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    SubjectRe: [PATCH] introduce lower_32_bits() macro
    On Fri, 25 Jul 2008 12:06:27 -0400
    "H. Peter Anvin" <hpa@zytor.com> wrote:

    > Joerg Roedel wrote:
    > > The file kernel.h contains the upper_32_bits macro. This patch adds the other
    > > part, the lower_32_bits macro. Its first use will be in the driver for AMD
    > > IOMMU.
    > >
    > > Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
    > > ---
    > > include/linux/kernel.h | 6 ++++++
    > > 1 files changed, 6 insertions(+), 0 deletions(-)
    > >
    > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    > > index f9cd7a5..6fd2977 100644
    > > --- a/include/linux/kernel.h
    > > +++ b/include/linux/kernel.h
    > > @@ -73,6 +73,12 @@ extern const char linux_proc_banner[];
    > > */
    > > #define upper_32_bits(n) ((u32)(((n) >> 16) >> 16))
    > >
    > > +/**
    > > + * lower_32_bits - return bits 0-31 of a number
    > > + * @n: the number we're accessing
    > > + */
    > > +#define lower_32_bits(n) ((n) & 0xffffffffULL)
    > > +
    >
    > NAK. These are assymmetric with regards to type, which is the *last*
    > thing we want.

    Yes, it will convert a 32-bit expression into a 64-bit one.

    > The symmetric definition would be ((u32)(n)), but that's already
    > idiomatic use, so why not use it as-is?

    There's some readability benefit. Sometimes it is hard to understand
    why some random open-coded cast was used. But I seem to recall that
    there was another reason why we decided we needed this. I forget, and
    so apparently did the changelog author ;)




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