ARM: tegra: pinmux: add note re: drive group field defines

Tegra's drive group registers have a remarkably inconsistent layout. The
current U-Boot driver doesn't take this into account at all. Add a
comment to describe the issue, so at least anyone debugging the driver
will be aware of this. To solve this, we'd need to add a per-drive-group
data structure describing the layout for the individual register. Since
we don't set up too many drive groups in U-Boot at present, this
hopefully isn't causing too much practical issue. Still, we probably need
to fix this sometime.

Wth Tegra210, the register layout becomes almost entirely consistent, so
this problem partially solves itself over time.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

arch/arm/mach-tegra/pinmux-common.c

@@ -347,6 +347,21 @@ void pinmux_config_pingrp_table(const struct pmux_pingrp_config *config,
 #define SCHMT_SHIFT	3
 #define LPMD_SHIFT	4
 #define LPMD_MASK	(3 << LPMD_SHIFT)
+/*
+ * Note that the following DRV* and SLW* defines are accurate for many drive
+ * groups on many SoCs. We really need a per-group data structure to solve
+ * this, since the fields are in different positions/sizes in different
+ * registers (for different groups).
+ *
+ * On Tegra30/114/124, the DRV*_SHIFT values vary.
+ * On Tegra30, the SLW*_SHIFT values vary.
+ * On Tegra30/114/124/210, the DRV*_MASK values vary, although the values
+ *   below are wide enough to cover the widest fields, and hopefully don't
+ *   interfere with any other fields.
+ * On Tegra30, the SLW*_MASK values vary, but we can't use a value that's
+ *   wide enough to cover all cases, since that would cause the field to
+ *   overlap with other fields in the narrower cases.
+ */
 #define DRVDN_SHIFT	12
 #define DRVDN_MASK	(0x7F << DRVDN_SHIFT)
 #define DRVUP_SHIFT	20