linux-kernel - Re: [PATCH v6 5/9] PCI: dwc: Implement .start_link(), .stop

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Message-ID: <3tuxxskusi5ck7cu2nqfkilqdqvzqjy77qgpvuo4nhcugdebug@geqxeslmxmdr>
Date: Sat, 27 Sep 2025 09:22:41 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, chaitanya chundru <quic_krichai@...cinc.com>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	cros-qcom-dts-watchers@...omium.org, Jingoo Han <jingoohan1@...il.com>, 
	Bartosz Golaszewski <brgl@...ev.pl>, Krzysztof Wilczyński <kwilczynski@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, quic_vbadigan@...cnic.com, 
	amitk@...nel.org, linux-pci@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, jorge.ramirez@....qualcomm.com, 
	linux-arm-kernel@...ts.infradead.org, Dmitry Baryshkov <lumag@...nel.org>
Subject: Re: [PATCH v6 5/9] PCI: dwc: Implement .start_link(), .stop_link()
 hooks

On Fri, Sep 26, 2025 at 03:39:16PM -0500, Bjorn Helgaas wrote:
> On Fri, Sep 26, 2025 at 07:09:17PM +0530, Krishna Chaitanya Chundru wrote:
> > On 9/25/2025 10:55 PM, Bjorn Helgaas wrote:
> > > On Thu, Sep 25, 2025 at 09:49:16PM +0530, Manivannan Sadhasivam wrote:
> > > > On Thu, Sep 25, 2025 at 09:54:16AM -0500, Bjorn Helgaas wrote:
> > > > > On Thu, Aug 28, 2025 at 05:39:02PM +0530, Krishna Chaitanya Chundru wrote:
> > > > > > Implement stop_link() and  start_link() function op for dwc drivers.
> > > > > > 
> > > > > > Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
> > > > > > ---
> > > > > >   drivers/pci/controller/dwc/pcie-designware-host.c | 18 ++++++++++++++++++
> > > > > >   1 file changed, 18 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > index 952f8594b501254d2b2de5d5e056e16d2aa8d4b7..bcdc4a0e4b4747f2d62e1b67bc1aeda16e35acdd 100644
> > > > > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > @@ -722,10 +722,28 @@ void __iomem *dw_pcie_own_conf_map_bus(struct pci_bus *bus, unsigned int devfn,
> > > > > >   }
> > > > > >   EXPORT_SYMBOL_GPL(dw_pcie_own_conf_map_bus);
> > > > > > +static int dw_pcie_op_start_link(struct pci_bus *bus)
> > > > > > +{
> > > > > > +	struct dw_pcie_rp *pp = bus->sysdata;
> > > > > > +	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
> > > > > > +
> > > > > > +	return dw_pcie_host_start_link(pci);
> > > > > 
> > > > > This takes a pci_bus *, which could be any PCI bus, but this only
> > > > > works for root buses because it affects the link from a Root Port.
> > > > > 
> > > > > I know the TC9563 is directly below the Root Port in the current
> > > > > topology, but it seems like the ability to configure a Switch with
> > > > > I2C or similar is potentially of general interest, even if the
> > > > > switch is deeper in the hierarchy.
> > > > > 
> > > > > Is there a generic way to inhibit link training, e.g., with the
> > > > > Link Disable bit in the Link Control register?  If so, this could
> > > > > potentially be done in a way that would work for any vendor and
> > > > > for any Downstream Port, including Root Ports and Switch
> > > > > Downstream Ports.
> > > > 
> > > > FWIW, the link should not be stopped for a single device, since it
> > > > could affect other devices in the bus. Imagine if this switch is
> > > > connected to one of the downstream port of another switch. Then
> > > > stopping and starting the link will affect other devices connected
> > > > to the upstream switch as well.
> > > 
> > > Link Disable would affect all devices downstream of the bridge where
> > > it is set, same as dw_pcie_op_stop_link().
> > > 
> > > > This driver is doing it right now just because, there is no other
> > > > way to control the switch state machine. Ideally, we would want the
> > > > PERST# to be in asserted stage to keep the device from starting the
> > > > state machine, then program the registers over I2C and deassert
> > > > PERST#. This will work across all of the host controller drivers (if
> > > > they support pwrctrl framework).
> > > 
> > > I don't think there's a way to implement .start_link() and
> > > .stop_link() for ACPI unless it's by using Link Disable, which is why
> > > I asked about this.  If Link Disable *does* work, it would be a very
> > > generic way to do this because it's part of the PCIe base spec.
> > 
> > We did test as you suggested but unfortunately the setting are not
> > getting reflected we need to explicitly assert perst to make sure
> > pcie is in reset state while applying these settings.
> 
> Maybe ".stop_link()" is the wrong name if what's actually required is
> PERST#?
> 

If we rename this callback to foo_perst(), then it will be similar to my Pwrctrl
PERST# integration series [1]. I'm wondering why shouldn't we merge it instead
and get rid of this callback from this series for good?

- Mani

[1] https://lore.kernel.org/linux-pci/20250912-pci-pwrctrl-perst-v3-0-3c0ac62b032c@oss.qualcomm.com/

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