linux-kernel - Re: [PATCH 1/1] pci: pciehp: Handle MRL interrupts to enable slot for hotplug.

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Date:   Fri, 20 Nov 2020 11:33:37 -0800
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     "Raj, Ashok" <ashok.raj@...el.com>, Lukas Wunner <lukas@...ner.de>
Cc:     linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] pci: pciehp: Handle MRL interrupts to enable slot for
 hotplug.

Hi,

On 11/19/20 2:08 PM, Raj, Ashok wrote:
>> If an Attention Button is present, the current behavior is to bring up
>> the hotplug slot as soon as presence or link is detected.  We don't wait
>> for a button press.  This is intended as a convience feature to bring up
>> slots as quickly as possible, but the behavior can be changed if the
>> button press and 5 second delay is preferred.
> No personal preference, I thought that is how the code in Linux was
> earlier.
>
> Looks like we still don't subscribe to PDC if ATTN is present? So you don't
> get an event until someone presses the button to process hotplug right?
>
> pciehp_hpc.c:pcie_enable_notification()
> {
> ....
>
>           * Always enable link events: thus link-up and link-down shall
>           * always be treated as hotplug and unplug respectively. Enable
>           * presence detect only if Attention Button is not present.
>           */
>          cmd = PCI_EXP_SLTCTL_DLLSCE;
>          if (ATTN_BUTTN(ctrl))
>                  cmd |= PCI_EXP_SLTCTL_ABPE;
>          else
>                  cmd |= PCI_EXP_SLTCTL_PDCE;
> ....
> }
>
>
> Looks like we still wait for button press to process. When you don't have a
> power control yes the DLLSC would kick in and we would process them. but if
> you have power control, you won't turn on until the button press?
>
Yes, as per current logic, if attention button is present, then we don't
subscribe to PDC event changes. we wait for button press  to turn on/off
the slot.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer