linux-kernel - Re: [PATCH] PCI: pciehp: Add check for DL_ACTIVE bit in pciehp_check_link

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Message-ID: <7309c7f3-a895-e4aa-578e-444c5d5734d6@amd.com>
Date:   Thu, 8 Oct 2020 12:43:17 +0530
From:   Sanjay R Mehta <sanmehta@....com>
To:     Lukas Wunner <lukas@...ner.de>,
        Sanjay R Mehta <Sanju.Mehta@....com>
Cc:     bhelgaas@...gle.com, andriy.shevchenko@...ux.intel.com,
        stuart.w.hayes@...il.com, mr.nuke.me@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: pciehp: Add check for DL_ACTIVE bit in
 pciehp_check_link_status()



On 10/7/2020 1:08 AM, Lukas Wunner wrote:
> [CAUTION: External Email]
> 
> On Tue, Oct 06, 2020 at 01:24:28PM -0500, Sanjay R Mehta wrote:
>> if DL_ACTIVE bit is set it means that there is no need to check
>> PCI_EXP_LNKSTA_LT bit, as DL_ACTIVE would have set only if the link
>> is already trained. Hence adding a check which takes care of this
>> scenario.
> 
> Sorry for being dense but I don't understand this at all:
> 
> The PCI_EXP_DPC_CAP_DL_ACTIVE bit which you check here indicates
> that the port is capable of sending an ERR_COR interrupt whenever
> the link transitions from inactive to active.
> 
> What is the connection to the PCI_EXP_LNKSTA_LT bit (which indicates
> that the link is still being trained)?
> 
> Also, the negation of a bitwise AND is always either 0 or 1
> (!(lnk_status & PCI_EXP_DPC_CAP_DL_ACTIVE)), so bit 0 is set or not set.
> However PCI_EXP_LNKSTA_LT is bit 11.  A bitwise AND of bit 11 and 0 is
> always 0, so the expression can never be 1.
> 
> Am I missing something?
> 
Please accept my sincere apologies for sending the wrong patch.

I am supposed to use PCI_EXP_LNKSTA_DLLLA bit in my patch but have used PCI_EXP_DPC_CAP_DL_ACTIVE.

The correct code should be as below,

-     if ((lnk_status & PCI_EXP_LNKSTA_LT) ||
+     if (((lnk_status & PCI_EXP_LNKSTA_LT) &
+          !(lnk_status & PCI_EXP_LNKSTA_DLLLA )) ||

Is it right? please share your feedback, if I am wrong. Will send out V2 patch, once you confirm on this.


> Thanks,
> 
> Lukas
> 
>> Signed-off-by: Sanjay R Mehta <sanju.mehta@....com>
>> ---
>>  drivers/pci/hotplug/pciehp_hpc.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
>> index 53433b3..81d1348 100644
>> --- a/drivers/pci/hotplug/pciehp_hpc.c
>> +++ b/drivers/pci/hotplug/pciehp_hpc.c
>> @@ -309,7 +309,8 @@ int pciehp_check_link_status(struct controller *ctrl)
>>
>>       pcie_capability_read_word(pdev, PCI_EXP_LNKSTA, &lnk_status);
>>       ctrl_dbg(ctrl, "%s: lnk_status = %x\n", __func__, lnk_status);
>> -     if ((lnk_status & PCI_EXP_LNKSTA_LT) ||
>> +     if (((lnk_status & PCI_EXP_LNKSTA_LT) &
>> +          !(lnk_status & PCI_EXP_DPC_CAP_DL_ACTIVE)) ||
>>           !(lnk_status & PCI_EXP_LNKSTA_NLW)) {
>>               ctrl_err(ctrl, "link training error: status %#06x\n",
>>                        lnk_status);
>> --
>> 2.7.4