Controller and method for statistical allocation of multichannel direct memory access bandwidth

A DMA controller and a method for statistical allocation of multichannel DMA bandwidth. In one embodiment, the DMA controller includes: (1) channel interfaces including respective counters and configured to provide request signals, priority signals and counter value signals representing current values of the counters at a given time and (2) a grant control unit coupled to the channel interfaces and configured to grant DMA access to one of the channel interfaces based on values of the priority signals and the counter value signals.

TECHNICAL FIELD

This application is directed, in general, to direct memory access (DMA) and, more specifically, to a controller and method for statistical allocation of multichannel DMA bandwidth.

BACKGROUND

In the simplest computer systems, the processor manages the movement of data between the memory and peripheral devices, such as graphics subsystems and ports. Unfortunately, this burdens the processor with not only processing, but moving, all data. As a result, the processor's speed frequently limits the computer's overall performance. More complex computer systems employ direct memory access (DMA). In DMA, a DMA controller separate from the processor moves data between the memory and peripheral devices. The processor's role is therefore reduced, and the computer's overall performance is enhanced.

In computer systems having multiple peripheral devices, each peripheral device is assigned a DMA channel, and allocation (called “granting”) of DMA bandwidth (sometimes expressed in terms of “time slots”) between or among the channels becomes a challenge. In those systems in which the channels are of equal priority, bandwidth is granted based on a round-robin algorithm. In those systems in which the channels are of unequal priority, higher-priority channels are granted bandwidth until they no longer require it. Only then is bandwidth granted to lower-priority channels. The disadvantage of the latter approach is that the lower-priority channels may receive insufficient bandwidth.

One example of the latter approach is found in U.S. Patent Publication 2006/0004931, in which memory access bandwidth within a digital camera is allocated among several channels by assigning each channel a “tokens per snapshot” (TPS) value. Each channel has a DMA engine and a DMA entry queue. If the channel wishes to access the memory, then a DMA entry is pushed onto the DMA entry queue of the channel. An arbiter uses the TPS values to select DMA entries off the various queues for incorporation into a “snapshot.” The arbiter then selects DMA entries from the snapshot in an order for servicing such that memory access overhead in accessing the memory is reduced. Only after all DMA entries of the snapshot have been serviced is another snapshot of entries selected. Maximum latency in servicing a queue is controlled by assigning each queue a time-out value (TOV). If a queue times out, then that queue is moved up in the order of servicing.

In U.S. Pat. No. 6,430,194, bus access is arbitrated among modules connected to a common bus. Each module has a priority level and an arbitration number assigned to it. More than one module can have the same priority level. For each priority level, the arbitration numbers assigned are unique. When two or more modules attempt bus access at the same time, the one with the higher priority level wins access. If the priority levels are the same but one module has already accessed the bus, the module that has been waiting wins access. If the modules have the same priority level and have been waiting then the module with the highest arbitration number wins access.

U.S. Pat. No. 7,085,875 discloses a modular switch, comprising a plurality of backplane sub-buses; a plurality of cards which are each allocated one or more of the backplane sub-buses and a controller that dynamically allocates the backplane sub-buses to the plurality of cards, based on the bandwidth needs of the cards. Preferably, the bandwidth capacity of substantially all the backplane sub-buses is less than the sum of the maximal transmission bandwidth capacities of the cards.

In U.S. Pat. No. 7,360,068, a dynamically reconfigurable processing unit includes a microprocessor and an embedded flash memory for nonvolatile storage of code, data and bitstreams. The embedded flash memory includes a field programmable gate array (FPGA) port. The reconfigurable processing unit further includes a direct memory access (DMA) channel, and an SRAM embedded FPGA for FPGA reconfigurations. The SRAM embedded FPGA has an FPGA programming interface connected to the FPGA port of the flash memory through the DMA channel interface.

PCT Application No. WO/2002/039631 discloses a method of prioritizing network resources in a network that includes providing the network with a high priority channel and a low priority channel. The high priority channel has insufficient bandwidth resources to transmit a message on the high priority channel. The high priority channel reserves bandwidth resources from a local free list. If this is insufficient, the high priority channel preempts bandwidth resources of the low priority channel. If this is insufficient to send the message, the high priority channel obtains bandwidth resources from the nodes in the network so the message can be send on the high priority channel.

SUMMARY

One aspect provides a DMA controller. In one embodiment, the DMA controller includes: (1) channel interfaces including respective counters and configured to provide request signals, priority signals and counter value signals representing current values of the counters at a given time and (2) a grant control unit coupled to the channel interfaces and configured to grant DMA access to one of the channel interfaces based on values of the priority signals and the counter value signals.

Another aspect provides a method of statistically allocating multichannel DMA bandwidth. In one embodiment, the method includes: (1) providing request signals, priority signals and counter value signals representing current values for counters of channel interfaces at a given time and (2) granting DMA access to one of the channel interfaces based on values of the priority signals and the counter value signals.

DETAILED DESCRIPTION

As described above, in computer systems having multiple channels of unequal priority, higher-priority channels are granted bandwidth until they no longer require it. Only then is bandwidth granted to lower-priority channels. Again, the disadvantage of this approach is that the lower-priority channels may receive insufficient bandwidth.

Introduced herein are various embodiments of controllers and methods for allocating DMA bandwidth that can yield a better overall system performance by allowing all channels to receive a time slot for transferring their data. Conventional approaches do not allow priority to be given to channels according to their weight while continuing to guarantee that lower priority channels are granted at least an occasional time slot.

FIG. 1is a block diagram of a computer system employing DMA in which a controller or method for statistical allocation of multichannel DMA bandwidth may be incorporated or carried out. The system employs a processor100and a memory110, coupled together by a bus120. The processor100, memory110and bus120may be of any conventional or later-developed type. As described above, the system employs DMA to relieve the processor100of having to manage at least some transfers of data into or out of the memory110. Accordingly, a DMA controller130is provided for such purpose.

As those skilled in the art understand, the DMA controller130is configured to grant to various peripheral devices (e.g., a peripheral device1140-1, a peripheral device2,140-2and a peripheral device N140-N) temporary access to the memory110via the bus120. Temporary access will sometimes be referred to herein in terms of one or more “slots.” Since multiple peripheral devices exist in the embodiment ofFIG. 1, a contention for resources (expressed in terms of bandwidth) also exists. Among other things, the DMA controller130is configured to resolve the contention such that overall performance is at or near its highest possible level. To achieve this, the DMA controller130includes a grant control unit (GCU)210, and each peripheral device includes a DMA channel interface. More specifically, peripheral device1140-1includes a DMA channel interface220-1, peripheral device2140-2includes a DMA channel interface220-2, and peripheral device N140-N includes a DMA channel interface220-N. The DMA channel interfaces220-1,220-2,220-N and the GCU210cooperate with one another to manage DMA bandwidth. The DMA controller130also includes a GCU210. Various embodiments of the GCU210and the DMA channel interfaces220-1,220-2,220-N will now be described in greater detail.

FIG. 2is a block diagram of one embodiment of a controller for statistical allocation of multichannel DMA bandwidth.FIG. 2shows the GCU210ofFIG. 1and groups the DMA channel interfaces220-1,220-2,220-N ofFIG. 1together into n channel interfaces220. In the embodiment ofFIG. 2, each channel is assigned its own, unique priority. Each of the n channel interfaces220includes a counter221configured to provide a request signal and a signal representing its current value at a given time to the GCU210. The counter221is further configured to receive a priority signal, which represents the channel's priority and will be designed herein as Pn. The counter221is still further configured to decrement upon receipt of a grant signal, i.e., when the GCU210grants DMA access to that channel. The counter221is yet further configured to receive a reset signal from the GCU220, whereupon the counter221is reset to the initial value.

The GCU220includes a multiplexer211configured to receive and select among a plurality of request signals received from the n channel interfaces220. The GCU220further includes a mathematical function block (an adder212in the context ofFIG. 2) configured to receive a plurality of counter value signals received from the n channel interfaces220and apply a mathematical function to their values to yield a result. In the illustrated embodiment, the adder212is configured to receive the plurality of counter value signals and add at least some of their values together to yield a sum. In a more specific embodiment, only the values of the counter value signals corresponding to active ones of the channels are added. The GCU220still further includes a priority select module213configured to receive and select among the priority signals received from the n channel interfaces220based on the values of the various priority signals and the sum of the counter value signals as received from a register214. In a more specific embodiment, the priority select module213makes its selection based on the ratios of each of the values of the various priority signals to the sum of the active ones of the channels.

Various embodiments will now be described by the priority select module213may use this ratio to grant requests. One example embodiment calls for sequential execution and operates as follows. First, each counter is initialized with its priority. An example priority for a particular channel220may be six. Afterwards, the counters are decremented as the GCU210grants access to corresponding channels. Channels having the same counter value at any given time may then be changed round-robin or by any other evenhanded scheme.

Another example embodiment calls for nonsequential execution and operates as follows. First, the GCU210generates a table containing a list of each active channel220. The table has a number of entries equal to SP, and each channel220has Pn entries in the table. The GCU210also generates a pseudorandom integer number R in the group [1 . . . SP], and the GCU210grants access to the channel220entered at address R. For example, if two channels having respective priorities of three and two exist, Table 1, below, results:

In general, the illustrated embodiment of the controller operates as follows. Each channel220has its own priority (Pn). Upon initialization, the counter221for each channel is loaded with its respective Pn. Then, each channel220needing DMA access (i.e., active channel) sends its request along with its Pn. The GCU210then gathers the requests from all channels, and calculates the SP value (sum of Pn) and latches it. The priority select module picks the next channel to be granted sequentially or nonsequentially as described above or by another execution technique. The channel that was granted access then decrements its counter. When all the channel's counters220are cleared, the counters are reset and reloaded with their respective Pn.

FIG. 3is a flow diagram of one embodiment of a method of statistically allocating multichannel DMA bandwidth. The method begins in a start step305. In a step310, the counters of each channel are loaded with their respective Pn. In a step315, each active channel sends its request along with its Pn. In a step320, the requests from all channels are gathered. In a step325, SP (the sum of active channel Pn) is calculated. In a step330, The next channel to be granted access is selected based on ratios of Pn to SP. In a step335, the counter corresponding to the channel that was granted access is then decremented.