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

[Philippe Bergheaud]

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. 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.

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.

Philippe