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    SubjectRe: [PATCH 2/7] ftrace, ppc: convert to new dynamic ftrace arch API

    On Mon, 17 Nov 2008, Paul Mackerras wrote:

    > Steven Rostedt writes:
    > > +static int test_24bit_addr(unsigned long ip, unsigned long addr)
    > > +{
    > > + unsigned long diff;
    > > +
    > > + /* can we get to addr from ip in 24 bits? */
    > > + diff = ip > addr ? ip - addr : addr - ip;
    > > +
    > > + return !(diff & ((unsigned long)-1 << 24));
    > Since a branch instruction can reach +/- 32MB, the 24 needs to be 25.
    > It's 25 not 26 since you have effectively accounted for one bit by
    > taking the absolute value of the offset. Also, this will say that an
    > offset of -0x2000000 is out of range whereas it is just in range.
    > That probably doesn't matter unless you're relying on this to give
    > exactly the same answer in all cases as tests done elsewhere (e.g. in
    > module_64.c).

    I'll update this to be correct.

    > Note that the address difference being too large isn't the only reason
    > for a procedure call to go via a trampoline on ppc64. A trampoline is
    > also needed when the called function uses a different TOC from the
    > caller. The most obvious example of this is when the caller is in a
    > module and the callee is in the main kernel or another module.
    > Currently all functions in the main kernel use the same TOC, but the
    > linker is capable of dividing up a large executable into groups of
    > functions and assigning a different TOC to each group, in order to
    > avoid any of the TOCs getting too large (a TOC is limited to 64kB).
    > If the linker does that, it automatically inserts trampolines to
    > handle the TOC change.

    OK, I'll deal with that at a another time. If this does happen in core
    kernel code, it will produce a nice warning and nicely shutdown ftrace,
    without causing any harm. Then I can deal with the bug report.

    -- Steve

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