firmware: cs_dsp: Take pwr_lock around reading controls debugfs

In cs_dsp_debugfs_read_controls_show() take the pwr_lock mutex
around the list walk. This protects against debugfs returning
a partial set of new controls if those controls are being added
to the list while it is being walked.

Controls are never deleted from this list, and are only added to
the end of the list. So there was never a danger of following a
stale pointer to garbage.

The worst case was that the printed list is truncated if it saw an
entry that was the list end just before a new entry was appended to
the list.

With the original code, the truncated list from the debugfs could
show only _some_ of the new entries. This could be confusing because
it appears that some new entries are missing.

Adding the mutex means that the debugfs read provides an atomic view.
Either it shows the old content before any of the new controls were
added; or it shows the new content after all the new controls are
added.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20251127113238.1251352-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: rfvirgil

drivers/firmware/cirrus/cs_dsp.c

@@ -438,6 +438,8 @@ static int cs_dsp_debugfs_read_controls_show(struct seq_file *s, void *ignored)
 	struct cs_dsp_coeff_ctl *ctl;
 	unsigned int reg;
 
+	guard(mutex)(&dsp->pwr_lock);
+
 	list_for_each_entry(ctl, &dsp->ctl_list, list) {
 		cs_dsp_coeff_base_reg(ctl, &reg, 0);
 		seq_printf(s, "%22.*s: %#8x %s:%08x %#8x %s %#8x %#4x %c%c%c%c %s %s\n",