[PATCH RFC] uio: allow use on nommu systems

[PATCH RFC] uio: allow use on nommu systems

[Rich Felker]

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
 	help
 	  Enable this to allow the userspace driver core code to be
 	  built.  This code allows userspace programs easy access to
-- 
2.8.1

Re: [PATCH RFC] uio: allow use on nommu systems

[Greg Kroah-Hartman]

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?

thanks,

greg k-h

Re: [PATCH RFC] uio: allow use on nommu systems

[Rich Felker]

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

Re: [PATCH RFC] uio: allow use on nommu systems

[Greg Kroah-Hartman]

On Thu, Jul 14, 2016 at 09:58:29PM -0400, Rich Felker wrote:
> 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.

Try working that out and then we can talk :)

thanks,

greg k-h