[PATCH v4 1/2] cxl: Set the PBCQ Tunnel BAR register when enabling capi mode

[Philippe Bergheaud]

On 15/05/2018 07:30, Michael Ellerman wrote:
> Philippe Bergheaud <felix at linux.ibm.com> writes:
>
>> On 14/05/2018 12:51, Michael Ellerman wrote:
>>> Philippe Bergheaud <felix at linux.ibm.com> writes:
>>>
>>>> Skiboot used to set the default Tunnel BAR register value when capi mode
>>>> was enabled. This approach was ok for the cxl driver, but prevented other
>>>> drivers from choosing different values.
>>>>
>>>> Skiboot versions > 5.11 will not set the default value any longer. This
>>>> patch modifies the cxl driver to set/reset the Tunnel BAR register when
>>>> entering/exiting the cxl mode, with pnv_pci_set_tunnel_bar().
>>>>
>>>> That should work with old skiboot (since we are re-writing the value
>>>> already set) and new skiboot.
>>> But doesn't that mean new skiboot can't boot any old kernel? That seems
>>> undesirable.
>> Yes, with new skiboot, all kernels will boot.
> OK.
>
>> Capi mode tunnelled operations did not work until linux-4.17-rc1 (Apr
>> 15, 2008). With new skiboot, kernels since linux-4.17-rc1 will loose
>          ^
>          1
>
>> tunneled operations in capi mode, until this patch set is merged.
> So it would be preferable if I put this patch into 4.17, so that the
> 4.17 release works. Am I right?

Yes.

>
>> Capi mode tunneled operations are not used in any IBM or partner GA
>> products. The feature will be fully supported after this patch set and
>> the libcxl PR https://github.com/ibm-capi/libcxl/pull/29 are merged.
> OK. In general I ignore that sort of stuff because people also ignore it
> when they're filing bugs against the kernel. So if it's in the tree we
> want it to work (as much as possible), regardless of what's been GA'ed,
> or is supported officially etc.
>
> cheers
>