linux-kernel - Re: [PATCH 2/2] PCI: uniphier: Add checking whether PERST# is deasserted

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Message-Id: <20191108163026.0DFB.4A936039@socionext.com>
Date:   Fri, 08 Nov 2019 16:30:27 +0900
From:   Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
To:     Andrew Murray <andrew.murray@....com>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-pci@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <masami.hiramatsu@...aro.org>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Kishon Vijay Abraham I <kishon@...com>
Subject: Re: [PATCH 2/2] PCI: uniphier: Add checking whether PERST# is deasserted

Hi Andrew,
+CC Kishon

On Thu, 7 Nov 2019 12:46:17 +0000 <andrew.murray@....com> wrote:

> On Thu, Nov 07, 2019 at 08:52:39PM +0900, Kunihiko Hayashi wrote:
> > Hi Andrew,
> > Thank you for your comments.
> > 
> > On Thu, 7 Nov 2019 10:02:08 +0000 <andrew.murray@....com> wrote:
> > 
> > > On Thu, Nov 07, 2019 at 01:58:15PM +0900, Kunihiko Hayashi wrote:
> > > > When PERST# is asserted once, EP configuration will be initialized.
> > > 
> > > I don't quite understand this - does the EP/RC mode depend on how often
> > > PERST# is toggled?
> > 
> > I think of connecting this RC controller and EP based on `Linux PCI
> > endpoint framework' in another machine.
> > 
> > While this RC driver is probing, the EP driver might be also probing and
> > configurating itself using configfs. If PERST# is toggled after the EP
> > has done its configuration, this configuration will be lost.
> > 
> > I expect that the EP configurates after RC has toggled PERST#, however,
> > there is no way to synchronize both of them.
> > 
> 
> OK I understand where you are coming from now. Please ensure the commit
> message gives this rationale.

I'll explain about that in the commit message next.

> However, If I understand correctly, doesn't your solution only work some
> of the time? What happens if you boot both machines at the same time,
> and PERST# isn't asserted prior to the kernel booting?

I think it contains an annoying problem.

If PERST# isn't toggled prior to the kernel booting, PERST# remains asserted
and the RC driver can't access PCI bus.

As a result, this patch works and deasserts PERST# (and EP configuration will
be lost). So boot sequence needs to include deasserting PERST#.

> The only way you can ensure the EP is started after the RC is initialised
> is to start the EP after the RC is initialised.

Yes, it's the only soution for now.

> I'm not sure what the solution is here, but it feels like this approach
> only partially solves it.

Surely relying on outside of the driver doesn't seem to be a complete solution.

If there is the way that `Linux PCI endpoint framework' assumes,
I'd like to follow it, however, I can't find the other way.

Thank you,

---
Best Regards,
Kunihiko Hayashi