[PATCH 2/2] soundwire: honor clock_reg_supported in the clock scaling check

[Jorijn van der Graaf]

sdw_slave_set_frequency() treats class_id and prop.clock_reg_supported
as equivalent evidence that a slave implements the bus-clock base and
scale registers, but the bank-switch reprogramming path checks class_id
alone, so a class-0 slave that declared the registers never gets the
next-bank scale written there. The registers are SoundWire 1.2, not
SDCA, so a device may well implement them without setting the class
field.

Extend the helper to honor clock_reg_supported, as discussed with
Pierre-Louis in the WCD9378 review. This also makes a link whose
peripherals all declare clock_reg_supported eligible for dynamic clock
scaling in the generic bandwidth allocation, which is what declaring
the registers means.

Link: https://lore.kernel.org/all/5717102b-f7ab-42b2-8065-064d94dd2bee@xxxxxxxxx/
Assisted-by: Claude:claude-fable-5
Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@xxxxxxxxxxxxx>
---
Pierre-Louis agreed the helper should be extended in the WCD9378 v1
thread [1]. On the qcom bus this only adds next-bank scale writes of
the same value on bank switches (the clock is fixed); verified on the
Fairphone 6 that capture still works with them.

One behavior change I cannot test: the helper also feeds
is_clock_scaling_supported() in the generic bandwidth allocation, so an
Intel link whose peripherals all pass the check - max98363 is the
in-tree clock_reg_supported case - becomes eligible for dynamic clock
scaling where it previously ran at a fixed clock. Only configurations
that fail the bandwidth check today can select a different frequency; I
could not test that combination, flagging it for the Intel side.

[1] https://lore.kernel.org/all/5717102b-f7ab-42b2-8065-064d94dd2bee@xxxxxxxxx/

 drivers/soundwire/bus.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
index 0490777fa406..d94e44b59050 100644
--- a/drivers/soundwire/bus.c
+++ b/drivers/soundwire/bus.c
@@ -817,8 +817,11 @@ bool is_clock_scaling_supported_by_slave(struct sdw_slave *slave)
 	/*
 	 * Dynamic scaling is a defined by SDCA. However, some devices expose the class ID but
 	 * can't support dynamic scaling. We might need a quirk to handle such devices.
+	 * The clock base and scale registers themselves are SoundWire 1.2, so a device
+	 * may implement them without setting the class field; the driver says so with
+	 * clock_reg_supported.
 	 */
-	return slave->id.class_id;
+	return slave->id.class_id || slave->prop.clock_reg_supported;
 }
 EXPORT_SYMBOL(is_clock_scaling_supported_by_slave);
 
-- 
2.55.0