> From: [email protected] > [mailto:[email protected]] On Behalf Of Daniel Karling > Sent: Wednesday, October 15, 2014 2:08 PM > > Hi, > > I noticed that the macro for extracting the number of scratchpad > buffers to allocate from the HCSPARAMS2 register differs from what is > defined in the specification. > > Current macro is in xhci.h: > > /* bits 27:31 number of Scratchpad buffers SW must allocate for the HW */ > #define HCS_MAX_SCRATCHPAD(p) (((p) >> 27) & 0x1f) > > This macro is used in scratchpad_alloc and scratchpad_free in xhci-mem.c. > > According to "eXtensible Host Controller Interface for Universal > Serial Bus (xHCI)" revision 1.1, section 5.3.4, bits 27:31 are only > the five low order bits and there are five high order bits to get from > bits 21:25. > > Is there any particular reason for why the macro is as it is, or is this a > bug? > > Note: I haven't seen any host controllers actually requesting so many > scratchpad buffers that any of the high bits have to be used, but I > thought it might cause issues in the future.
That must be a relatively recent addition to the spec. In the original v1.0 spec, only bits 31:27 are specified for this. -- Paul
