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    SubjectRe: [RFC][PATCH] bcmai: introduce AI driver
    On Wed, 2011-04-06 at 23:01 +0200, Rafał Miłecki wrote: 
    > W dniu 6 kwietnia 2011 22:57 użytkownik Michael Büsch <> napisał:
    > > On Wed, 2011-04-06 at 22:42 +0200, Rafał Miłecki wrote:
    > >> 2011/4/6 Rafał Miłecki <>:
    > >> > If we want to have two drivers working on two (different) cores
    > >> > simultaneously, we will have to add trivial mutex to group core
    > >> > switching with core operation (read/write).
    > >>
    > >> With a little of work we could avoid switching and mutexes on no-host
    > >> boards. MMIO is not limited to one core at once in such a case.
    > >
    > > I don't think that this is a problem at all.
    > > All that magic does happen inside of the bus I/O handlers.
    > > Just like SSB does it.
    > > From a driver point of view, the I/O functions just need to
    > > be atomic.
    > >
    > > For SSB it's not always 100% atomic, but we're always safe
    > > due to some assumptions being made. But this is an SSB implementation
    > > detail that is different from AXI. So don't look too closely
    > > at the SSB implementation of the I/O functions. You certainly want
    > > to implement them slightly differently in AXI. SSB currently doesn't
    > > make use of the additional sliding windows, because they are not
    > > available in the majority of SSB devices.
    > >
    > > The AXI bus subsystem will manage the sliding windows and the driver
    > > doesn't know about the details.
    > Sure, I've meant mutex inside bcmai (or whatever name), not on the driver side!
    > In BCMAI:
    > bcmai_read() {
    > mutex_get();
    > switch_core();
    > ioread();
    > mutex_release();
    > }

    Yeah that basically is the idea. But it's a little bit harder than that.
    The problem is that the mutex cannot be taken in interrupt context.
    A spinlock probably is a bit hairy, too, depending on how heavy
    a core switch is on AXI.

    On SSB we workaround this with some (dirty but working) assumptions.

    On AXI you probably can do lockless I/O, if you use the two windows
    (how many windows are there?) in a clever way to avoid core switching
    completely after the system was initialized.

    Greetings Michael.

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