linux-kernel - Re: [PATCH 2/6] device/pci: add cmdmem cap to pci

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Date:   Tue, 31 Mar 2020 17:00:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     vkoul@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, gregkh@...uxfoundation.org,
        arnd@...db.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        dmaengine@...r.kernel.org, dan.j.williams@...el.com,
        ashok.raj@...el.com, fenghua.yu@...el.com,
        linux-pci@...r.kernel.org, tony.luck@...el.com, jing.lin@...el.com,
        sanjay.k.kumar@...el.com
Subject: Re: [PATCH 2/6] device/pci: add cmdmem cap to pci_dev

On Tue, Mar 31, 2020 at 10:59:44AM -0700, Dave Jiang wrote:
> On 3/31/2020 9:03 AM, Bjorn Helgaas wrote:
> > On Mon, Mar 30, 2020 at 02:27:00PM -0700, Dave Jiang wrote:
> > > Since the current accelerator devices do not have standard PCIe capability
> > > enumeration for accepting ENQCMDS yet, for now an attribute of pdev->cmdmem has
> > > been added to struct pci_dev.  Currently a PCI quirk must be used for the
> > > devices that have such cap until the PCI cap is standardized. Add a helper
> > > function to provide the check if a device supports the cmdmem capability.
> > > 
> > > Such capability is expected to be added to PCIe device cap enumeration in
> > > the future.
> > This needs some sort of thumbnail description of what "synchronous
> > write notification" and "cmdmem" mean.
> 
> I will add more explanation.
> 
> > Do you have a pointer to a PCI-SIG ECR or similar?
> 
> Deferrable Memory Write (DMWr) ECR
> 
> https://members.pcisig.com/wg/PCI-SIG/document/13747
> 
> From what I'm told it should be available for public review by EOW.

Please use terminology from the spec instead of things like
"synchronous write notification".

AIUI, ENQCMDS is an x86 instruction.  That would have no meaning in
the PCIe domain.

I'm not committing to acking any part of this before the ECR is
accepted, but if you're adding support for the feature described by
the ECR, you might as well add support for discovering the DMWr
capability via Device Capabilities 2 as described in the ECR.

If you have hardware that supports DMWr but doesn't advertise it via
Device Capabilities 2, you can always have a quirk that works around
that lack.