Re: [PATCH 2/5] PCI: dwc: Add support for retaining link during host init

From: Bjorn Helgaas <helgaas@kernel.org>
To: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Cc: "Vinod Koul" <vkoul@kernel.org>,
	"Neil Armstrong" <neil.armstrong@linaro.org>,
	"Philipp Zabel" <p.zabel@pengutronix.de>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Manivannan Sadhasivam" <mani@kernel.org>,
	"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
	"Krzysztof Wilczyński" <kwilczynski@kernel.org>,
	"Rob Herring" <robh@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	linux-arm-msm@vger.kernel.org, linux-phy@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH 2/5] PCI: dwc: Add support for retaining link during host init
Date: Fri, 9 Jan 2026 09:53:50 -0600	[thread overview]
Message-ID: <20260109155350.GA546142@bhelgaas> (raw)
In-Reply-To: <20260109-link_retain-v1-2-7e6782230f4b@oss.qualcomm.com>

On Fri, Jan 09, 2026 at 12:51:07PM +0530, Krishna Chaitanya Chundru wrote:
> Some platforms keep the PCIe link up across bootloader and kernel
> handoff. In such cases, reinitializing the root complex is unnecessary
> if the DWC glue drivers wants to retain the PCIe link.
> 
> Introduce a link_retain flag in struct dw_pcie_rp to indicate that
> the link should be preserved. When this flag is set by DWC glue drivers,
> skip dw_pcie_setup_rc() and only initialize MSI, avoiding redundant
> configuration steps.

It sounds like this adds an assumption that the bootloader
initialization is the same as what dw_pcie_setup_rc() would do.  This
assumption also applies to future changes in dw_pcie_setup_rc().

It looks like you mention an issue like this in [PATCH 4/5]; DBI & ATU
base being different than "HLOS" (whatever that is).  This sounds like
a maintenance issue keeping bootloader and kernel driver assumptions
synchronized.

Is there something in dw_pcie_setup_rc() that takes a lot of time or
forces a link retrain?  You mentioned some clock and GENPD issues in
the cover letter, but I don't see the connection between those and
dw_pcie_setup_rc().  If there is a connection, please include it in
this commit log and include a code comment about why
dw_pcie_setup_rc() is being skipped.

> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
> ---
>  drivers/pci/controller/dwc/pcie-designware-host.c | 11 ++++++++---
>  drivers/pci/controller/dwc/pcie-designware.h      |  1 +
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> index 372207c33a857b4c98572bb1e9b61fa0080bc871..d050df3f22e9507749a8f2fedd4c24fca43fb410 100644
> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> @@ -655,9 +655,14 @@ int dw_pcie_host_init(struct dw_pcie_rp *pp)
>  	if (ret)
>  		goto err_free_msi;
>  
> -	ret = dw_pcie_setup_rc(pp);
> -	if (ret)
> -		goto err_remove_edma;
> +	if (!pp->link_retain) {

Use positive logic if possible (test "pp->link_retain" instead of
"!pp->link_retain").

I suspect this would be more maintainable if you identified specific
things *inside* dw_pcie_setup_rc() that need to be skipped, and you
added tests there.

> +		ret = dw_pcie_setup_rc(pp);
> +		if (ret)
> +			goto err_remove_edma;
> +	} else {
> +		dw_pcie_msi_init(pp);
> +	}
> +
>  
>  	if (!dw_pcie_link_up(pci)) {
>  		ret = dw_pcie_start_link(pci);
> diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> index 31685951a080456b8834aab2bf79a36c78f46639..8acab751b66a06e8322e027ab55dc0ecfdcf634c 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.h
> +++ b/drivers/pci/controller/dwc/pcie-designware.h
> @@ -439,6 +439,7 @@ struct dw_pcie_rp {
>  	struct pci_config_window *cfg;
>  	bool			ecam_enabled;
>  	bool			native_ecam;
> +	bool			link_retain;
>  };
>  
>  struct dw_pcie_ep_ops {
> 
> -- 
> 2.34.1
> 