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Annotation of driverkit/notes/DMARaces2, revision 1.1.1.1

1.1       root        1: descriptorList_t       *head, *tail;   // software's list of descriptors
                      2: 
                      3: /*
                      4:  * Returns CHAN_RUNNING/ CHAN_NOT_RUNNING.  CHAN_NOT_RUNNING implies that
                      5:  * a 'load descriptor' operation is required to start the channel.
                      6:  */
                      7: emptyIndication_t enqueueDescriptors(descriptorList_t *descFrame)
                      8: {
                      9:        if (channel's DNLE == 0) {
                     10:                /*
                     11:                 * hw out of descriptors, channel MUST be stopped.
                     12:                 * Set-up hardware to start with this descriptor if
                     13:                 * current list is empty, else enqueue on end of 
                     14:                 * (non-running) list. 
                     15:                 */
                     16:                if (tail == NULL)
                     17:                        head = descFrame;
                     18:                else
                     19:                        tail->next = descFrame;
                     20:                tail = tailOf(descFrame);
                     21:                hw_current_desc = descFrame;
                     22:                hw_count = -1;
                     23:                return CHAN_NOT_RUNNING;
                     24:        } else {
                     25:                /*
                     26:                 * Channel has descriptors to process.
                     27:                 * It may, or may not be currently running.
                     28:                 *
                     29:                 * First, just do what's necessary to enqueue descriptors
                     30:                 * in memory.
                     31:                 *
                     32:                 * Software queue CAN'T be empty....
                     33:                 */
                     34:                tail->next = descFrame;
                     35:                tail = tailOf(descFrame);
                     36:                
                     37:                /*
                     38:                 * Now, need to check to see if the memory enqueue "made it".
                     39:                 *
                     40:                 * The first test is to check the hw_next_desc.
                     41:                 *
                     42:                 * If the channel is running and hw_next_desc is non-null,
                     43:                 * then the update must have been in time (channel is running
                     44:                 * with more descriptors ready).
                     45:                 *
                     46:                 * Of course, if the channel is not running, there's no race
                     47:                 * (we wouldn't be here...??)
                     48:                 */
                     49:                if (hw_next_desc != NULL)
                     50:                        return CHAN_RUNNING;
                     51:                /*
                     52:                 * At this point, the channel MUST have been running since
                     53:                 * the last time this routine was called because that's the 
                     54:                 * only way that hw_next_desc can become null.
                     55:                 *
                     56:                 * Race to update hw_next_desc before the transfer engine
                     57:                 * needs to advance.
                     58:                 */
                     59:                hw_next_desc = descFrame;
                     60:                
                     61:                /*
                     62:                 * Have to do an implementation-dependent test for desc list
                     63:                 * not empty - updates to TE registers are atomic, but the
                     64:                 * update of the channel status register is not. Current
                     65:                 * desc_list_not_empty() test is to read channel's DNLE bit
                     66:                 * 'n' times here; any read of 0 is a failure (i.e., it's 
                     67:                 * empty).
                     68:                 */
                     69:                if (desc_list_not_empty() == 1)
                     70:                        return CHAN_RUNNING;
                     71:                        
                     72:                /*
                     73:                 * Lost the race.  The channel MUST be stopped.
                     74:                 *
                     75:                 * Set-up the channel so that a driver ENABLE will restart
                     76:                 * it with descFrame.
                     77:                 */
                     78:                hw_current_desc = descFrame;
                     79:                hw_count = -1;
                     80:                return CHAN_NOT_RUNNING;
                     81:        }
                     82: }
                     83: 

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