[PATCH v2 1/2] ALSA: docs: Add Marian M2 driver documentation

[Ivan Orlov <ivan.orlov0322@gmail.com>]

Add documentation for the new MARIAN Seraph M2 sound card. It covers
current development status, available controls of the card and
information about the integrated loopback.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
---
V1 -> V2:
- Remove redundant documentation fix from the next patch in the series

 Documentation/sound/cards/index.rst     |   1 +
 Documentation/sound/cards/marian-m2.rst | 104 ++++++++++++++++++++++++
 2 files changed, 105 insertions(+)
 create mode 100644 Documentation/sound/cards/marian-m2.rst

diff --git a/Documentation/sound/cards/index.rst b/Documentation/sound/cards/index.rst
index e68bbb13c384..e873592d8d00 100644
--- a/Documentation/sound/cards/index.rst
+++ b/Documentation/sound/cards/index.rst
@@ -19,3 +19,4 @@ Card-Specific Information
    serial-u16550
    img-spdif-in
    pcmtest
+   marian-m2
diff --git a/Documentation/sound/cards/marian-m2.rst b/Documentation/sound/cards/marian-m2.rst
new file mode 100644
index 000000000000..bf12445e20d7
--- /dev/null
+++ b/Documentation/sound/cards/marian-m2.rst
@@ -0,0 +1,104 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+MARIAN Seraph M2 Driver
+=======================
+
+Sep 18, 2023
+
+Ivan Orlov <ivan.orlov0322@gmail.com>
+
+STATE OF DEVELOPMENT
+====================
+
+This driver is based on the driver written by Florian Faber in 2012, which seemed to work fine.
+However, the initial code contained multiple issues, which had to be solved before sending the
+driver upstream.
+
+The vendor lost the full documentation, so what we have here was recovered from drafts and found
+after experiments with the card.
+
+What seems to be working fine:
+- Playback and capture for all supported rates
+- Integrated loopback (with some exceptions, see below)
+
+MEMORY MODEL
+============
+
+The hardware requires one huge contiguous DMA space to be allocated. After allocation, the bus address of
+this buffer should be written to the hardware register.
+
+We can split this space into two parts: the first one contains samples for capture, another one contains
+play samples:
+
+CAPTURE_CH_0, CAPTURE_CH_1, ..., CAPTURE_CH_127 | PLAY_CH_0, PLAY_CH_1, ..., PLAY_CH_127
+
+The card supports the non-interleaved access mode only, so samples for each channel lay together:
+
+C0, C0, ..., C0, C1, C1, ..., C1, ..., C127 | C0, C0, ..., C0, C1, C1, ..., C1, ..., C127
+
+The count of samples per each channel buffer needs to be set explicitly, so the address of the first
+byte of the playback data depends on this value. The playback buffer starts where the capture buffer ends.
+It makes the arbitrary period count/buffer size feature impossible to implement, and the driver supports only
+2 periods per buffer.
+
+Controls
+========
+
+Input 1 Sync
+    0 - No signal, 1 - valid MADI signal found, 2 - Synced with MADI signal
+Input 2 Sync
+    0 - No signal, 1 - valid MADI signal found, 2 - Synced with MADI signal
+
+Input 1 Channel Mode
+    0 - 56 channels, 1 - 64 channels
+Input 2 Channel Mode
+    0 - 56 channels, 1 - 64 channels
+
+Input 1 Frame Mode
+    0 - 48 kHz, 1 - 96 kHz
+Input 2 Frame Mode
+    0 - 48 kHz, 1 - 96 kHz
+
+Input 1 Frequency
+    Measured frequency on Input 1
+Input 2 Frequency
+    Measured frequency on Input 2
+
+Output 1 Channel Mode
+    0 - 56 channels, 1 - 64 channels
+Output 2 Channel Mode
+    0 - 56 channels, 1 - 64 channels
+
+Output 1 Frame Mode
+    0 - 48 kHz, 1 - 96 kHz
+Output 2 Frame Mode
+    0 - 48 kHz, 1 - 96 kHz
+
+Clock Source
+    Internal/Sync Bus, Input Port 1, Input Port 2
+
+Speed mode
+    VCO clock range (slow/fast)
+
+DCO Frequency (Hz)
+
+DCO Frequency (ms)
+
+DCO Detune
+    Range: -200...200
+
+Loopback
+    Enable/Disable integrated loopback
+
+
+Loopback
+========
+
+The card contains integrated loopback. When it is enabled, it sets the hardware’s DAW-in memory pointer
+to the hardware’s DAW-out memory. So, what you play is what you record.
+
+You can enable the integrated loopback using the corresponding control.
+
+The loopback seems to work well on lower rates (like 28000). However, when the rate goes higher, I observe
+multiple mistakes in recorded byte ordering.
-- 
2.34.1