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

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Date: Wed, 17 Jan 2024 20:06:25 -0800
From: Davidlohr Bueso <dave@...olabs.net>
To: Ira Weiny <ira.weiny@...el.com>
Cc: Jonathan Cameron <jonathan.cameron@...wei.com>, 
	Dave Jiang <dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dan Williams <dan.j.williams@...el.com>, 
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Dave Larsen <davelarsen58@...il.com>, Fan Ni <fan.ni@...sung.com>
Subject: Re: [PATCH v2] cxl/pci: Skip irq features if MSI/MSI-X are not
 supported

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@...il.com>
>Reviewed-by: Dave Jiang <dave.jiang@...el.com>
>Reviewed-by: Fan Ni <fan.ni@...sung.com>
>Signed-off-by: Ira Weiny <ira.weiny@...el.com>

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

Reviewed-and-tested-by: Davidlohr Bueso <dave@...olabs.net>