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    SubjectRe: Using kernel_fpu_begin() in device driver - feasible or not?
    Alan Cox wrote:
    >
    > > 1. Would it be sufficient to just bracket all fpu-using code code by
    > > kernel_fpu_begin()/kernel_fpu_end()? If not, what problems could I
    > > run into?
    >
    > You can do that providing you dont
    >
    > > 2. Would it be OK to go to sleep inside such a region, or should I
    > > take care to avoid that?
    >
    > You can't sleep in such a region - there is nowhere left to store the
    > FPU context
    >
    > > 3. Perhaps I should call init_fpu() at some point as well? If so,
    > > should it be done before or after kernel_fpu_begin()?
    >
    > After
    >
    > > 4. Is there any difference between doing this in the context of a user
    > > process (implementation of an ioctl) compared to doing it in a
    > > daemonized kernel thread (created by a loadable kernel module)?
    >
    > The kernel thread is actually easier, you can happily corrupt its user
    > FPU context by sleeping since you are the only FPU user for the thread.
    > Not nice, not portable but should work fine on x86 without any of the
    > above for the moment.
    >
    > You should probably also test the FPU is present and handle it accordingly
    > with polite messages not an oops 8)

    So are kernel_fpu_begin()/kernel_fpu_end() (and also init_fpu()) necessary in a kernel thread at all
    ? Does kernel_fpu_begin()/kernel_fpu_end() take care of SSE and MMX registers too (I'm aware of the
    extra "emms" needed in the MMX case) ?

    Regards,
    --
    Steffen Persvold | Scalable Linux Systems | Try out the world's best
    mailto:sp@scali.com | http://www.scali.com | performing MPI implementation:
    Tel: (+47) 2262 8950 | Olaf Helsets vei 6 | - ScaMPI 1.13.8 -
    Fax: (+47) 2262 8951 | N0621 Oslo, NORWAY | >320MBytes/s and <4uS latency
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