ASoC: dmaengine: Introduce module option prealloc_buffer_size_kbytes

Currently, The fixed 512KB prealloc buffer size is too larger for
tiny memory kernel (such as 16MB memory). This patch adds the module
option "prealloc_buffer_size_kbytes" to specify prealloc buffer size.

It's suitable for cards which use the generic dmaengine pcm driver
with no config.
Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Link: https://lore.kernel.org/r/1632394246-59341-1-git-send-email-sugar.zhang@rock-chips.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/sound/alsa-configuration.rst

@@ -100,6 +100,15 @@ amidi_map
     MIDI device number maps assigned to the 2st OSS device;
     Default: 1
 
+Module snd-soc-core
+-------------------
+
+The soc core module. It is used by all ALSA card drivers.
+It takes the following options which have global effects.
+
+prealloc_buffer_size_kbytes
+    Specify prealloc buffer size in kbytes (default: 512).
+
 Common parameters for top sound card modules
 --------------------------------------------
 

sound/soc/soc-generic-dmaengine-pcm.c

@@ -15,6 +15,10 @@
 
 #include <sound/dmaengine_pcm.h>
 
+static unsigned int prealloc_buffer_size_kbytes = 512;
+module_param(prealloc_buffer_size_kbytes, uint, 0444);
+MODULE_PARM_DESC(prealloc_buffer_size_kbytes, "Preallocate DMA buffer size (KB).");
+
 /*
  * The platforms dmaengine driver does not support reporting the amount of
  * bytes that are still left to transfer.
@@ -237,7 +241,7 @@ static int dmaengine_pcm_new(struct snd_soc_component *component,
 		prealloc_buffer_size = config->prealloc_buffer_size;
 		max_buffer_size = config->pcm_hardware->buffer_bytes_max;
 	} else {
-		prealloc_buffer_size = 512 * 1024;
+		prealloc_buffer_size = prealloc_buffer_size_kbytes * 1024;
 		max_buffer_size = SIZE_MAX;
 	}