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charset=us-ascii Content-Disposition: inline In-Reply-To: <4a3f9494-27a2-47d6-bdef-0b1bcbd99903@oss.qualcomm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250926_133919_304873_9BBB687F X-CRM114-Status: GOOD ( 42.36 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 > > > > > --- > > > > > 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#? This feels like the kind of problem we are likely to see again in different topologies, e.g., a switch in an external enclosure that needs configuration.