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

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Date: Thu, 18 Jan 2024 12:42:03 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Ira Weiny <ira.weiny@...el.com>, Davidlohr Bueso <dave@...olabs.net>,
	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>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Dave Larsen
	<davelarsen58@...il.com>, Fan Ni <fan.ni@...sung.com>, Ira Weiny
	<ira.weiny@...el.com>
Subject: Re: [PATCH v2] cxl/pci: Skip irq features if MSI/MSI-X are not
 supported

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>
> ---
> Changes in v2:
> - [djbw: add reported by and info about the card.]
> - [djbw: skip error reporting in the mailbox case.]
> - [djbw: clean up event message]
> - [iweiny: pick up tags]
> - Link to v1: https://lore.kernel.org/r/20240111-dont-fail-irq-v1-1-802c22a79ecb@intel.com
> ---
>  drivers/cxl/pci.c | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)

Looks good, I consider this suitable as post -rc1 material. Will get it
queued once the merge window closes.