Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 1/2] spapr: Add support for hwrng when available

[Thomas Huth <thuth@redhat.com>]

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On 10/09/15 12:40, David Gibson wrote:
> On Thu, Sep 10, 2015 at 09:33:21AM +0200, Thomas Huth wrote:
>> On 09/09/15 23:10, Thomas Huth wrote:
>>> On 08/09/15 07:15, David Gibson wrote:
>> ...
>>>> At this point rather than just implementing them as discrete machine
>>>> options, I suspect it will be more maintainable to split out the
>>>> h-random implementation as a pseudo-device with its own qdev and so
>>>> forth.  We already do similarly for the RTAS time of day functions
>>>> (spapr-rtc).
>>>
>>> I gave that I try, but it does not work as expected. To be able to
>>> specify the options, I'd need to instantiate this device with the
>>> "-device" option, right? Something like:
>>>
>>> 	-device spapr-rng,backend=rng0,usekvm=0
>>>
>>> Now this does not work when I use TYPE_SYS_BUS_DEVICE as parent class
>>> like it is done for spapr-rtc, since the user apparently can not plug
>>> device to this bus on machine spapr (you can also not plug an spapr-rtc
>>> device this way!).
>>>
>>> The spapr-vlan, spapr-vty, etc. devices are TYPE_VIO_SPAPR_DEVICE, so I
>>> also tried that instead, but then the rng device suddenly shows up under
>>> /vdevice in the device tree - that's also not what we want, I guess.
>>
>> I did some more tests, and I think I can get this working with one small
>> modification to spapr_vio.c
...
>> i.e. when the dt_name has not been set, the device won't be added to the
>> /vdevice device tree node. If that's acceptable, I'll continue with this
>> approach.
> 
> A bit hacky.
> 
> I think it would be preferable to build it under SysBus by default,
> like spapr-rtc.  Properties can be set on the device using -global (or
> -set, but -global is easier).

If anyhow possible, I'd prefere to use "-device" for this instead, because

a) it's easier to use for the user, for example you can simply use
   "-device spapr-rng,?" to get the list of properties - this
   does not seem to work with spapr-rtc (it has a "date" property
   which does not show up in the help text?)

b) unlike the rtc device which is always instantiated, the rng
   device is rather optional, so it is IMHO more intuitive if
   created via the -device option.

So I'd like to give it a try with the TYPE_VIO_SPAPR_DEVICE first ... if
you then still don't like the patches at all, I can still rework them to
use TYPE_SYS_BUS_DEVICE instead.

 Thomas



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