關於PS3改機的問題 - 改機

※ 引述《tsairay (火の紅寶石)》之銘言：
: http://moss.csc.ncsu.edu/~mueller/cluster/ps3/doc/LinuxKernelOverview.html
: 看來看去就只有NIC有獨立的DMAC,所以能有背景下載
: 前景遊戲照玩的功能
: Giga Bit Ethernet(GbE) (Module name: gelic_net)
: PS3 has built-in giga bit ethernet controller. Unlike conventional PC’s
: ethernet controllers, it is not PCI device. It is connected to companion
: chip directly. To access or setup the chip, guest OSes should use dedicated
: hypervisor call. This controller has its dedicated DMA controller,
: thus transfer data is automatically sent/received without PPE interventions.
: There is also dedicated hypervisor call to set up DMAC.
: 而USB host controller是直接被PPE access的
: USB
: PS3 equips industry standard USB EHCI host controller and its companion
: OHCI host controllers. Although most USB host controllers are PCI devices,
: PS3 USB host controller exists in the companion chip. The PCI probe routines
: of Linux are modified so that PS3 USB host controller can imitate PCI device.
: After initialization, all operational registers of USB host controllers are
: directly accessed by PPE, so no modification to original Linux USB host
: controller driver is needed. PS3 has internal high speed USB hub.
以上這段只說明 PS3 的 usb host controller 是模擬成 PCI device
並沒說明他不支援 DMA，而你高亮度的地方也只說明 CPU 可直接操作 usb host
controller 的 operational registers，這裡也沒說明 PCI device 的 data 是怎麼
傳送的，CPU 直接存取 PCI device 的暫存器並不能代表沒使用 DMA。

: 至於visor說的DMA,我想指的是這個
: http://en.wikipedia.org/wiki/Cell_%28microprocessor%29
: the Cell processor marries the SPEs and the PPE via EIB to give access,
: via fully cache coherent DMA (direct memory access), to both main memory
: and to other external data storage.To make the best of EIB, and to overlap
: computation and data transfer, each of the nine processing elements
: (PPE and SPEs) is equipped with a DMA engine.
這邊有說明啦 PPC 有一個內建的 DMA engine 了，至於他有沒有用他去存取 USB，
我是有看到 linux for ps3 裡的 usb driver 有啟動 DMA 機制，我還沒看到其他資料
有說他是用 PPE 直接讀取 usb 的 data，至於 PS3 的 OS 我更無法得知了。

: 而這個DMA engine的目的不是為了降低在I/O時CPU的負擔
: 而是用CPU去加速I/O
: http://en.wikipedia.org/wiki/Direct_memory_access
: DMA engine
: In addition to hardware interaction, DMA can also be used to offload
: expensive memory operations, such as large copies or scatter-gather
: operations, from the CPU to a dedicated DMA engine. Intel includes such
: engines on high-end servers, called I/O Acceleration Technology (IOAT).
以上你的講法會讓懂 DMA 的人笑死.....

: ============================================================================
: 綜合以上幾點,可以證明PS3的USB傳輸是會耗用CPU資源的
: 想不耗用資源就只能像NIC那樣多加一個DMAC
不管怎樣，讀取 USB 一定會耗用 CPU 資源，畢竟 CPU 一定要下指令要 USB host
controller 去讀取哪個裝置上的資料，但是到了資料傳輸時，就可以省去 CPU 的時間
轉交由 DMAC 幫你讀取然後將資料轉存到 DRAM 上，DMA主要是不讓 CPU 去等待 device
回應的時間然後可以做其他事情，也可以減少 CPU 從 device 讀資料出來再寫回記憶體的
指令數量，並不是什麼用 CPU 去加速 I/O....

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