ALSA: hda: No preallocation on x86 platforms

Like many other drivers, HD-audio drivers also do PCM buffer
preallocation to assure the buffer pages allocated at the early boot
stage.  This step is useful for platforms that may fail to allocate
the PCM hardware buffers -- which is mostly for either large
continuous pages or with the specific DMA mask (like emu10k1).

OTOH, when a buffer is allocated as SG-buffer and the DMA mask is
either 32 or 64 bits, the allocation almost never fails unless it hits
the real OOM situation.  In such a case, we don't need the
preallocation inevitably unlike the cases above.

That said, we may drop the preallocation for HD-audio that does
allocate via SG-buffers, and the patch achieves it.

However, there is one caveat: the buffer allocation behavior depends
on CONFIG_SND_DMA_SGBUF, and it falls back to the continuous pages
when it's not set.  And, currently this SG buffer allocation is
enabled only on x86 platforms.  So, covering those fall-outs, the
patch adjusts CONFIG_SND_HDA_PREALLOC_SIZE depending on the condition,
and keeps the old behavior as-is for non-x86 platforms.

On x86, the kconfig item is no longer adjustable but always set to
zero for disabling the preallocation.  You can still enable the
preallocation via procfs interface at any time later, too.

Link: https://lore.kernel.org/r/20200120124423.11862-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/Kconfig

@@ -21,14 +21,16 @@ config SND_HDA_EXT_CORE
        select SND_HDA_CORE
 
 config SND_HDA_PREALLOC_SIZE
-	int "Pre-allocated buffer size for HD-audio driver"
+	int "Pre-allocated buffer size for HD-audio driver" if !SND_DMA_SGBUF
 	range 0 32768
-	default 64
+	default 0 if SND_DMA_SGBUF
+	default 64 if !SND_DMA_SGBUF
 	help
 	  Specifies the default pre-allocated buffer-size in kB for the
 	  HD-audio driver.  A larger buffer (e.g. 2048) is preferred
 	  for systems using PulseAudio.  The default 64 is chosen just
 	  for compatibility reasons.
+	  On x86 systems, the default is zero as we need no preallocation.
 
 	  Note that the pre-allocation size can be changed dynamically
 	  via a proc file (/proc/asound/card*/pcm*/sub*/prealloc), too.