Messages in this thread | | | Date | Fri, 2 Sep 2011 09:53:14 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [PATCH 0/3] RFC: addition to DMA API |
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On Fri, 2 Sep 2011, Ming Lei wrote:
> >> I am not sure if there are other devices except for EHCI(maybe have uhci/ohci). > > > > Yes: UHCI, OHCI, EHCI, and XHCI all poll memory constantly. > > In fact, the flush may be not required for ohci and xhci case, since there is > already one mmio register writing at the end of .enqueue path in ohci/xhci > driver.(just a glance at the code of ohci/xhci, please correct if I am wrong)
I don't know about xhci, but you're right about ohci. However, there's no guarantee that the mmio write will always remain there; somebody might change the code so that the write takes place only when it is needed instead of every time.
> For UHCI, looks like it has not been used on ARM, so maybe can ignore it. > UHCI is to support a slow usb 1.1 transfer, so I am wondering if the flush > can produce a obvious performance boost.
Believe me, even with USB-1.1 a 20-ms delay will be noticeable.
> So looks like the flush only makes sense on EHCI.
Assuming ARM is the only architecture that needs it.
> If the above is not wrong, is it really needed to introduce a general DMA API > only for EHCI?
Maybe not. I'm hoping that people will identify the underlying cause for these delayed write-backs and fix it. Then no changes at all would be needed in the USB stack.
Alan Stern
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