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

[Lorenzo Pieralisi]

On Sat, Sep 17, 2022 at 10:05:59AM +0100, Marc Zyngier wrote:
> Hi Lorenzo,
> 
> On Fri, 09 Sep 2022 15:57:11 +0100,
> Lorenzo Pieralisi <lpieralisi at 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 at 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.
> > 
> > That aside, the interaction of the timer and the IRQ domain code
> > must be reviewed by Marc and Thomas to make sure this is not
> > a gross violation of the respective subsystems usage.
> 
> I don't see anything being a "gross violation" here, at least from an
> interrupt subsystem perspective. In a way, this is synthesising an
> interrupt on the back of some other event, and as long as the context
> is somehow appropriate (something that looks like an interrupt when
> pretending there is one), this should be OK. Other subsystems such as
> i2c GPIO expanders do similar things.

Right, thanks.

> The one thing I'm dubious about is the frequency of the timer. Asking
> for a poll of the link every jiffy is bound to be expensive, and it
> would be good to relax this as much as possible, specially on low-end
> HW such as this, where every cycle counts. It is always going to be a
> "best effort" thing, and the commit message doesn't say what's the
> actual grace period to handle this (the spec probably has one).

AFAICS, the code does not poll the link. It sets a timer only if
the link is checked (eg upon PCI bus forced rescan or config access)
the link is up and it was down, to emulate a HP IRQ.

> I guess this patch could do with being split between handling link
> down and link up events, but that's for you to decide.

It is fine for me as-is even though its logic could be simplified
by the split.

Thanks,
Lorenzo

> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.
> 
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