linux-kernel - Re: [PATCH v3 1/3] PCI: dwc: Skip waiting for link up if vendor drivers can detect Link up event

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Message-ID: <20241115062925.kuclg4w5wnticyvd@thinkpad>
Date: Fri, 15 Nov 2024 11:59:25 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Krishna chaitanya chundru <quic_krichai@...cinc.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, quic_mrana@...cinc.com,
	quic_vbadigan@...cinc.com
Subject: Re: [PATCH v3 1/3] PCI: dwc: Skip waiting for link up if vendor
 drivers can detect Link up event

On Fri, Nov 01, 2024 at 10:26:38AM -0500, Bjorn Andersson wrote:
> On Fri, Nov 01, 2024 at 05:04:12PM GMT, Krishna chaitanya chundru wrote:
> > If the vendor drivers can detect the Link up event using mechanisms
> > such as Link up IRQ and can the driver can enumerate downstream devices
> > instead of waiting here, then waiting for Link up during probe is not
> > needed here, which optimizes the boot time.
> > 
> > So skip waiting for link to be up if the driver supports 'linkup_irq'.
> > 
> > Signed-off-by: Krishna chaitanya chundru <quic_krichai@...cinc.com>
> > ---
> >  drivers/pci/controller/dwc/pcie-designware-host.c | 10 ++++++++--
> >  drivers/pci/controller/dwc/pcie-designware.h      |  1 +
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > index 3e41865c7290..26418873ce14 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > @@ -530,8 +530,14 @@ int dw_pcie_host_init(struct dw_pcie_rp *pp)
> >  			goto err_remove_edma;
> >  	}
> >  
> > -	/* Ignore errors, the link may come up later */
> > -	dw_pcie_wait_for_link(pci);
> > +	/*
> > +	 * Note: The link up delay is skipped only when a link up IRQ is present.
> > +	 * This flag should not be used to bypass the link up delay for arbitrary
> > +	 * reasons.
> 
> Perhaps by improving the naming of the variable, you don't need 3 lines
> of comment describing the conditional.
> 
> > +	 */
> > +	if (!pp->linkup_irq)
> > +		/* Ignore errors, the link may come up later */
> 
> Does this mean that we will be able to start handling these errors?
> 
> > +		dw_pcie_wait_for_link(pci);
> >  
> >  	bridge->sysdata = pp;
> >  
> > diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> > index 347ab74ac35a..539c6d106bb0 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware.h
> > +++ b/drivers/pci/controller/dwc/pcie-designware.h
> > @@ -379,6 +379,7 @@ struct dw_pcie_rp {
> >  	bool			use_atu_msg;
> >  	int			msg_atu_index;
> >  	struct resource		*msg_res;
> > +	bool			linkup_irq;
> 
> Please name this for what it is, rather than some property from which
> some other decision should be derived. (And then you need a comment to
> describe how people should interpret and use it)
> 
> Also, "linkup_irq" sound like an int carrying the interrupt number, not
> a boolean.
> 
> 
> Please call it "use_async_linkup", "use_linkup_irq" or something.
> 

"use_linkup_irq" sounds good to me. But I do like to keep the note above as
there were incidents that people tried to avoid this delay as a "workaround" to
unrelated problems.

- Mani

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