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

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Message-Id: <20191204190547.333C.4A936039@socionext.com>
Date:   Wed, 04 Dec 2019 19:05:47 +0900
From:   Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <andrew.murray@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     <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, 22 Nov 2019 20:53:16 +0900 <hayashi.kunihiko@...ionext.com> wrote:

> Hello Lorenzo,
> 
> On Thu, 21 Nov 2019 16:47:05 +0000 <lorenzo.pieralisi@....com> wrote:
> 
> > 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.
> 
> I tried to connect between the following boards, and do pci-epf-test:
>  - "RC board": UniPhier ld20 board that has DWC RC controller
>  - "EP board": UniPhier legacy board that has DWC EP controller
> 
> This EP has power-on-state configuration, but it's necessary to set
> class ID, BAR sizes, etc. after starting up.
> 
> In case of that starting up RC board before EP board, the RC driver
> can't establish link. So we need to boot EP board first.

At that point, I've considered why RC can't establish link,
and found that the waitng time was too short.

- EP/RC: power on both boards

- RC: start up the kernel on RC board

- RC: wait for link up (long time enough)

- EP: start up the kernel on EP board

- EP: configurate pci-epf-test

When the endpoint  configuration is done and the EP driver enables LTSSM,
the RC driver will quit from waiting for link up.

Currently DWC RC driver calls dwc_pcie_wait_for_link(), however,
the function tries to link up 10 times only, that is defined
as LINK_WAIT_MAX_RETRIES in pcie-designware.h, it's too short
to configurate the endpoint.

Now the patch to bypass PERST# is not necessary.

Instead for DWC RC drivers, I think that the number of retries
should be changed according to the usage.
And the same issue remains with other RC drivers.

Thank you,

---
Best Regards,
Kunihiko Hayashi