On Sat, Sep 4, 2010 at 4:46 PM, Ming Lei <tom.leiming@xxxxxxxxx> wrote: > > Seems there is very few such cases (buffer is not aligned to the > HC controller requirement) now, why not fix it in upper driver (do the > bounce and PIO in interface driver instead of usbcore)? > What is the "upper driver"? (the Host Controller Driver or the usb "function" driver (usbnet, etc etc). [Is there an accepted non ambiguous term for this more explicit than just "usb driver"?] I don't think fixing this in the "function" driver is right - there are too many of them and they shouldn't need to be concerned with this type of low level issue. Whether they should be able to discover the low level requirements to perform _optional_ optimisations is another question. Here are the tradeoffs I see between usbcore and HCD implementation: Advantages of usbcore implementation: * Single point of implementation for all HCDs that may need it Currently I think imx21-hcd is the only in tree one but at least two others not yet in tree have similar requirements. Someone also reported using this patch for musb but I think there's another patch out there for that. There is already a precedent in that usbcore manages bounce buffers for HCDs requiring "local" memory (HCD_LOCAL_MEM) * For core mapped buffers no need to sync the buffer before copying it (since the core can copy the buffer before mapping it) Advantages of HCD implementation: * HCD can make "intelligent" decisions (such as using bounce buffer for large unaligned transfers and PIO for small ones) * If bounce buffer is implemented in the core it will be allocated at URB submission time rather than when the hardware needs it. This will waste memory if multiple URBs are queued for the same endpoint (each will have a bounce buffer but only one per endpoint will ever be needed at one time) I've only just realized this second point and I think it's a good reason for me to drop this patch and implement it in the HCD. Unless anyone wants to convince me otherwise :) Regards, Martin -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
