Re: [Xen-devel] Problems when using latest git tree to boot xen on OMAP5

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-10-09 at 12:29 +0100, Julien Grall wrote:
> On 10/09/2013 12:19 PM, Ian Campbell wrote:
> > On Wed, 2013-10-09 at 12:15 +0100, Julien Grall wrote:
> >> On 10/09/2013 09:11 AM, Ian Campbell wrote:
> >>> Rather than whitelisting and mapping disabled devices through perhaps we
> >>> should implement them as read 0xf (or 0x0) and write ignore?
> >>
> >> We can't assume that these values (0x0 or 0xf) won't affect dom0 behaviour.
> > 
> > 0xff is what you would typically get back if there was no actual
> > hardware present.
> > 
> >> What about a new quirk to map all disabled device in dom0
> >> (PLATFORM_QUIRK_DOM0_MAP_DISABLED_DEVICE)?
> > 
> > Why is this not the default behaviour?
> 
> From our discussion a months a ago (http://patches.linaro.org/19590/),
> status = "disabled" means : "not present/wired-up on this variant of
> the platform".

Actually ePAPR says:
        Indicates that the device is not presently operational, but it
        might
        become operational in the future (for example, something is not
        plugged in, or switched off).
        Refer to the device binding for details on what disabled means
        for
        a given device.

This doesn't strictly speaking say "and you must not prod any associated
MMIO".

> As Linux should cope with this status, why do we need to map these
> devices in DOM0?

Because in practice Linux doesn't actually cope like you asserted it
would.

> If we go further, with passthrough we may want remove device from the
> device tree. If Linux has hardcoded device (but also in the device tree)
> it will likely fail.

If/when we come to be passing through a device which Linux is poking
despite it being disabled then we have a kernel bug which needs fixing,
nothing more, and there is no reason for this to be a factor in deciding
what to do by default.

Ian.


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