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

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Message-ID: <20191121164705.GA14229@e121166-lin.cambridge.arm.com>
Date:   Thu, 21 Nov 2019 16:47:05 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
Cc:     Andrew Murray <andrew.murray@....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

On Fri, Nov 08, 2019 at 04:30:27PM +0900, Kunihiko Hayashi wrote:
> > 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#.

I am sorry but I have lost you. Can you explain to us why checking
that PERST# is deasserted guarantees you that:

- The EP has bootstrapped
- It is safe not to toggle it again (and also skip
  uniphier_pcie_ltssm_enable())

Please provide details of the HW configuration so that we understand
what's actually supposed to happen and why this patch fixes the
issue you are facing.

Thanks,
Lorenzo