1 files changed, 51 insertions, 0 deletions

diff --git a/drivers/ata/sata_sx4.c b/drivers/ata/sata_sx4.c
index ff0a78dc8b86..3d83ee273388 100644
--- a/drivers/ata/sata_sx4.c
+++ b/drivers/ata/sata_sx4.c
@@ -30,6 +30,54 @@
  *
  */
 
+/*
+        Theory of operation
+        -------------------
+
+        The SX4 (PDC20621) chip features a single Host DMA (HDMA) copy
+        engine, DIMM memory, and four ATA engines (one per SATA port).
+        Data is copied to/from DIMM memory by the HDMA engine, before
+        handing off to one (or more) of the ATA engines.  The ATA
+        engines operate solely on DIMM memory.
+
+        The SX4 behaves like a PATA chip, with no SATA controls or
+        knowledge whatsoever, leading to the presumption that
+        PATA<->SATA bridges exist on SX4 boards, external to the
+        PDC20621 chip itself.
+
+        The chip is quite capable, supporting an XOR engine and linked
+        hardware commands (permits a string to transactions to be
+        submitted and waited-on as a single unit), and an optional
+        microprocessor.
+
+        The limiting factor is largely software.  This Linux driver was
+        written to multiplex the single HDMA engine to copy disk
+        transactions into a fixed DIMM memory space, from where an ATA
+        engine takes over.  As a result, each WRITE looks like this:
+
+                submit HDMA packet to hardware
+                hardware copies data from system memory to DIMM
+                hardware raises interrupt
+
+                submit ATA packet to hardware
+                hardware executes ATA WRITE command, w/ data in DIMM
+                hardware raises interrupt
+        
+        and each READ looks like this:
+
+                submit ATA packet to hardware
+                hardware executes ATA READ command, w/ data in DIMM
+                hardware raises interrupt
+        
+                submit HDMA packet to hardware
+                hardware copies data from DIMM to system memory
+                hardware raises interrupt
+
+        This is a very slow, lock-step way of doing things that can
+        certainly be improved by motivated kernel hackers.
+
+ */
+
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/pci.h>
@@ -58,6 +106,8 @@ enum {
         PDC_INT_SEQMASK         = 0x40, /* Mask of asserted SEQ INTs */
         PDC_HDMA_CTLSTAT        = 0x12C, /* Host DMA control / status */
 
+        PDC_CTLSTAT             = 0x60, /* IDEn control / status */
+
         PDC_20621_SEQCTL        = 0x400,
         PDC_20621_SEQMASK       = 0x480,
         PDC_20621_GENERAL_CTL   = 0x484,
@@ -89,6 +139,7 @@ enum {
 
         PDC_MASK_INT            = (1 << 10), /* HDMA/ATA mask int */
         PDC_RESET               = (1 << 11), /* HDMA/ATA reset */
+        PDC_DMA_ENABLE          = (1 << 7),  /* DMA start/stop */
 
         PDC_MAX_HDMA            = 32,
         PDC_HDMA_Q_MASK         = (PDC_MAX_HDMA - 1),