[PATCH v2] soundwire: intel_auxdevice: add kernel parameter for mclk divider

[Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>]

From: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>

Add a kernel parameter to work-around discrepancies between hardware
and platform firmware, it's not unusual to see e.g. 38.4MHz listed in
_DSD properties as the SoundWire clock source, but the hardware may be
based on a 19.2 MHz mclk source.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
Signed-off-by: Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>
---
v2: Add description of the kernel parameter in in kernel-parameters.txt.
---
 Documentation/admin-guide/kernel-parameters.rst |  1 +
 Documentation/admin-guide/kernel-parameters.txt |  4 ++++
 drivers/soundwire/intel_auxdevice.c             | 12 ++++++++++--
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
index fdea7c26ef80..a64130fd19bf 100644
--- a/Documentation/admin-guide/kernel-parameters.rst
+++ b/Documentation/admin-guide/kernel-parameters.rst
@@ -159,6 +159,7 @@ is applicable::
 	SCSI	Appropriate SCSI support is enabled.
 			A lot of drivers have their options described inside
 			the Documentation/scsi/ sub-directory.
+        SDW     SoundWire support is enabled.
 	SECURITY Different security models are enabled.
 	SELINUX SELinux support is enabled.
 	SERIAL	Serial support is enabled.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1518343bbe22..d7aee9ed5a1d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6060,6 +6060,10 @@
 			non-zero "wait" parameter.  See weight_single
 			and weight_many.
 
+	sdw_mclk_divider=[SDW]
+			Specify the MCLK divider for Intel SoundWire buses in
+			case the BIOS does not provide the clock rate properly.
+
 	skew_tick=	[KNL,EARLY] Offset the periodic timer tick per cpu to mitigate
 			xtime_lock contention on larger systems, and/or RCU lock
 			contention on all systems with CONFIG_MAXSMP set.
diff --git a/drivers/soundwire/intel_auxdevice.c b/drivers/soundwire/intel_auxdevice.c
index ae689d5d1ab9..599954d92752 100644
--- a/drivers/soundwire/intel_auxdevice.c
+++ b/drivers/soundwire/intel_auxdevice.c
@@ -41,6 +41,10 @@ static int md_flags;
 module_param_named(sdw_md_flags, md_flags, int, 0444);
 MODULE_PARM_DESC(sdw_md_flags, "SoundWire Intel Master device flags (0x0 all off)");
 
+static int mclk_divider;
+module_param_named(sdw_mclk_divider, mclk_divider, int, 0444);
+MODULE_PARM_DESC(sdw_mclk_divider, "SoundWire Intel mclk divider");
+
 struct wake_capable_part {
 	const u16 mfg_id;
 	const u16 part_id;
@@ -142,8 +146,12 @@ static int sdw_master_read_intel_prop(struct sdw_bus *bus)
 				 "intel-sdw-ip-clock",
 				 &prop->mclk_freq);
 
-	/* the values reported by BIOS are the 2x clock, not the bus clock */
-	prop->mclk_freq /= 2;
+	if (mclk_divider)
+		/* use kernel parameter for BIOS or board work-arounds */
+		prop->mclk_freq /= mclk_divider;
+	else
+		/* the values reported by BIOS are the 2x clock, not the bus clock */
+		prop->mclk_freq /= 2;
 
 	fwnode_property_read_u32(link,
 				 "intel-quirk-mask",
-- 
2.43.0