linux-kernel - Re: [PATCH v5 7/9] PCI: PCI: Add pcie_link_is_active() to determine if the PCIe link is active

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Message-ID: <9cc84d20-90e8-069d-db0f-21386c5b0a98@oss.qualcomm.com>
Date: Mon, 14 Apr 2025 09:51:34 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Lukas Wunner <lukas@...ner.de>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczy??ski <kw@...ux.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.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>, 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,
        Dmitry Baryshkov <lumag@...nel.org>
Subject: Re: [PATCH v5 7/9] PCI: PCI: Add pcie_link_is_active() to determine
 if the PCIe link is active



On 4/13/2025 10:44 PM, Lukas Wunner wrote:
> On Sat, Apr 12, 2025 at 05:52:56AM +0200, Lukas Wunner wrote:
>> On Sat, Apr 12, 2025 at 07:19:56AM +0530, Krishna Chaitanya Chundru wrote:
>>> Introduce a common API to check if the PCIe link is active, replacing
>>> duplicate code in multiple locations.
>>>
>>> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
>>
>> Reviewed-by: Lukas Wunner <lukas@...ner.de>
> 
> Looking at this with a fresh pair of eyeballs, I realize there's an issue
> here, so unfortunately I have to retract the Reviewed-by:
> 
> pcie_link_is_active() differs from the existing pciehp_check_link_active()
> in that it returns 0 not only if the link is down, but also if the
> Config Space read returns with an error.
> 
> In particular, if Config Space of a hotplug bridge is inaccessible,
> 0 is returned instead of -ENODEV with this patch.  That can happen if
> the hotplug bridge itself has been hot-removed, which is common with
> Thunderbolt, but also on servers with nested PCIe switches.
> 
> The existing invocations of pciehp_check_link_active() do the right
> thing if the hotplug bridge has been hot-removed, but after this patch
> they no longer do.  For example in this hunk ...
> 
>>> --- a/drivers/pci/hotplug/pciehp_hpc.c
>>> +++ b/drivers/pci/hotplug/pciehp_hpc.c
>>> @@ -584,7 +557,7 @@ static void pciehp_ignore_dpc_link_change(struct controller *ctrl,
>>>   	 * Synthesize it to ensure that it is acted on.
>>>   	 */
>>>   	down_read_nested(&ctrl->reset_lock, ctrl->depth);
>>> -	if (!pciehp_check_link_active(ctrl))
>>> +	if (!pcie_link_is_active(ctrl_dev(ctrl)))
>>>   		pciehp_request(ctrl, PCI_EXP_SLTSTA_DLLSC);
>>>   	up_read(&ctrl->reset_lock);
>>>   }
> 
> ... pciehp_request() will be called if the hotplug bridge was
> hot-removed, which isn't the right thing to do.  The current
> behavior is to do nothing.
> 
> I realize I steered you in the wrong direction because in my
> review of your v4 I asked why pcie_link_is_active() doesn't
> return a bool:
> 
> https://lore.kernel.org/all/Z72TRBvpzizcgm9S@wunner.de/
> 
> So I sincerely apologize for that!  You actually did the right
> thing in v4 by returning a negative int if the Config Space read
> returned an error.
> 

Ok, No problem. I will revert v4 patch i.e returning int instead of
bool. I will wait for few days for any other new review comments.

- Krishna Chaitanya.
> Thanks,
> 
> Lukas