DMA doorbell

A method of direct memory access (DMA), comprising:

• preparing a first list of descriptors for execution by a DMA engine, each of the descriptors in the list comprising an instruction for execution by the DMA engine and a link to a succeeding one of the descriptors, except for a final descriptor in the list, which has a null link;
• submitting a first notification to the DMA engine that the first list has been prepared, so that the DMA engine will execute the descriptors in the first list responsive to the first notification;
• preparing a second list of the descriptors for execution by the DMA engine;
• modifying the link in the final descriptor in the first list so as to indicate a first descriptor in the second list;
• submitting a second notification to the DMA engine that the second list has been prepared, so that the DMA engine will execute the descriptors in the second list responsive to the second notification, substantially irrespective of an execution state of the DMA engine when it receives the second notification;
• wherein submitting the second notification comprises causing the DMA engine to reread at least a portion of only the final descriptor in the first list so as to locate thereby the first descriptor in the second list.

Apparatus for direct memory access (DMA), comprising:

• a DMA engine, adapted to transfer data between a data target and a data source responsive to descriptors submitted thereto;
• and a descriptor processor, adapted to prepare a first list of the descriptors for execution by the DMA engine, each of the descriptors in the list comprising an instruction for execution by the DMA engine and a link to a succeeding one of the descriptors, except for a final descriptor in the list, which has a null link, and coupled to submit a first notification to the DMA engine that the first list has been prepared, so that the DMA engine will execute the descriptors in the first list responsive to the first notification, and which is further adapted to prepare a second list of the descriptors for execution by the DMA engine and to modify the link in the final descriptor in the first list so as to indicate a first descriptor in the second list, and which is further coupled to submit a second notification to the DMA engine that the second list has been prepared, causing the DMA engine to execute the descriptors in the second list responsive to the second notification, substantially irrespective of an execution state of the DMA engine when it receives the second notification;
• wherein responsive to the second notification the DMA engine is operative to reread at least a portion of only the final descriptor in the first list so as to locate thereby the first descriptor in the second list.

A method of direct memory access (DMA), comprising:

• receiving a first notification at a DMA engine that a first list of descriptors has been prepared, each of the descriptors in the list comprising an instruction for execution by the DMA engine and a link to a succeeding one of the descriptors, except for a final descriptor in the list, which has a null link;
• reading and executing the descriptors in the first list using the DMA engine;
• receiving a second notification at the DMA engine that a second list of the descriptors has been prepared;
• rereading at least a part of the final descriptor in the first list to determine a changed value of the link, indicating to the DMA engine a first descriptor in the second list;
• and reading and executing the descriptors in the second list using the DMA engine responsive to the changed value of the link;
• wherein receiving the first and second notifications comprises receiving the notifications that the first and second lists have been stored at respective addresses in a memory, and wherein reading the descriptors comprises reading the descriptors from the memory;
• and
• wherein executing the descriptors comprises marking each of the descriptors as having been executed after executing each of the descriptors, and wherein marking each of the descriptors comprises marking the final descriptor in a manner that is distinctive from the marking of the other descriptors.

Apparatus for direct memory access (DMA), comprising a DMA engine, which is coupled to receive a first notification that a first list of descriptors has been prepared, each of the descriptors in the list comprising an instruction for execution by the DMA engine and a link to a succeeding one of the descriptors, except for a final descriptor in the list, which has a null link, and which is adapted to read and execute the descriptors in the first list, and which is further coupled to receive a second notification a second list of the descriptors has been prepared, and which is further adapted to reread at least a part of the final descriptor in the first list to determine a changed value of the link, indicating a first descriptor in the second list, and to read and execute the descriptors in the second list using the DMA engine responsive to the changed value of the link;

• the apparatus further comprising a memory, which is coupled to store the first and second lists, wherein the first and second notifications comprise notifications that the first and second lists have been stored at respective addresses in a memory, and wherein the DMA engine is coupled to read the descriptors from the memory;
• and
• wherein the DMA engine is adapted to mark each of the descriptors with an indication that the descriptor has been executed, and to mark the final descriptor in a manner that is distinctive from the other descriptors.