[Skiboot] [PATCH 2/2] phb4: set PBCQ Tunnel BAR for tunneled operations

[Benjamin Herrenschmidt]

On Mon, 2017-11-13 at 11:45 +0100, Philippe Bergheaud wrote:
> On 07/11/2017 21:18, Benjamin Herrenschmidt wrote:
> > On Tue, 2017-11-07 at 14:19 +0100, Philippe Bergheaud wrote:
> > > I understand that the Tunnel BAR value is chosen by the device logic,
> > > within the regular BARs assigned to the device at power-on (an entire
> > > BAR, or a subset). The Tunnel BAR value is fetched by the device driver,
> > > that passes it to skiboot, to set the PBCQ Tunnel BAR Response register.
> > > 
> > > I think that this patch is required. I cannot see how to reverse the
> > > information flow.
> > 
> > Why would it be chosen by the device ? I don't understand...
> > 
> 
> There can be more than one device connected to the PHB. But as there
> is only one PBCQ Tunnel BAR Response register, only one device will
> be able to use tunneled operations at a certain time.

Why ? I don't understand. What is special about those tunneled
operations that require that sort of mutual exclusion ?

>  I have chosen
> a first-come first-serve policy, where a device will:
> 
> 1. Acquire tunneled operations the by setting the register with
>     its own specific BAR value, chosen within the range of its BARs,
>     where tunneled operation responses will be expected.

Ok I obsiously don't understand how tunneled operations work. Can you
elaborate a bit ?

> 2. Release tunneled operations after use by resetting the PBCQ Tunnel
>     BAR response register, authenticating itself by passing its device-
>     specific BAR value again (as required by the API).
> 
> The register value indicates which device should receive the tunneled
> operation responses. The value of the register must be dynamically
> modified by devices to point to their own MMIO range. It cannot be
> statically set by skiboot.

Oh so basically we expect a "response" to the device ? That smells like
a disgusting HW hack to me ...

Ben.