Patent Application
20060123180
The present invention pertains to the field of computer system design. More particularly, the present invention relates to a USB schedule prefetcher that allows a processor to enter a power-saving mode during periods when no USB activities are scheduled.
A computer system may be equipped with a Universal Serial Bus (USB). USB ports allow USB-enabled devices to connect and communicate with the computer system. Examples of electronic devices that communicate with computer systems through USB ports include digital cameras, keyboards, hard drives, and printers.
A USB host is in charge of the USB bus in a computer system. The USB host is a collection of software and hardware inside the computer system that supports the USB bus. The USB host is typically responsible for identifying devices that are connected to a USB port. The USB host may then load any needed device drivers dynamically. Finally, the USB host may periodically poll each of the attached devices for data communications.
a is an embodiment of a flowchart for a prefetch engine filling a future activity vector.
b is an embodiment of a flowchart of a periodic direct memory access (DMA) engine accessing memory.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
USB data may be delivered isochronously. Software usually schedules a USB periodic list to communicate data transfer and interrupt information to a USB host controller. Such a periodic linked list schedule structure may be stored in the main memory of a computer system.
The USB host controller may be located in a chipset/controller hub. USB data may be transferred in approximately 125 microsecond-granular frames. However, the granularity of the frames is not so limited. As an exemplary range, frames may have a temporal length of 75 to 175 microseconds. The USB host controller may access the linked list structure directly from main memory. However, continuous access of main memory may require continuous snooping of a processor's cache. As a result, the processor may be prevented from being placed in a low power mode.
A processor is in the “C0” state if the processor is operating at full power. The processor is in the “C1” state if the processor gates some internal clocks. The processor is in the “C2” state if an external device drives a pin to the processor to stop internal clocks. However, in the C2 state, the processor cache may still be snooped. The “C3” state is similar to the “C2” state. In the C3 state, however, the cache may not be snooped. Finally, the processor is in the “C4” state if internal clocks are stopped and the processor voltage level is decreased. The C0, C1, C2, C3, and C4 states may be similar to or equal to the processor states defined by the Advanced Configuration and Power Interface (ACPI) specification.
For one embodiment of the invention,
Software of the computer system schedules a USB periodic list. The periodic list instructs a USB host controller when to run interrupt and isochronous transfers to and from a USB port. The periodic list is stored in memory 130. Memory 130 may be Dynamic Random Access Memory (DRAM) or any other commonly used random access memory (RAM). USB data may be transferred from chipset 120 to USB port 140 in approximately 125 microsecond granular frames. For this embodiment of the invention, the schedule prefetcher 125 in chipset 120 checks for frames where the periodic list has activity scheduled. Furthermore, schedule prefetcher 125 tracks the frames where USB activities are scheduled. Memory 130 is subsequently accessed by a periodic DMA engine during frames having scheduled USB activities. The periodic DMA engine will be discussed in more detail in reference to
Schedule prefetcher 210 may access structures in memory via memory bus 260. The memory bus 260 may be coupled to a dynamic random access memory (DRAM). Schedule prefetcher 210 may read the structures and check for scheduled activity in frames, the frames being pointed to by a software generated periodic list with USB scheduled activities. Schedule prefetcher 210 may then mark frames having USB activities scheduled as “active” and frames not having USB activities schedules as “not active.” Schedule prefetcher 210 may store the results in the future activity vector 220. The results may be stored as bits, such as activity bits that represent whether activity for a corresponding frame is scheduled (active) or not scheduled (inactive/not active).
For example, each bit represents a 125 microsecond granular frame. Schedule prefetcher 210, if a frame has USB activity scheduled, sets a corresponding bit in the future activity vector 220. Similarly, schedule prefetcher 210, if the frame has no USB activity scheduled, clears the corresponding bit in future activity vector 220. Future activity vector 220 may be a circular first in first out (FIFO) structure having associated pointers to track storage locations in the FIFO structure.
Frame index counter 230 tracks the frames accessed-by periodic DMA engine 250. Frame index counter 230 may advance, or retard based on the design, the frame index approximately every 125 microseconds. Pause logic 240 reads the frame data from future activity vector 220 as the frame index counter 230 advances the frame index. The activity bit associated with the current frame index is used to determine whether or not to disable/pause the periodic DMA engine 250 for that frame.
Pause logic 240 pauses the data communication to the periodic DMA engine whenever the current frame's activity bit is cleared. As a result, the periodic DMA engine 250 may access memory only during frames where USB traffic is scheduled. By creating long enough idle times on a processor and memory, in the presence of connected, but inactive USB devices, the processor may enter a C3, a C4, or other power management state. As a specific example, if an activity bit within future activity vector 220 is set/cleared to represent no scheduled activity for the current frame by schedule prefetcher 210, then pause logic 240 disables/pauses periodic DMA engine 250. Therefore, periodic DMA engine 250 does not access memory and potentially cause a cache snoop that would disallow processor 110 from entering a low power state.
Schedule prefetcher 210 opportunistically refills future activity vector 220 while the processor and memory are busy anyway. As an example, prefetcher 210 accesses system memory and refills the future activity vector 220 in bursts during times when the system memory is in use anyway, thereby allowing long periods of idle time on the memory and cache interfaces.
a is an embodiment of a flowchart for a schedule prefetcher filling a future activity vector. The system is powered-up in operation 310. An operating system may schedule interrupt and isochronous transfers for a USB port in operation 320. The USB transfers may be scheduled in approximately 125 microsecond granular frames and stored in main memory. Next, a schedule prefetcher may access and parse the USB periodic linked list schedule structures in main memory in operation 330. The transfer frames are marked as “active” or “inactive” in operation 340, based on whether activity is scheduled in the structures of the linked list. The results are then stored in a future activity vector. If the future activity vector is full then schedule prefetcher is paused in operation 355. However, if the future activity vector is not full, then the frame index counter advances, as normal, and the schedule prefetcher accesses and parses the next frame in memory. Once an opportunistic prefetch occurs in operation 365 or the future activity vector is no longer full, schedule prefetcher accesses and parses the next structure in memory, returning to operation 330.
Turning to
Once frame index counter is advanced in operation 370, which occurs approximately every 125 microseconds, data is read from the future activity vector by the periodic DMA engine in operation 375. If there is not USB traffic scheduled in the frame, as represented by the future activity vector, the frame index counter is advanced again in a return to operation 370 without the periodic DMA engine accessing memory. In contrast, if activity is scheduled in the frame, as represented by the future activity vector, memory is accessed in operation 385. The ability to not access memory in a frame where no activity is scheduled enables a processor to be placed in a low power mode, because unnecessary memory accesses and cache snoops are not committed.
DMA engine schedule prefetcher 410 may access structures in memory via memory bus 460. Memory bus 460 may be coupled to a DRAM. DMA engine schedule prefetcher 410 may read the structures and check for frames where a software generated periodic list has USB activities scheduled. DMA engine schedule prefetcher 410 may then mark frames having USB activities scheduled as “active” and frames not having USB activities schedules as “not active.” DMA engine schedule prefetcher 410 may store the results in future activity vector 420. DMA engine schedule prefetcher 410 may set a bit in future activity vector 420, if a frame is marked active. Similarly, DMA engine schedule prefetcher 410 may clear a bit in future activity vector 420, if a frame is marked not active. Future activity vector 420 may be a circular first in first out (FIFO) structure having associated pointers to track storage locations in the FIFO structure.
In addition to storing active and inactive bits in future activity vector 420, identified active control structures may be stored in random access memory 415.
The frame index counter 430 tracks the current frame for Periodic DMA Engine 450 and its associated Pause Logic 440. The frame index information is also passed to Schedule Prefetcher 410 in order to update Future Activity FIFO 420 before it is needed. Frame index counter 430 may advance the frame index approximately 125 microseconds at a time. Pause logic 440 reads the frame data from future activity vector 420, as frame index counter 430 advances the frame index. The data is passed from pause logic 440 to periodic DMA engine 450. However, pause logic 440 may pause the data communication to periodic DMA engine 450 whenever the current frame's activity bit is cleared. As a result, periodic DMA engine 450 may access memory during frames where USB traffic is scheduled. However, DMA engine 450 may read active control structures from random access memory 415 rather then re-reading the structures through memory bus 460. By creating long enough idle times on a processor and memory complex in the presence of connected, but inactive USB devices, the processor may enter a C3 a C4, or other power management state.
If DMA engine schedule prefetcher 410 fills future activity vector 420, pause logic 420 may pause schedule prefetcher 355. Alternatively, if future activity vector 420 is not full, DMA engine schedule prefetcher accesses and parses the next frame in memory 415.
In the foregoing specification the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modification and changes may be made thereto without departure from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.
