linux-kernel - Re: [PATCH v15,RESEND 22/23] PCI: starfive: Offload the NVMe timeout workaround to host drivers.

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Date: Wed, 13 Mar 2024 20:51:29 -0600
From: Keith Busch <kbusch@...nel.org>
To: Kevin Xie <kevin.xie@...rfivetech.com>
Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Minda Chen <minda.chen@...rfivetech.com>,
	Conor Dooley <conor@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"daire.mcnamara@...rochip.com" <daire.mcnamara@...rochip.com>,
	"emil.renner.berthing@...onical.com" <emil.renner.berthing@...onical.com>,
	"krzysztof.kozlowski+dt@...aro.org" <krzysztof.kozlowski+dt@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	"aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
	"p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
	Mason Huo <mason.huo@...rfivetech.com>,
	Leyfoon Tan <leyfoon.tan@...rfivetech.com>
Subject: Re: [PATCH v15,RESEND 22/23] PCI: starfive: Offload the NVMe timeout
 workaround to host drivers.

On Thu, Mar 14, 2024 at 02:18:38AM +0000, Kevin Xie wrote:
> > Re: [PATCH v15,RESEND 22/23] PCI: starfive: Offload the NVMe timeout
> > workaround to host drivers.
> > 
> > On Mon, Mar 04, 2024 at 10:08:06AM -0800, Palmer Dabbelt wrote:
> > > On Thu, 29 Feb 2024 07:08:43 PST (-0800), lpieralisi@...nel.org wrote:
> > > > On Tue, Feb 27, 2024 at 06:35:21PM +0800, Minda Chen wrote:
> > > > > From: Kevin Xie <kevin.xie@...rfivetech.com>
> > > > >
> > > > > As the Starfive JH7110 hardware can't keep two inbound post write
> > > > > in order all the time, such as MSI messages and NVMe completions.
> > > > > If the NVMe completion update later than the MSI, an NVMe IRQ handle
> > will miss.
> > > >
> > > > Please explain what the problem is and what "NVMe completions" means
> > > > given that you are talking about posted writes.
> 
> Sorry, we made a casual conclusion here.
> Not any two of inbound post requests can`t be kept in order in JH7110 SoC, 
> the only one case we found is NVMe completions with MSI interrupts.
> To be more precise, they are the pending status in nvme_completion struct and
> nvme_irq handler in nvme/host/pci.c.
> 
> We have shown the original workaround patch before:
> https://lore.kernel.org/lkml/CAJM55Z9HtBSyCq7rDEDFdw644pOWCKJfPqhmi3SD1x6p3g2SLQ@mail.gmail.com/
> We put it in our github branch and works fine for a long time.
> Looking forward to better advices from someone familiar with NVMe drivers.

So this platform treats strictly ordered writes the same as if relaxed
ordering was enabled? I am not sure if we could reasonably work around
such behavior. An arbitrary delay is likely too long for most cases, and
too short for the worst case.

I suppose we could quirk a non-posted transaction in the interrupt
handler to force flush pending memory updates, but that will noticeably
harm your nvme performance. Maybe if you constrain such behavior to the
spurious IRQ_NONE condition, then it might be okay? I don't know.