[RFC PATCH] restrict /dev/mem to idle io memory ranges

[Dan Williams]

On Fri, Nov 20, 2015 at 12:12 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Fri, Nov 20, 2015 at 09:31:33AM -0800, Dan Williams wrote:
>> This effectively promotes IORESOURCE_BUSY to IORESOURCE_EXCLUSIVE
>> semantics by default.  If userspace really believes it is safe to access
>> the memory region it can also perform the extra step of disabling an
>> active driver.  This protects device address ranges with read side
>> effects and otherwise directs userspace to use the driver.
>
> I'm happy with this as long as we retain the option to disable this
> new behaviour.
>
> The reason being, when developing a driver, it is _very_ useful to
> be able to poke around in the device's (and system memory) address
> spaces with tools like devmem2 to work out what's going on when
> things go wrong.
>
> To put it another way, I think it's a good idea to disable access to
> these regions on production systems, but for driver development, we
> want to retain the ability to poke around in physical address space
> in any way we so desire.
>

Sounds ok to me, but I do think it's a good idea to default it to the
same value as STRICT_DEVMEM.  Perhaps:

bool "Filter I/O access to /dev/mem" if EXPERT
default STRICT_DEVMEM

When this in do we even need IORESOURCE_EXCLUSIVE?  It's barely used.