Re: [PATCH 1/2] USB: Allow HCDs to specifiy their DMA alignment requirements

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On Sat, Sep 4, 2010 at 3:38 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Not so.  The USB audio driver does this.  It allocates a coherent
> buffer and then logically divides the buffer up into multiple pieces,
> each of which gets used with its own URB.
>
Ok so I see, thanks for pointing this out.

>
> Why would a driver want to sync the buffers manually when usbcore
> already takes care of all that?  Besides, manual syncing won't mix with
> usbmon.
>
However Documentation/usb/dma.txt says:
- Some drivers may prefer to work with the model that they're mapping large
  buffers, synchronizing their safe re-use.  (If there's no re-use, then let
  usbcore do the map/unmap.)  Large periodic transfers make good examples
  here, since it's cheaper to just synchronize the buffer than to unmap it
  each time an urb completes and then re-map it on during resubmission.

Is this out of date then?

> Why go out of your way to create complexity?  Just assume that
> driver-managed mappings must always be coherent.
>
That's fine by me. I certainly have no desire to create complexity.

Regards,
Martin
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