Re: [Xen-devel] [PATCH OSSTEST v2 18/20] Osstest/Debian: Add "clk_ignore_unused" to default command line

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-11-11 at 17:50 +0100, Julien Grall wrote:
> Hi,
> 
> Somehow I missed this email.
> 
> On 30/10/2014 13:33, Ian Campbell wrote:
> > create !
> > title it arm: domain 0 disables clocks which are in fact being used
> > thanks
> >
> > On Wed, 2014-10-29 at 16:39 +0000, Ian Jackson wrote:
> >> Ian Campbell writes ("[PATCH OSSTEST v2 18/20] Osstest/Debian: Add 
> >> "clk_ignore_unused" to default command line"):
> >>> This stops dom0 from messing with clocks which it should and is required 
> >>> on
> >>> some platforms. It's harmless even when not needed.
> >>
> >> This is pretty odd.  Doesn't this correspond to some kind of bug, in
> >> the dom0 kernel perhaps ?
> >
> > dom0 is not aware that some clocks are actually in use (e.g. by the
> > hypervisor), so the bug is probably in the lack of some interface to
> > communicate this from Xen to dom0, or some other mechanism to gate this.
> 
> In reality, Xen is only using the clock of the UART. Even though, I 
> think this would also happen with platform device passthrough.
> 
> For the latter, it would be fairly easy via a PV drivers. But for 
> UART... gating the clock could be a nightmare because every platform 
> have their own way to enable/disable the clock. Futhermore, the clock 
> could be shared with multiple device...
> 
> I'm wondering if we could expose the UART to DOM0 without marking as 
> disabled. It would avoid DOM0 to mess up the clock while everything 
> would be catch via the vuart implementation in Xen.

Perhaps we could propagate any clock related properties from the UART
node into the Xen hypervisor node (or a subnode)? The Xen Linux code
would then consume those and mark the relevant clocks as in use. I've
not looked into the clock bindings to know how plausible this might be.

Ian.


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