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

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 11/12/2014 10:07 AM, Ian Campbell wrote:
> 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.

It looks like the bindings for clock is well defined and fairly common
(Documentation/devicetree/bindins/clock/clock-bindings.txt). So your
solution sounds plausible.

Regards,

-- 
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel