In message <[EMAIL PROTECTED]>, Hans Petter Selasky writes: >On Sunday 02 July 2006 14:05, Ian Dowse wrote: >> This data structure requires the associated data buffer to be >> contiguous (relative to virtual memory), but allows the physical >> memory pages to be non-contiguous. Seven page pointers are provided >> to support the expression of 8 isochronous transfers. The seven >> pointers allow for 3 (transactions) * 1024 (maximum packet size) >> * 8 (transaction records) (24576 bytes) to be moved with this >> data structure, regardless of the alignment offset of the first >> page. > >3 * 1024 bytes = 0xC00 bytes > >8 * 0xC00 = 0x6000 bytes maximum > >According to this you need "6" "EHCI pages", because "6 * 0x1000 = 0x6000". >The seventh "EHCI page" is just there to allow one to start at any page >offset. There is no eight "EHCI page". > >The only solution I see, is to have a double layer ITD. The first layer have >the 4 first transfers activated, and the second layer have the 4 last >transfers activated. > >A little more complicated, but not impossible.
The trick is that if the 0x6000 bytes are contiguous in virtual memory then they never span more than 6 pages so one iTD is enough. i.e. you can just do malloc(0x6000) and you don't need multi-page physically contiguous buffers or extra memory-memory copies regardless of how the virtual buffer maps to physical pages. This seems to be the general extent of scatter-gather support offered by the various USB host controllers (modulo various caveats such as assuming pages are >= 4k, handling physical addresses > 4GB on non-IOMMU hardware and UHCI's lack of support for mid-packet non-contiguous page boundaries). Ian _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
