linux-kernel - Re: [PATCH v7 2/4] PCI: qcom-ep: Add support for D-state change notification

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Message-ID: <20240711184842.GA285502@bhelgaas>
Date: Thu, 11 Jul 2024 13:48:42 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jonathan Corbet <corbet@....net>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Rob Herring <robh@...nel.org>, linux-pci@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, mhi@...ts.linux.dev,
	quic_vbadigan@...cinc.com, quic_ramkri@...cinc.com,
	quic_nitegupt@...cinc.com, quic_skananth@...cinc.com,
	quic_parass@...cinc.com, Manivannan Sadhasivam <mani@...nel.org>
Subject: Re: [PATCH v7 2/4] PCI: qcom-ep: Add support for D-state change
 notification

On Thu, Jul 11, 2024 at 11:57:35AM +0530, Krishna Chaitanya Chundru wrote:
> On 7/10/2024 5:41 PM, Bjorn Helgaas wrote:
> > On Wed, Jul 10, 2024 at 04:38:15PM +0530, Krishna chaitanya chundru wrote:
> > > Add support to pass D-state change notification to Endpoint
> > > function driver.
> ...

> > I don't understand the connection between PERST# state and the device
> > D state.  D3cold is defined to mean main power is absent.  Is the
> > endpoint firmware still running when main power is absent?
>
> Host as part of its d3cold sequence will assert the perst. so we are
> reading perst to know the link the state.

I think it's true that when the device is in D3cold, PERST# will be
asserted (PCIe CEM r5.0, sec 2.2.3, fig 2-6).

But I don't think it's necessarily true that when PERST# is asserted,
the device is in D3cold.  For example, PCIe Mini CEM r2.1, sec
3.2.5.3, says "The system may also use PERST# to cause a warm reset of
the add-in card."  In a warm reset, the component remains powered up,
i.e., it is not in D3cold (PCIe r6.0, sec 6.6.1).

I would think the endpoint firmware would be able to directly read the
state of main power or the LTSSM state of the link, without having to
use PERST# to infer it.

I guess the ultimate point of figuring out D3hot vs D3cold is to
figure out whether to use PME or WAKE#?  I'm a little bit dubious
about that being racy, as I mentioned elsewhere.  If there were a way
to attempt PME and fall back to WAKE# if you can determine that PME
failed, maybe that would be safer and obviate the need for the D-state
tracking?

> Qcom devices are drawing power from the PCIe, so even when PCIe is in
> D3cold endpoint firmware can still run.
> 
> - Krishna Chaitanya.
> > > Reviewed-by: Manivannan Sadhasivam <mani@...nel.org>
> > > Signed-off-by: Krishna chaitanya chundru <quic_krichai@...cinc.com>
> > > ---
> > >   drivers/pci/controller/dwc/pcie-qcom-ep.c | 8 +++++++-
> > >   1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/pci/controller/dwc/pcie-qcom-ep.c b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > index 236229f66c80..817fad805c51 100644
> > > --- a/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > +++ b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > > @@ -648,6 +648,7 @@ static irqreturn_t qcom_pcie_ep_global_irq_thread(int irq, void *data)
> > >   	struct device *dev = pci->dev;
> > >   	u32 status = readl_relaxed(pcie_ep->parf + PARF_INT_ALL_STATUS);
> > >   	u32 mask = readl_relaxed(pcie_ep->parf + PARF_INT_ALL_MASK);
> > > +	pci_power_t state;
> > >   	u32 dstate, val;
> > >   	writel_relaxed(status, pcie_ep->parf + PARF_INT_ALL_CLEAR);
> > > @@ -671,11 +672,16 @@ static irqreturn_t qcom_pcie_ep_global_irq_thread(int irq, void *data)
> > >   		dstate = dw_pcie_readl_dbi(pci, DBI_CON_STATUS) &
> > >   					   DBI_CON_STATUS_POWER_STATE_MASK;
> > >   		dev_dbg(dev, "Received D%d state event\n", dstate);
> > > -		if (dstate == 3) {
> > > +		state = dstate;
> > > +		if (dstate == PCI_D3hot) {
> > >   			val = readl_relaxed(pcie_ep->parf + PARF_PM_CTRL);
> > >   			val |= PARF_PM_CTRL_REQ_EXIT_L1;
> > >   			writel_relaxed(val, pcie_ep->parf + PARF_PM_CTRL);
> > > +
> > > +			if (gpiod_get_value(pcie_ep->reset))
> > > +				state = PCI_D3cold;
> > >   		}
> > > +		pci_epc_dstate_notify(pci->ep.epc, state);
> > >   	} else if (FIELD_GET(PARF_INT_ALL_LINK_UP, status)) {
> > >   		dev_dbg(dev, "Received Linkup event. Enumeration complete!\n");
> > >   		dw_pcie_ep_linkup(&pci->ep);
> > > 
> > > -- 
> > > 2.42.0
> > >