linux-amazon-biscuit: remove dummy mmap from pcm audio driver (MR 6233)

More detail in the included patch but tl;dr patches a dummy mmap
function in the PCM audio driver that causes a kernel panic
when used by pulse.

[ci:skip-build]: already built successfully in CI

device/testing/linux-amazon-biscuit/APKBUILD

@@ -3,7 +3,7 @@
 
 pkgname=linux-amazon-biscuit
 pkgver=3.18.19
-pkgrel=0
+pkgrel=1
 pkgdesc="Amazon Echo Dot (2nd gen) kernel fork"
 arch="armv7"
 _carch="arm"
@@ -44,6 +44,7 @@ source="
 	replace-missing-code-from-mainline.patch
 	replace-solaris-style-directive.patch
 	x509-certificate-list.patch
+	biscuit-remove-dummy-audio-mmap.patch
 "
 builddir="$srcdir/$_repository-$_commit"
 _outdir="out"
@@ -73,4 +74,5 @@ f0a0591e0008fa5465f479677112b81ef3dac88c26e0aa358bb504abdb920d3682613dfc51183130
 9ae0f1ca06c6084c0577578721968cd4b154d9cb9fb82af3b30732dd4ca2e857cae58c285d0f529e0b2c827f1cc2a12426698065e2d0aab1b23e2208fee70bc5  replace-missing-code-from-mainline.patch
 a4196e0184b3bb1bc8d0cf4fae931dab623bb9cd02732758bb8ac1d0588682960f58d4dba8ccebb1377e853ca5c135d2a700750228d54ac95a33d1cd44533bca  replace-solaris-style-directive.patch
 756bfc37c903085bec2e016f146cfc795da5495d8b095124a83f9d92931882ca63c5a1c418016dbcd19a18357c1e783dc7547bb115bb186751615ce9268007f2  x509-certificate-list.patch
+99a8649fbeda1a6343e994d65b391c684e0ccfa8c5d7c3f7e584ee16bfc98e1b7dcca359fd6541f341f54f883708a8bc69a50fde459fedf05ad6ea5ab133bfc9  biscuit-remove-dummy-audio-mmap.patch
 "

device/testing/linux-amazon-biscuit/biscuit-remove-dummy-audio-mmap.patch

@@ -0,0 +1,49 @@
+remove mmap capability from amazon mt8163 pcm audio driver
+
+For reasons known only to MediaTek, the pcm driver used by most amazon devices
+of this era provide a fake zero page address whenever a mmap is attempted.
+Naturally, this causes a kernel panic when an audio server like pulseaudio
+attempts to mmap the sound device using the fake address. Remove this dummy
+page function and remove mmap flags from the driver description so pulse
+handles the device correctly.
+
+Verified on amazon-biscuit (Echo Dot 2G) but may also apply to other MT8163
+devices like karnak and douglas. MT8127 devices like ford appear to pull this
+dummy page trick as well, so a modified patch may work there.
+diff --git a/sound/soc/mediatek/mt_soc_audio_8163_amzn/mt_soc_pcm_dl1_i2s0Dl1.c b/sound/soc/mediatek/mt_soc_audio_8163_amzn/mt_soc_pcm_dl1_i2s0Dl1.c
+index b68f2fa..afaaba8 100644
+--- a/sound/soc/mediatek/mt_soc_audio_8163_amzn/mt_soc_pcm_dl1_i2s0Dl1.c
++++ b/sound/soc/mediatek/mt_soc_audio_8163_amzn/mt_soc_pcm_dl1_i2s0Dl1.c
+@@ -124,8 +124,7 @@ static const struct snd_kcontrol_new Audio_snd_I2S0dl1_controls[] = {
+ 
+ static struct snd_pcm_hardware mtk_I2S0dl1_hardware = {
+ 
+-	.info = (SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_INTERLEAVED |
+-		SNDRV_PCM_INFO_RESUME | SNDRV_PCM_INFO_MMAP_VALID),
++	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_RESUME),
+ 	.formats = SND_SOC_ADV_MT_FMTS,
+ 	.rates = SOC_HIGH_USE_RATE,
+ 	.rate_min = SOC_HIGH_USE_RATE_MIN,
+@@ -888,14 +887,6 @@ static int mtk_pcm_I2S0dl1_silence(struct snd_pcm_substream *substream,
+ 	return 0;
+ }
+ 
+-static void *dummy_page[2];
+-
+-static struct page *mtk_I2S0dl1_pcm_page(struct snd_pcm_substream *substream, unsigned long offset)
+-{
+-	pr_debug("%s\n", __func__);
+-	return virt_to_page(dummy_page[substream->stream]);	/* the same page */
+-}
+-
+ static struct snd_pcm_ops mtk_I2S0dl1_ops = {
+ 
+ 	.open = mtk_pcm_I2S0dl1_open,
+@@ -908,7 +899,6 @@ static struct snd_pcm_ops mtk_I2S0dl1_ops = {
+ 	.pointer = mtk_pcm_I2S0dl1_pointer,
+ 	.copy = mtk_pcm_I2S0dl1_copy,
+ 	.silence = mtk_pcm_I2S0dl1_silence,
+-	.page = mtk_I2S0dl1_pcm_page,
+ };
+ 
+ static struct snd_soc_platform_driver mtk_I2S0dl1_soc_platform = {