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Date: Sun, 19 Apr 2009 18:20:00 -0700 From: "H. Peter Anvin" <hpa@...or.com> To: Roland Dreier <rdreier@...co.com> CC: Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>, "Robert P. J. Day" <rpjday@...shcourse.ca>, Hitoshi Mitake <h.mitake@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars Roland Dreier wrote: > > Notice that it reads from addr+4 *before* it reads from addr, rather > than after as in your example (and in fact your example depends on > undefined compiler semantics, since there is no sequence point between > the two operands of the | operator). Now, I don't know that hardware, > so I don't know if it makes a difference, but the niu example I gave in > my original email shows that given hardware with clear-on-read > registers, the order does very much matter. > At least for x86, the order should be low-high, because that is the order that those two transactions would be seen on a 32-bit bus downstream from the CPU if the CPU issued a 64-bit transaction. The only sane way to handle this as something other than per-driver hacks would be something like: #include <linux/io64.h> /* Any 64-bit I/O OK */ #include <linux/io64lh.h> /* Low-high splitting OK */ #include <linux/io64hl.h> /* High-low splitting OK */ #include <linux/io64atomic.h> /* 64-bit I/O must be atomic */ ... i.e. letting the driver choose what fallback method it will accept. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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