linux-kernel - Re: [PATCH] pci/doe: add a 1 second retry window to pci

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Message-ID: <ZvwRjbRIrkCSjwQI@PC2K9PVX.TheFacebook.com>
Date: Tue, 1 Oct 2024 11:13:17 -0400
From: Gregory Price <gourry@...rry.net>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Dan Williams <dan.j.williams@...el.com>, linux-pci@...r.kernel.org,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	bhelgaas@...gle.com, dave@...olabs.net, dave.jiang@...el.com,
	vishal.l.verma@...el.com, lukas@...ner.de
Subject: Re: [PATCH] pci/doe: add a 1 second retry window to pci_doe

On Mon, Sep 16, 2024 at 10:15:57AM +0100, Jonathan Cameron wrote:
> On Fri, 13 Sep 2024 22:32:28 -0700
> Dan Williams <dan.j.williams@...el.com> wrote:
> 
> > [ add linux-pci and Lukas ]
> > 
> > Gregory Price wrote:
> > > Depending on the device, sometimes firmware clears the busy flag
> > > later than expected.  This can cause the device to appear busy when
> > > calling multiple commands in quick sucession. Add a 1 second retry
> > > window to all doe commands that end with -EBUSY.  
> > 
> > I would have expected this to be handled as part of finishing off
> > pci_doe_recv_resp() not retrying on a new submission.
> > 
> > It also occurs to me that instead of warning "another entity is sending conflicting
> > requests" message, the doe core should just ensure that it is the only
> > agent using the mailbox. Something like hold the PCI config lock over
> > DOE transactions. Then it will remove ambiguity of "conflicting agent"
> > vs "device is slow to clear BUSY".
> > 
> 
> I believe we put that dance in to not fail too horribly
> if a firmware was messing with the DOE behind our backs rather than
> another OS level actor was messing with it.
> 
> We wouldn't expect firmware to be using a DOE that Linux wants, but
> the problem is the discovery protocol which the firmware might run
> to find the DOE it does want to use.
> 
> My memory might be wrong though as this was a while back.
> 
> Jonathan

Just following up here, it sounds like everyone is unsure of this change.

I can confirm that this handles the CDAT retry issue I am seeing, and that
the BUSY bit is set upon entry into the initial call. Only 1 or 2 retries
are attempted before it is cleared and returns successfully.

I'd explored putting the retry logic in the CDAT code that calls into here,
but that just seemed wrong.  Is there a suggestion or a nak here?

Trying to find a path forward.

~Gregory