ALSA: pcm_native - fix runtime->boundary calculation

The code in pcm_lib updating runtime->hw_ptr_interrupt expects
that runtime->boundary is divisible with runtime->period_size.
Thanks are going to Clemens Ladisch for the notice.

Fix the runtime->boundary calculation using buffer_size * period_size
as base and find a least common multiple for 32bit platforms when
the expression might overflow.
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

sound/core/pcm_native.c

@@ -27,6 +27,7 @@
 #include <linux/pm_qos_params.h>
 #include <linux/uio.h>
 #include <linux/dma-mapping.h>
+#include <linux/math64.h>
 #include <sound/core.h>
 #include <sound/control.h>
 #include <sound/info.h>
@@ -366,6 +367,38 @@ static int period_to_usecs(struct snd_pcm_runtime *runtime)
 	return usecs;
 }
 
+static int calc_boundary(struct snd_pcm_runtime *runtime)
+{
+	u_int64_t boundary;
+
+	boundary = (u_int64_t)runtime->buffer_size *
+		   (u_int64_t)runtime->period_size;
+#if BITS_PER_LONG < 64
+	/* try to find lowest common multiple for buffer and period */
+	if (boundary > LONG_MAX - runtime->buffer_size) {
+		u_int32_t remainder = -1;
+		u_int32_t divident = runtime->buffer_size;
+		u_int32_t divisor = runtime->period_size;
+		while (remainder) {
+			remainder = divident % divisor;
+			if (remainder) {
+				divident = divisor;
+				divisor = remainder;
+			}
+		}
+		boundary = div_u64(boundary, divisor);
+		if (boundary > LONG_MAX - runtime->buffer_size)
+			return -ERANGE;
+	}
+#endif
+	if (boundary == 0)
+		return -ERANGE;
+	runtime->boundary = boundary;
+	while (runtime->boundary * 2 <= LONG_MAX - runtime->buffer_size)
+		runtime->boundary *= 2;
+	return 0;
+}
+
 static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 			     struct snd_pcm_hw_params *params)
 {
@@ -441,9 +474,9 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 	runtime->stop_threshold = runtime->buffer_size;
 	runtime->silence_threshold = 0;
 	runtime->silence_size = 0;
-	runtime->boundary = runtime->buffer_size;
-	while (runtime->boundary * 2 <= LONG_MAX - runtime->buffer_size)
-		runtime->boundary *= 2;
+	err = calc_boundary(runtime);
+	if (err < 0)
+		goto _error;
 
 	snd_pcm_timer_resolution_change(substream);
 	runtime->status->state = SNDRV_PCM_STATE_SETUP;