Re: [PATCH 03/11] PCI: aardvark: Add support for DLLSC and hotplug interrupt

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Fri, Sep 16, 2022 at 06:23:02PM +0200, Marek Behún wrote:
> On Fri, 9 Sep 2022 16:57:11 +0200
> Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:
> 
> > [+Marc, Thomas - I can't merge this code without them reviewing it,
> > I am not sure at all you can mix the timer/IRQ code the way you do]
> > 
> > On Thu, Aug 18, 2022 at 03:51:32PM +0200, Marek Behún wrote:
> > > From: Pali Rohár <pali@kernel.org>
> > > 
> > > Add support for Data Link Layer State Change in the emulated slot
> > > registers and hotplug interrupt via the emulated root bridge.
> > > 
> > > This is mainly useful for when an error causes link down event. With
> > > this change, drivers can try recovery.
> > > 
> > > Link down state change can be implemented because Aardvark supports Link
> > > Down event interrupt. Use it for signaling that Data Link Layer Link is
> > > not active anymore via Hot-Plug Interrupt on emulated root bridge.
> > > 
> > > Link up interrupt is not available on Aardvark, but we check for whether
> > > link is up in the advk_pcie_link_up() function. By triggering Hot-Plug
> > > Interrupt from this function we achieve Link up event, so long as the
> > > function is called (which it is after probe and when rescanning).
> > > Although it is not ideal, it is better than nothing.  
> > 
> > So before even coming to the code review: this patch does two things.
> > 
> > 1) It adds support for handling the Link down state
> > 2) It adds some code to emulate a Link-up event
> > 
> > Now, for (2). IIUC you are adding code to make sure that an HP
> > event is triggered if advk_pcie_link_up() is called and it
> > detects a Link-down->Link-up transition, that has to be notified
> > through an HP event.
> > 
> > If that's correct, you have to explain to me please what this is
> > actually achieving and a specific scenario where we want this to be
> > implemented, in fine details; then we add it to the commit log.
> 
> Hello Lorenzo, sorry for not replying earlier.
> 
> Would something like this be sufficient?
> 
>   DLLSC is needed by the pciehp driver, which handles hotplug, but also
>   link state change events. AFAIK no Aardvark devices support hotplug,
>   but link state change events are required for graceful driver
>   unbinding in case of link down event.
> 
>   So with this change we achieve graceful driver unbind for example
>   when WiFi card PCIe link goes down (we've seen this with ath10k and
>   mt76 WiFi cards). Before the WiFi driver started spitting out errors,
>   or even taking the whole system down.
> 
>   Since after link goes down, it can come back up if the WiFi card can
>   recover (or if reset pin is used to reset the card), we need to be
>   able to recognize link up event. Since AFAIK Aardvark does not have
>   interrupt for link up event, the best thing we can do is simulate it
>   - whenever we read the link state, find it is up, and have cached
>   value saying it is down, we trigger the link up event. We read link
>   state whenever the configuration space is read, for example by
>   writing 1 to /sys/bus/pci/rescan.

Better, certainly. Question, also related to Marc's query. Do you
rely on the hotplug (emulated IRQ) to be run _before_ carrying on
with PCI config space accesses following a link-up detection ?

How was the jiffies + 1 expiration time determined ? I assume you
want to run the emulated HP IRQ asap - the question though is
how fast should it be ?

Lorenzo
> 
> Marek
> 
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