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+
+                                   HARP Notes
+                                   1998-09-14
+
+This is a list of currently known incompatibilities and miscellaneous gotchas 
+in HARP.  
+
+To report new items, please send mail to harp-bugs@magic.net.
+
+================================================================================
+
+
+Efficient Driver and DMA sizes
+==============================
+
+The Efficient adapter moves PDUs between host memory and adapter memory with 
+the help of DMA descriptor lists. Each DMA descriptor consists of two words. 
+Word 0 contains a DMA type identifier and a repetition count. Word 1 contains 
+the physical (not virtual) host buffer address. Each DMA type is really an 
+encoding of the burst size for the DMA. (See /usr/src/sys/dev/hea/eni.h for
+more on the DMA types.) HARP was originally developed using burst sizes of 
+8_WORD, 4_WORD, and 1_WORD sizes. Each DMA request would be built to first 
+move as much data as possible using an 8_WORD burst. This should leave 0-7 
+words left over. If there were more than 3 words remaining, a 4_WORD DMA burst 
+would be scheduled. The remaining data must then be 0-3 words in length and 
+would be moved with 1_WORD bursts. The use of large burst sizes makes more 
+efficient use of DMA by performing the same amount of work in fewer cycles.
+
+Several users have reported problems with DMA which were characterized by error
+messages of the form:
+
+	"eni_output: Transmit drain queue is full. Resources will be lost."
+or
+	"eni_output: not enough room in DMA queue".
+
+It was determined that these systems do not support the use of four- or 
+eight-word DMA bursts.  To resolve this problem, HARP now #ifdef's around the 
+8_WORD and 4_WORD DMA setup and #undef's both values by default. This results 
+in the default operation of the Efficient driver to use only 1_WORD DMA bursts.
+
+If you wish to experiment with larger DMA bursts, you can edit the file
+/usr/src/sys/dev/hea/eni_transmit.c and change the #undef to a #define for 
+DMA_USE_8WORD and/or DMA_USE_4WORD. You will need to rebuild and install your 
+kernel for this change to take effect.
+
+We are exploring solutions which would allow HARP to determine which DMA bursts
+are supported by the system at run-time.  This would allow the Efficient device
+driver to make use of larger, more efficient burst sizes where supported 
+without halting on systems which can't support the larger sizes.
+
+
+
+	@(#) $Id: NOTES,v 1.1 1997/08/22 22:11:17 mks Exp $
+