linux-kernel - Re: [RFC PATCH] PCI: pciehp: Replace fixed delay with polling for slot power-off

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Message-ID: <b95a60f8-0e1d-4c81-8d5a-e2ea7d083780@hj-micro.com>
Date: Fri, 20 Jun 2025 13:54:05 +0800
From: Hongbo Yao <andy.xu@...micro.com>
To: Lukas Wunner <lukas@...ner.de>
Cc: bhelgaas@...gle.com, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 peter.du@...micro.com, jemma.zhang@...micro.com
Subject: Re: [RFC PATCH] PCI: pciehp: Replace fixed delay with polling for
 slot power-off



在 2025/6/19 19:52, Lukas Wunner 写道:
> On Thu, Jun 19, 2025 at 05:32:28PM +0800, Hongbo Yao wrote:
>> Fixed 1-second delay in remove_board() fails to accommodate certain
>> hardware like multi-host OCP cards, which exhibit longer power-off
>> latencies.
> 
> Please name the affected product(s).
> 
> They don't seem to comply to the spec.  How prevalent are they?
> If there are only few deployed, quirks like this are probably
> best addressed by an out-of-tree patch.
> 

Hi Lukas,
Thank you for reviewing the patch.

The affected hardware configuration:
 - Host system: Arm Neoverse N2 based server
 - Multi-host OCP card: Mellanox Technologies MT2910 Family [ConnectX-7]


>> Logs before fix:
>> [157.778307] pcieport 0003:00:00.0: pciehp: pending interrupts 0x0001 from Slot Status
>> [157.778321] pcieport 0003:00:00.0: pciehp: Slot(31): Attention button pressed
>> [157.785445] pcieport 0003:00:00.0: pciehp: Slot(31): Powering off due to button press
>> [157.798931] pcieport 000b:00:02.0: pciehp: pending interrupts 0x0001 from Slot Status
> 
> This log excerpt mixes messages from two separate hotplug ports
> (0003:00:00.0 and 000b:00:02.0).  Are these hotplug ports related?
> If not, please reduce the log excerpt to a single hotplug port
> to avoid confusion.
> 
Sorry for not providing adequate context in the patch submission.

Yes, these two hotplug ports are related - they are part of the same
physical multi-host OCP card.

Key points:
1. The OCP card has two independent PCIe endpoints
2. Each endpoint connected to a PCIe root port:
   - Endpoint 1 → Port 0003:00:00.0
   - Endpoint 2 → Port 000b:00:02.0
3. Both endpoints share a common power domain
4. Full power-off occurs only after BOTH endpoints are powered down
5. DLLSC is triggered only after complete power-off

Critical log events:
[157.778307] Both ports: Attention button pressed
[167.540342] Port 0003:00:00.0 power off command issued
[172.289366] Port 000b:00:02.0 power off command issued
[172.302385] Card fully powered off, trigger AER interrupts and DLLSC

Full power-off occurs only after BOTH ports complete their sequences,
taking about 5s total.

>> --- a/drivers/pci/hotplug/pciehp_ctrl.c
>> +++ b/drivers/pci/hotplug/pciehp_ctrl.c
>> @@ -30,6 +30,25 @@
>>  #define SAFE_REMOVAL	 true
>>  #define SURPRISE_REMOVAL false
>>  
>> +static void pciehp_wait_for_link_inactive(struct controller *ctrl)
>> +{
>> +	u16 lnk_status;
>> +	int timeout = 10000, step = 20;
>> +
>> +	do {
>> +		pcie_capability_read_word(ctrl->pcie->port, PCI_EXP_LNKSTA,
>> +					  &lnk_status);
>> +
>> +		if (!(lnk_status & PCI_EXP_LNKSTA_DLLLA))
>> +			return;
>> +
>> +		msleep(step);
>> +		timeout -= step;
>> +	} while (timeout >= 0);
>> +
>> +	ctrl_dbg(ctrl, "Timeout waiting for link inactive state\n");
>> +}
> 
> Any chance you can use one of the existing helpers, such as
> pcie_wait_for_link()?
> 
> Is the 10 second delay chosen arbitrarily or how did you come up
> with it?  How much time do the affected products really need?
>
Ok,  I will try to use pcie_wait_for_link().

The 10-second timeout was determined from actual log observations. The
power-off process for the multi-host OCP card takes approximately 5-9
seconds in our measurements.

>> @@ -119,8 +138,11 @@ static void remove_board(struct controller *ctrl, bool safe_removal)
>>  		 * After turning power off, we must wait for at least 1 second
>>  		 * before taking any action that relies on power having been
>>  		 * removed from the slot/adapter.
>> +		 *
>> +		 * Extended wait with polling to ensure hardware has completed
>> +		 * power-off sequence.
>>  		 */
>> -		msleep(1000);
>> +		pciehp_wait_for_link_inactive(ctrl);
>>  
>>  		/* Ignore link or presence changes caused by power off */
>>  		atomic_and(~(PCI_EXP_SLTSTA_DLLSC | PCI_EXP_SLTSTA_PDC),
> 
> Please keep the msleep(1000), that's the minimum we need to wait
> per PCIe r6.3 sec 6.7.1.8.
> 
> Please make the extra wait for link down conditional on
> ctrl->pcie->port->link_active_reporting.  (DLLLA reporting is
> optional for hotplug ports conforming to older spec revisions.)
> 
Thank you for the valuable suggestion. i'll  revise the patch

Best regards,
Hongbo.> Thanks,
> 
> Lukas
> 
>