[PATCH alsa-lib 2/2] pcm: dmix: make lockless operation optional

[Takashi Iwai]

The recent overlooked bug about the unconditional semaphore usage in
the dmix implies that basically we've had no problem with the locking
in the practical usages over years.  Although the lockless operation
has a clear merit, it's a much higher CPU usage (especially on some
uncached pages), and it might lead to a potential deadlock in theory
(which is hard to reproduce at will, though).

This patch introduces a new configure option "--enable-lockless-dmix"
or "--disable-lockless-dmix" to let user choose the default dmix
operation mode, then the default value for the dmix
"direct_memory_access" option is set based on it.

A big question is which operation mode should be default: it's hard to
decide, but in this patch, I bet for disabling the lockless in the
end as the performance loss is significant.

But the user can enable the lockless operation at any time; at build
time, with the configure option above, and at run time, by specifying
the dmix "direct_memory_access" option, too.

Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 configure.ac         | 13 +++++++++++++
 src/pcm/pcm_direct.c |  4 ++++
 2 files changed, 17 insertions(+)

diff --git a/configure.ac b/configure.ac
index 4577c417037a..75d037d5509a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -629,6 +629,19 @@ if test "$build_pcm_mmap_emul" = "yes"; then
   AC_DEFINE([BUILD_PCM_PLUGIN_MMAP_EMUL], "1", [Build PCM mmap-emul plugin])
 fi
 
+if test "$build_pcm_dmix" = "yes"; then
+AC_MSG_CHECKING(for default lockless dmix)
+AC_ARG_ENABLE(lockless-dmix,
+  AS_HELP_STRING([--enable-lockless-dmix],
+    [use lockless dmix as default on x86]),
+  lockless_dmix="$enableval", lockless_dmix="no")
+if test "$lockless_dmix" = "yes"; then
+  AC_MSG_RESULT(yes)
+  AC_DEFINE([LOCKLESS_DMIX_DEFAULT], "1", [Lockless dmix as default])
+else
+  AC_MSG_RESULT(no)
+fi
+fi
 
 dnl Create PCM plugin symbol list for static library
 rm -f "$srcdir"/src/pcm/pcm_symbols_list.c
diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index 665340954cf3..c38ba3190f1a 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -1892,7 +1892,11 @@ int snd_pcm_direct_parse_open_conf(snd_config_t *root, snd_config_t *conf,
 	rec->slowptr = 1;
 	rec->max_periods = 0;
 	rec->var_periodsize = 0;
+#ifdef LOCKLESS_DMIX_DEFAULT
 	rec->direct_memory_access = 1;
+#else
+	rec->direct_memory_access = 0;
+#endif
 	rec->hw_ptr_alignment = SND_PCM_HW_PTR_ALIGNMENT_AUTO;
 	rec->tstamp_type = -1;
 
-- 
2.16.4