linux-kernel - Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card connected

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Message-ID: <20200710091800.GA3419@e121166-lin.cambridge.arm.com>
Date:   Fri, 10 Jul 2020 10:18:00 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Pali Rohár <pali@...nel.org>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Marek Behún <marek.behun@....cz>,
        Remi Pommarel <repk@...plefau.lt>,
        Tomasz Maciej Nowak <tmn505@...il.com>,
        Xogium <contact@...ium.me>, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card
 connected

On Thu, Jul 09, 2020 at 05:09:59PM +0200, Pali Rohár wrote:

[...]

> > I understand that but the bridge bus resource can be trimmed to just
> > contain the root bus because that's the only one where there is a
> > chance you can enumerate a device.
> 
> It is possible to register only root bridge without endpoint?

It is possible to register the root bridge with a trimmed IORESOURCE_BUS
so that you don't enumerate anything other than the root port.

> > I would like to get Bjorn's opinion on this, I don't like these "link is
> > up" checks in config accessors (they are racy and honestly it is a
> > run-time check that does not make much sense, either it is always
> > true/false or it is inevitably racy)
> 
> It is runtime check, but does not have to be always true/false. I have
> tested more Compex wifi cards and under certain conditions they
> "disappear" from the bus during usage.

I would be very grateful if you could describe what happens in HW
when these conditions trigger - I would like to understand if this
issue is aardvark specific or it isn't.

> So I think it still make sense to do this "fast" check as it is only
> optimization.

I will merge this patch but I'd also like to understand the underlying
issue better.

Thanks,
Lorenzo