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Annotation of driverkit/notes/DMARaces2, revision 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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