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

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Date:   Tue, 6 Oct 2020 21:38:30 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     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 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?

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