Re: [PATCH v2] cxl/pci: Skip irq features if MSI/MSI-X are not supported

[Davidlohr Bueso]

On Wed, 17 Jan 2024, Ira Weiny wrote:

> CXL 3.1 Section 3.1.1 states:
>
> 	"A Function on a CXL device must not generate INTx messages if
> 	that Function participates in CXL.cache protocol or CXL.mem
> 	protocols."
>
> The generic CXL memory driver only supports devices which use the
> CXL.mem protocol.  The current driver attempts to allocate MSI/MSI-X
> vectors in anticipation of their need for mailbox interrupts or event
> processing.  However, the above requirement does not require a device to
> support interrupts, only that they use MSI/MSI-X.  For example, a device
> may disable mailbox interrupts and either be configured for firmware
> first or skip event processing and function.
>
> Dave Larsen reported that the following Intel / Agilex card does not
> support interrupts on function 0.
>
> 	CXL: Intel Corporation Device 0ddb (rev 01) (prog-if 10 [CXL Memory Device (CXL 2.x)])
>
> Rather than fail device probe if interrupts are not supported; flag that
> irqs are not enabled and avoid features which require interrupts.
> Emit messages appropriate for the situation to aid in debugging should
> device behavior be unexpected due to a failure to allocate vectors.
>
> Note that it is possible for a device to have host based event
> processing through polling.  However, the driver does not support
> polling and it is not anticipated to be generally required.  Leave that
> functionality to a future patch if such a device comes along.
>
> Reported-by: Dave Larsen <davelarsen58@xxxxxxxxx>
> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>
> Reviewed-by: Fan Ni <fan.ni@xxxxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>

Yes, I suspected this might come up at some point.

Reviewed-and-tested-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>