Date: Tue, 9 Oct 2012 11:54:15 -0400 From: John Baldwin <jhb@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: freebsd-hackers@freebsd.org, Carl Delsey <carl.r.delsey@intel.com> Subject: Re: No bus_space_read_8 on x86 ? Message-ID: <201210091154.15873.jhb@freebsd.org> In-Reply-To: <8BC4C95F-2D10-46A5-89C8-74801BB4E23A@bsdimp.com> References: <506DC574.9010300@intel.com> <201210051208.45550.jhb@freebsd.org> <8BC4C95F-2D10-46A5-89C8-74801BB4E23A@bsdimp.com>
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On Monday, October 08, 2012 4:59:24 pm Warner Losh wrote: > > On Oct 5, 2012, at 10:08 AM, John Baldwin wrote: > > > On Thursday, October 04, 2012 1:20:52 pm Carl Delsey wrote: > >> I noticed that the bus_space_*_8 functions are unimplemented for x86. > >> Looking at the code, it seems this is intentional. > >> > >> Is this done because on 32-bit systems we don't know, in the general > >> case, whether to read the upper or lower 32-bits first? > >> > >> If that's the reason, I was thinking we could provide two > >> implementations for i386: bus_space_read_8_upper_first and > >> bus_space_read_8_lower_first. For amd64 we would just have bus_space_read_8 > >> > >> Anybody who wants to use bus_space_read_8 in their file would do > >> something like: > >> #define BUS_SPACE_8_BYTES LOWER_FIRST > >> or > >> #define BUS_SPACE_8_BYTES UPPER_FIRST > >> whichever is appropriate for their hardware. > >> > >> This would go in their source file before including bus.h and we would > >> take care of mapping to the correct implementation. > >> > >> With the prevalence of 64-bit registers these days, if we don't provide > >> an implementation, I expect many drivers will end up rolling their own. > >> > >> If this seems like a good idea, I'll happily whip up a patch and submit it. > > > > I think cxgb* already have an implementation. For amd64 we should certainly > > have bus_space_*_8(), at least for SYS_RES_MEMORY. I think they should fail > > for SYS_RES_IOPORT. I don't think we can force a compile-time error though, > > would just have to return -1 on reads or some such? > > I believe it was because bus reads weren't guaranteed to be atomic on i386. > don't know if that's still the case or a concern, but it was an intentional omission. True. If you are on a 32-bit system you can read the two 4 byte values and then build a 64-bit value. For 64-bit platforms we should offer bus_read_8() however. -- John Baldwin
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