From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E953B13BC3F; Fri, 26 Sep 2025 20:39:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758919159; cv=none; b=aZuaAH8MpheAM2ClY37CC++JAZjeUomXmtRIPib+nJTriem63lD8LRcdUOI5K6VieX0jBcJA8/SohQNcyevGOXR1EjE6sWPInI0yPuuSdL0eWBK0rpr6vEY7gMj72m2GbamhmT3HECFCjSmc7kYXvuasmMZbalCWMz1KrHHyFIo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758919159; c=relaxed/simple; bh=DN6Qwr15i5uFVBUupd5QlMIvaTum5HUKi8sCfH8FrpI=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition:In-Reply-To; b=hPhcyx7UNI5gsq63kqMCqq64VLP4b2BNRNjFJzpkWrkRlZ0W1g0v9t2slpnglLO3ZD+oJOKqLKF+I0tTwe4drfLBGchD52tERmjQcwAchtKZO5/SebsGle9Kc4CnR9nMZK1IYZ/RLITnhUQ3XsPUjii0DNvN0Ou7DGQSWR666H0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=siIOIe/J; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="siIOIe/J" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 42628C4CEF4; Fri, 26 Sep 2025 20:39:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1758919158; bh=DN6Qwr15i5uFVBUupd5QlMIvaTum5HUKi8sCfH8FrpI=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=siIOIe/Jddewm5lK6in8+WPYGB83DvHTHmJw3qcHCKgl88DH9uAFCnU/alvTGc771 HZ12gzaoeJTS2ZkiYCJxQzvubH66i1yvLvWqsXZThEA2S4AUp8IBYfDtF3wmLthmwl Zv8lVXGvZWxl0wCFVwRTCzbA3kl76iI3kx4k3oF027IRHXN1J6qdwSPh/+LecMZDVL IJ4rK4CW9SH0NU08vEURQ0FbvY3v57TZWJfd05qWly7IM3b6wd/kCvJcAt/i1j4SHI qQlk5c1Tb+Pp3R8+mV3n7mdHgce6ljgKFjzO3hdzrCwrBoTxvDZwZxk12Nzlxs7NlL 5yD7MXLqyUDtw== Date: Fri, 26 Sep 2025 15:39:16 -0500 From: Bjorn Helgaas To: Krishna Chaitanya Chundru Cc: Manivannan Sadhasivam , Bjorn Helgaas , Lorenzo Pieralisi , Rob Herring , Krzysztof Kozlowski , Conor Dooley , chaitanya chundru , Bjorn Andersson , Konrad Dybcio , cros-qcom-dts-watchers@chromium.org, Jingoo Han , Bartosz Golaszewski , Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= , Catalin Marinas , Will Deacon , quic_vbadigan@quicnic.com, amitk@kernel.org, linux-pci@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, jorge.ramirez@oss.qualcomm.com, linux-arm-kernel@lists.infradead.org, Dmitry Baryshkov Subject: Re: [PATCH v6 5/9] PCI: dwc: Implement .start_link(), .stop_link() hooks Message-ID: <20250926203916.GA2266029@bhelgaas> Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4a3f9494-27a2-47d6-bdef-0b1bcbd99903@oss.qualcomm.com> 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.