ALSA: emu10k1: improve cache behavior documentation

Resulting from more reverse engineering in the course of debugging.
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Message-ID: <20240406064830.1029573-15-oswald.buddenhagen@gmx.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/emu10k1.h

@@ -598,17 +598,25 @@ SUB_REG(PEFE, FILTERAMOUNT,	0x000000ff)	/* Filter envlope amount				*/
 // In stereo mode, the two channels' caches are concatenated into one,
 // and hold the interleaved frames.
 // The cache holds 64 frames, so the upper half is not used in 8-bit mode.
-// All registers mentioned below count in frames.
-// The cache is a ring buffer; CCR_READADDRESS operates modulo 64.
-// The cache is filled from (CCCA_CURRADDR - CCR_CACHEINVALIDSIZE)
-// into (CCR_READADDRESS - CCR_CACHEINVALIDSIZE).
+// All registers mentioned below count in frames. Shortcuts:
+//   CA = CCCA_CURRADDR, CRA = CCR_READADDRESS,
+//   CLA = CCR_CACHELOOPADDRHI:CLP_CACHELOOPADDR,
+//   CIS = CCR_CACHEINVALIDSIZE, LIS = CCR_LOOPINVALSIZE,
+//   CLF = CCR_CACHELOOPFLAG, LF = CCR_LOOPFLAG
+// The cache is a ring buffer; CRA operates modulo 64.
+// The cache is filled from (CA - CIS) into (CRA - CIS).
 // The engine has a fetch threshold of 32 bytes, so it tries to keep
-// CCR_CACHEINVALIDSIZE below 8 (16-bit stereo), 16 (16-bit mono,
-// 8-bit stereo), or 32 (8-bit mono). The actual transfers are pretty
-// unpredictable, especially if several voices are running.
-// Frames are consumed at CCR_READADDRESS, which is incremented afterwards,
-// along with CCCA_CURRADDR and CCR_CACHEINVALIDSIZE. This implies that the
-// actual playback position always lags CCCA_CURRADDR by exactly 64 frames.
+// CIS below 8 (16-bit stereo), 16 (16-bit mono, 8-bit stereo), or
+// 32 (8-bit mono). The actual transfers are pretty unpredictable,
+// especially if several voices are running.
+// Frames are consumed at CRA, which is incremented afterwards,
+// along with CA and CIS. This implies that the actual playback
+// position always lags CA by exactly 64 frames.
+// When CA reaches DSL_LOOPENDADDR, LF is set for one frame's time.
+// LF's rising edge causes the current values of CA and CIS to be
+// copied into CLA and LIS, resp., and CLF to be set.
+// If CLF is set, the first LIS of the CIS frames are instead
+// filled from (CLA - LIS), and CLF is subsequently reset.
 #define CD0			0x20		/* Cache data registers 0 .. 0x1f			*/
 
 #define PTB			0x40		/* Page table base register				*/