clk/Makefile
@@ -27,6 +27,7 @@ obj-$(CONFIG_ARCH_U8500) += ux500/
obj-$(CONFIG_ARCH_VT8500) += clk-vt8500.o
obj-$(CONFIG_ARCH_ZYNQ) += clk-zynq.o
obj-$(CONFIG_ARCH_TEGRA) += tegra/
+obj-$(CONFIG_ARCH_OMAP) += omap/

obj-$(CONFIG_X86) += x86/

diff --git a/drivers/clk/omap/Makefile b/drivers/clk/omap/Makefile
new file mode 100644
index 0000000..8195931
--- /dev/null
+++ b/drivers/clk/omap/Makefile
@@ -0,0 +1 @@
+obj-y += clk.o
diff --git a/drivers/clk/omap/clk.c b/drivers/clk/omap/clk.c
new file mode 100644
index 0000000..63a4cce
--- /dev/null
+++ b/drivers/clk/omap/clk.c
@@ -0,0 +1,94 @@
+/*
+ * Texas Instruments OMAP Clock driver
+ *
+ * Copyright (C) 2013 Texas Instruments Incorporated - http://www.ti.com/
+ * Nishanth Menon
+ * Tony Lindgren <tony@xxxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed "as is" WITHOUT ANY WARRANTY of any
+ * kind, whether express or implied; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/clkdev.h>
+#include <linux/clk-private.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of_device.h>
+#include <linux/platform_device.h>
+#include <linux/string.h>
+
+static const struct of_device_id omap_clk_of_match[] = {
+ {.compatible = "ti,omap-clock",},
+ {},
+};
+
+MODULE_DEVICE_TABLE(of, omap_clk_of_match);
+
+/**
+ * omap_clk_probe() - create link from DT definition to clock data
+ * @pdev: device node
+ *
+ * REVISIT: We are now assuming the following:
+ * 1. omap clock names end with _ck
+ * 2. omap clocks don't get removed
+ */
+static int omap_clk_probe(struct platform_device *pdev)
+{
+ struct clk *clk;
+ int res;
+
+ const struct of_device_id *match;
+ struct device_node *np = pdev->dev.of_node;
+ char clk_name[32];
+
+ match = of_match_device(omap_clk_of_match, &pdev->dev);
+
+ /* Set up things so consumer can call clk_get() with name */
+ snprintf(clk_name, 32, "%s_ck", np->name);
+ clk = clk_get(NULL, clk_name);
+ if (IS_ERR(clk)) {
+ res = PTR_ERR(clk);
+ dev_err(&pdev->dev, "could not get clock %s (%d)\n",
+ clk_name, res);
+ goto out;
+ }
+
+ /* This allows the driver to of_clk_get() */
+ res = of_clk_add_provider(np, of_clk_src_simple_get, clk);
+ if (res)
+ dev_err(&pdev->dev, "could not add provider for %s (%d)\n",
+ clk_name, res);
+
+ clk_put(clk);
+out:
+ return res;
+}
+
+MODULE_ALIAS("platform:omap_clk");
+
+/* We assume here that OMAP clocks will not be removed */
+static struct platform_driver omap_clk_driver = {
+ .probe = omap_clk_probe,
+ .driver = {
+ .name = "omap_clk",
+ .owner = THIS_MODULE,
+ .of_match_table = of_match_ptr(omap_clk_of_match),
+ },
+};
+
+static int __init omap_clk_init(void)
+{
+ return platform_driver_register(&omap_clk_driver);
+}
+
+arch_initcall(omap_clk_init);
+
+MODULE_DESCRIPTION("OMAP Clock driver");
+MODULE_AUTHOR("Texas Instruments Inc.");
+MODULE_LICENSE("GPL v2");
--
1.7.9.5

--
Regards,
Nishanth Menon
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