Basically, I did the same thing as "amixer -c 0 set 'Headphone Mic Boost',0 1", and make the mic pin hidden as well. Otherwise, user space tools will change the value.

According to Kailang, the "Audio hiss" also happens on Windows - but windows has Noise Suppression filter, so it's not as noticeable as other OS.

So there are two ways to workaround the issue - use something like diff below, or use noise cancellation filter in PA.

diff --git a/sound/pci/hda/patch_realtek.c b/sound/pci/hda/patch_realtek.c
index 1aa21d7e7245..23373d6dc5a4 100644
--- a/sound/pci/hda/patch_realtek.c
+++ b/sound/pci/hda/patch_realtek.c
@@ -4553,6 +4553,16 @@ static void alc271_hp_gate_mic_jack(struct hda_codec *codec,
        }
 }
 
+static void alc_fixup_dell_xps_9350(struct hda_codec *codec,
+                                   const struct hda_fixup *fix,
+                                   int action)
+{
+       if (action != HDA_FIXUP_ACT_PROBE)
+               return;
+
+       snd_hda_codec_amp_stereo(codec, 0x1a, HDA_INPUT, 0, HDA_AMP_VOLMASK, 1);
+}
+
 static void alc269_fixup_limit_int_mic_boost(struct hda_codec *codec,
                                             const struct hda_fixup *fix,
                                             int action)
@@ -4861,6 +4871,7 @@ enum {
        ALC298_FIXUP_DELL_AIO_MIC_NO_PRESENCE,
        ALC275_FIXUP_DELL_XPS,
        ALC256_FIXUP_DELL_XPS_13_HEADPHONE_NOISE,
+       ALC256_FIXUP_DELL_XPS_9350,
        ALC293_FIXUP_LENOVO_SPK_NOISE,
        ALC233_FIXUP_LENOVO_LINE2_MIC_HOTKEY,
        ALC255_FIXUP_DELL_SPK_NOISE,
@@ -5500,7 +5511,13 @@ static const struct hda_fixup alc269_fixups[] = {
                        {}
                },
                .chained = true,
-               .chain_id = ALC255_FIXUP_DELL1_MIC_NO_PRESENCE
+               .chain_id = ALC255_FIXUP_DELL2_MIC_NO_PRESENCE
+       },
+       [ALC256_FIXUP_DELL_XPS_9350] = {
+               .type = HDA_FIXUP_FUNC,
+               .v.func = alc_fixup_dell_xps_9350,
+               .chained = true,
+               .chain_id = ALC256_FIXUP_DELL_XPS_13_HEADPHONE_NOISE
        },
        [ALC293_FIXUP_LENOVO_SPK_NOISE] = {
                .type = HDA_FIXUP_FUNC,
@@ -5622,10 +5639,10 @@ static const struct snd_pci_quirk alc269_fixup_tbl[] = {
        SND_PCI_QUIRK(0x1028, 0x06de, "Dell", ALC293_FIXUP_DISABLE_AAMIX_MULTIJACK),
        SND_PCI_QUIRK(0x1028, 0x06df, "Dell", ALC293_FIXUP_DISABLE_AAMIX_MULTIJACK),
        SND_PCI_QUIRK(0x1028, 0x06e0, "Dell", ALC293_FIXUP_DISABLE_AAMIX_MULTIJACK),
-       SND_PCI_QUIRK(0x1028, 0x0704, "Dell XPS 13 9350", ALC256_FIXUP_DELL_XPS_13_HEADPHONE_NOISE),
+       SND_PCI_QUIRK(0x1028, 0x0704, "Dell XPS 13 9350", ALC256_FIXUP_DELL_XPS_9350),
        SND_PCI_QUIRK(0x1028, 0x0706, "Dell Inspiron 7559", ALC256_FIXUP_DELL_INSPIRON_7559_SUBWOOFER),
        SND_PCI_QUIRK(0x1028, 0x0725, "Dell Inspiron 3162", ALC255_FIXUP_DELL_SPK_NOISE),
-       SND_PCI_QUIRK(0x1028, 0x075b, "Dell XPS 13 9360", ALC256_FIXUP_DELL_XPS_13_HEADPHONE_NOISE),
+       SND_PCI_QUIRK(0x1028, 0x075b, "Dell XPS 13 9360", ALC256_FIXUP_DELL_XPS_9350),
        SND_PCI_QUIRK(0x1028, 0x075d, "Dell AIO", ALC298_FIXUP_SPK_VOLUME),
        SND_PCI_QUIRK(0x1028, 0x0798, "Dell Inspiron 17 7000 Gaming", ALC256_FIXUP_DELL_INSPIRON_7559_SUBWOOFER),
        SND_PCI_QUIRK(0x1028, 0x164a, "Dell", ALC293_FIXUP_DELL1_MIC_NO_PRESENCE),