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

[Michael Ellerman]

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?

> 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