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    SubjectRe: patch: delay_50ms() in ide-probe.c in 2.2.14
    Date
    From
     <arijort@valinux.com> wrote:
    >
    > Simple arithmetic tells me this patch
    >
    > a) produces same behavior as previous code, and

    No.

    > b) does it with fewer calculations.
    >
    > So it has to be good, right?

    No. Its bad and doesn't work.

    > But then how do we exit the while loop?

    In your case: never.

    > How does jiffies change?

    It is modified from the timer interrupt. It is also marked "volatile".
    This means that the compiler is NOT allowed to move the "move jiffies
    into a register" outside the while loop.

    So, without that volatile the compiler would indeed be allowed to
    do something like

    load jiffies, r4 ; jiffies temporarily in r4.
    add r4, 6, r5 ; calculate timeout into r5.

    0: sub r4, r5, r0
    jnz 0b

    (which if the compiler is really good actually optimizes even futher:
    0: jmp 0b
    )

    but with the volatile attribute on the "jiffies" variable, it will
    need to reload "jiffies" every time around the loop:

    load jiffies, r4 ; jiffies temporarily in r4.
    add r4, 6, r5 ; calculate timeout into r5.

    0: load jiffies, r4 ; reload jiffies in r4.
    sub r4, r5, r0
    jnz 0b

    And the timer interrupt will increment jiffies 100 times
    a second, until the thing times out.

    Roger.


    --
    ** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
    *-- BitWizard writes Linux device drivers for any device you may have! --*
    * Common sense is the collection of *
    ****** prejudices acquired by age eighteen. -- Albert Einstein ********

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