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

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Message-ID: <acbbd966-2406-4b25-baf1-7fd9b5720bc1@oss.qualcomm.com>
Date: Thu, 22 Jan 2026 14:25:53 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Vinod Koul <vkoul@...nel.org>, Neil Armstrong
 <neil.armstrong@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Jingoo Han <jingoohan1@...il.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kwilczynski@...nel.org>,
        Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 2/5] PCI: dwc: Add support for retaining link during host
 init



On 1/9/2026 9:23 PM, Bjorn Helgaas wrote:
> 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().
Yes the bootloader is expected to do everything what dw_pcie_setup_r() 
does.
> 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.
As the devicetree changes already pointing to different address from the 
boatloader,
I was trying use this method. As changing device tree properties now 
might not
be good, but no harm in doing so. I can skip this and make device tree 
changes.
> Is there something in dw_pcie_setup_rc() that takes a lot of time or
> forces a link retrain?
I don't think it might not take much time as it is few register writes, 
Just doesn't
want to do redundant register writes which are costly in general.
> 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.
The clock and GENPD issues have no direct relation ship with 
dw_pcie_setup_r(). we are skipping them as they are redundant. I will 
add a comment in next series on this. - Krishna Chaitanya.
>> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....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
>>