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SubjectRe: [PATCH RFC] uio: allow use on nommu systems
On Fri, Jul 15, 2016 at 10:42:37AM +0900, Greg Kroah-Hartman wrote:
> On Thu, Jul 14, 2016 at 08:53:06PM -0400, Rich Felker wrote:
> > mmap of uio devices does not seem to work without further
> > nommu-specific support, but interrupt handling already works, and
> > userspace drivers for nommu systems can simply use physical mmio
> > addresses from userspace directly anyway without mmap.
> >
> > Signed-off-by: Rich Felker <dalias@libc.org>
> > ---
> >
> > I don't particularly expect this to be accepted upstream as-is, but
> > since we're actually trying to use the UIO subsystem on nommu (J2),
> > I'd like to start a discussion of what an acceptable patch would
> > entail.
> >
> > The uio_pdrv_genirq driver was tested on J2 with this patch and
> > interrupt handling works as expected.
> >
> > drivers/uio/Kconfig | 1 -
> > 1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/uio/Kconfig b/drivers/uio/Kconfig
> > index 52c98ce..387b2bb 100644
> > --- a/drivers/uio/Kconfig
> > +++ b/drivers/uio/Kconfig
> > @@ -1,6 +1,5 @@
> > menuconfig UIO
> > tristate "Userspace I/O drivers"
> > - depends on MMU
>
> This doesn't seem to do much, don't you need to modify uio-specific
> drivers? Or given that the core uio code doesn't rely on mmu, is this
> safe?

As noted above, mmap of the device does not work (returns error); I
don't understand why. But select/read/write for interrupt handling
works fine, and the uio system is fully usable if you just do mmio via
the raw addresses from userspace (this is nommu after all). I tested
it with uio_pdrv_genirq bound to a fake DT node for the J2 IPI
interrupt, disabling SMP and generating IPI interrupts manually to
check that it worked.

Rich

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