From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>

When platform firmware exposes multiple supported bus frequencies, the
existing SoundWire support selects the maximum frequency. This is not
aligned with the SoundWire 1.2 directions: the MIPI recommendation is
to start at a 'safe' speed, compatible with the default frame rate and
shape, and only increase the clock when vendor and codec PHY
parameters are updated.

However, clock changes are not supported for now by the SoundWire
core, so in practice this patch has the effect of discarding
frequencies different to the implicit default. Dynamic clock changes
will be required at some point, and this limitation will be removed
after the core is updated, specifically to perform synchronous clock
scale changes on manager and peripheral sides with a bank switch.

On Intel LunarLake platforms with a 'standard' DSDT, this forces the
use of 4.8MHz. On older platforms this patch has no effect.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
---
 drivers/soundwire/intel_auxdevice.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/soundwire/intel_auxdevice.c b/drivers/soundwire/intel_auxdevice.c
index 54cb455ed870..64ca55c1c94b 100644
--- a/drivers/soundwire/intel_auxdevice.c
+++ b/drivers/soundwire/intel_auxdevice.c
@@ -209,9 +209,30 @@ static int sdw_master_read_intel_prop(struct sdw_bus *bus)
 
 static int intel_prop_read(struct sdw_bus *bus)
 {
+	struct sdw_master_prop *prop;
+
 	/* Initialize with default handler to read all DisCo properties */
 	sdw_master_read_prop(bus);
 
+	/*
+	 * Only one bus frequency is supported so far, filter
+	 * frequencies reported in the DSDT
+	 */
+	prop = &bus->prop;
+	if (prop->clk_freq && prop->num_clk_freq > 1) {
+		unsigned int default_bus_frequency;
+
+		default_bus_frequency =
+			prop->default_frame_rate *
+			prop->default_row *
+			prop->default_col /
+			SDW_DOUBLE_RATE_FACTOR;
+
+		prop->num_clk_freq = 1;
+		prop->clk_freq[0] = default_bus_frequency;
+		prop->max_clk_freq = default_bus_frequency;
+	}
+
 	/* read Intel-specific properties */
 	sdw_master_read_intel_prop(bus);
 
-- 
2.43.0

On Thu, 04 Jul 2024 08:34:11 +0800, Bard Liao wrote:
> When platform firmware exposes multiple supported bus frequencies, the
> existing SoundWire support selects the maximum frequency. This is not
> aligned with the SoundWire 1.2 directions: the MIPI recommendation is
> to start at a 'safe' speed, compatible with the default frame rate and
> shape, and only increase the clock when vendor and codec PHY
> parameters are updated.
> 
> [...]

Applied, thanks!

[1/1] soundwire: intel_auxdevice: start the bus at default frequency
      commit: c326356188f1dc2d7a2c55b30dac6a8b76087bc6

Best regards,
-- 
Vinod Koul <vkoul@kernel.org>