soundwire: cadence: add paranoid check on self-clearing bits

The Cadence IP exposes a small number of self-clearing bits in
the MCP_CONTROL and MCP_CONFIG_UPDATE registers.

We currently do not check that those bits are indeed cleared,
e.g. during resume operations. That could lead to resuming peripheral
devices too early.

In addition, if we happen to read these registers, update one of the
fields and write the register back, we may be writing stale data that
might have been cleared in hardware. These sort of race conditions
could lead to e.g. doing a hw_reset twice or stopping a clock that
just restarted. There is no clear way of avoiding these potential race
conditions other than making sure that these registers fields are
cleared before any read-modify-write sequence. If we detect this sort
of errors, we only log them since there is no clear recovery
possible. The only way out is likely to restart the IP with a
suspend/resume cycle.

Note that the checks are performed before updating the registers, as
well as after the Intel 'sync go' sequence in multi-link mode. That
should cover both the start and end of suspend/resume hardware
configurations. The Multi-Master mode gates the configuration updates
until the 'sync go' signal is asserted, so we only check on init and
after the end of the 'sync go' sequence.

The duration of the usleep_range() was defined by the GSYNC frequency
used in multi-master mode. With a 4kHz frequency, any configuration
change might be deferred by up to 250us. Extending the range to
1000-1500us should guarantee that the configuration change is
completed without any significant impact on the overall resume
time.

Suggested-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20210714051349.13064-1-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/soundwire/cadence_master.h b/drivers/soundwire/cadence_master.h
index 0e7f8b35bb21..b54c161a837f 100644
--- a/drivers/soundwire/cadence_master.h
+++ b/drivers/soundwire/cadence_master.h
@@ -184,4 +184,8 @@ int cdns_bus_conf(struct sdw_bus *bus, struct sdw_bus_params *params);
 
 int cdns_set_sdw_stream(struct snd_soc_dai *dai,
 			void *stream, bool pcm, int direction);
+
+void sdw_cdns_check_self_clearing_bits(struct sdw_cdns *cdns, const char *string,
+				       bool initial_delay, int reset_iterations);
+
 #endif /* __SDW_CADENCE_H */