linux-kernel - Re: [PATCH v3 2/3] PCI: pciehp: Wait for PDS if in-band presence is disabled

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Date:   Mon, 21 Oct 2019 13:08:00 -0500
From:   Stuart Hayes <stuart.w.hayes@...il.com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Austin Bolen <austin_bolen@...l.com>, keith.busch@...el.com,
        Alexandru Gagniuc <mr.nuke.me@...il.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Sinan Kaya <okaya@...nel.org>,
        Oza Pawandeep <poza@...eaurora.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukas@...ner.de
Subject: Re: [PATCH v3 2/3] PCI: pciehp: Wait for PDS if in-band presence is
 disabled



On 10/21/19 8:41 AM, Mika Westerberg wrote:
> On Thu, Oct 17, 2019 at 03:32:55PM -0400, Stuart Hayes wrote:
>> From: Alexandru Gagniuc <mr.nuke.me@...il.com>
>>
>> When inband presence is disabled, PDS may come up at any time, or not
>> at all. PDS being low may indicate that the card is still mating, and
>> we could expect contact bounce to bring down the link as well.
>>
>> It is reasonable to assume that most cards will mate in a hotplug slot
>> in about a second. Thus, when we know PDS only reflects out-of-band
>> presence, it's worthwhile to wait the extra second or so to make sure
>> the card is properly mated before loading the driver, and to prevent
>> the hotplug code from disabling a device if the presence detect change
>> goes active after the device is enabled.
>>
>> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@...il.com>
>> Signed-off-by: Stuart Hayes <stuart.w.hayes@...il.com>
> 
> Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> 
> One nit below.
> 
>> ---
>> v2:
>>   replace while(true) loop with do...while
>> v3
>>   remove unused variable declaration (pds)
>>   modify text of warning message
>>
>>  drivers/pci/hotplug/pciehp_hpc.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
>> index dc109d521f30..02eb811a014f 100644
>> --- a/drivers/pci/hotplug/pciehp_hpc.c
>> +++ b/drivers/pci/hotplug/pciehp_hpc.c
>> @@ -242,6 +242,22 @@ static bool pci_bus_check_dev(struct pci_bus *bus, int devfn)
>>  	return found;
>>  }
>>  
>> +static void pcie_wait_for_presence(struct pci_dev *pdev)
>> +{
>> +	int timeout = 1250;
>> +	u16 slot_status;
>> +
>> +	do {
>> +		pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &slot_status);
>> +		if (!!(slot_status & PCI_EXP_SLTSTA_PDS))
> 
> It is more readable if you write it like:
> 
> 		if (slot_status & PCI_EXP_SLTSTA_PDS)
> 

I agree, it is more readable, and the double bang shouldn't be needed for an 
"if" condition.  Thanks.

>> +			return;
>> +		msleep(10);
>> +		timeout -= 10;
>> +	} while (timeout > 0);
>> +
>> +	pci_info(pdev, "Timeout waiting for Presence Detect state to be set\n");
>> +}
>> +
>>  int pciehp_check_link_status(struct controller *ctrl)
>>  {
>>  	struct pci_dev *pdev = ctrl_dev(ctrl);
>> @@ -251,6 +267,9 @@ int pciehp_check_link_status(struct controller *ctrl)
>>  	if (!pcie_wait_for_link(pdev, true))
>>  		return -1;
>>  
>> +	if (ctrl->inband_presence_disabled)
>> +		pcie_wait_for_presence(pdev);
>> +
>>  	found = pci_bus_check_dev(ctrl->pcie->port->subordinate,
>>  					PCI_DEVFN(0, 0));
>>  
>> -- 
>> 2.18.1