ALSA: hda: Change AZX_MAX_BUF_SIZE from 1GB to 4MB

When SND_HDA_PREALLOC_SIZE is set to 0, applications can request as much
memory as there is allowed. With value of AZX_MAX_BUF_SIZE it is 1GB per
stream, which is not realistic use case. Change it 4MB.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=201251#c322
Suggested-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Link: https://lore.kernel.org/r/20210318160618.2504068-3-amadeuszx.slawinski@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/hda_register.h

@@ -140,8 +140,12 @@ enum { SDI0, SDI1, SDI2, SDI3, SDO0, SDO1, SDO2, SDO3 };
 #define BDL_SIZE		4096
 #define AZX_MAX_BDL_ENTRIES	(BDL_SIZE / 16)
 #define AZX_MAX_FRAG		32
-/* max buffer size - no h/w limit, you can increase as you like */
-#define AZX_MAX_BUF_SIZE	(1024*1024*1024)
+/*
+ * max buffer size - artificial 4MB limit per stream to avoid big allocations
+ * In theory it can be really big, but as it is per stream on systems with many streams memory could
+ * be quickly saturated if userspace requests maximum buffer size for each of them.
+ */
+#define AZX_MAX_BUF_SIZE	(4*1024*1024)
 
 /* RIRB int mask: overrun[2], response[0] */
 #define RIRB_INT_RESPONSE	0x01