Re: [PATCH] PCI: apple: Reset the port for 100ms on probe

From: "Pali Rohár" <pali@kernel.org>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org,
	kernel-team@android.com, Alyssa Rosenzweig <alyssa@rosenzweig.io>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH] PCI: apple: Reset the port for 100ms on probe
Date: Wed, 17 Nov 2021 21:28:59 +0100	[thread overview]
Message-ID: <20211117202859.2m5sqwz6xsjgldji@pali> (raw)
In-Reply-To: <20211117201245.GA1768803@bhelgaas>

Hello!

On Wednesday 17 November 2021 14:12:45 Bjorn Helgaas wrote:
> [+cc Pali]
> 
> On Wed, Nov 17, 2021 at 04:00:53PM +0000, Marc Zyngier wrote:
> > While the Apple PCIe driver works correctly when directly booted
> > from the firmware, it fails to initialise when the kernel is booted
> > from a bootloader using PCIe such as u-boot.
> > 
> > That's beacuse we're missing a proper reset of the port (we only
> > clear the reset, but never assert it).
> 
> s/beacuse/because/
> 
> > Bring the port back to life by wiggling the #PERST pin for 100ms
> > (as per the spec).
> 
> I cc'd Pali because I think he's interested in consolidating or
> somehow rationalizing delays like this.
> 
> If we have a specific spec reference here, I think it would help that
> effort.  I *think* it's PCIe r5.0, sec 6.6.1, which mentions the 100ms
> along with some additional constraints, like waiting 100ms after Link
> training completes for ports that support > 5.0 GT/s, whether
> Readiness Notifications are used, and CRS Software Visiblity.

This is not 100ms timeout "after link training completes".

Timeout in this patch is between flipping PERST# signal, so timeout
means how long needs to be endpoint card in reset state. And this
timeout cannot be controller specific. In past I have tried to find this
timeout in specifications, I was not able. Some summary is in my email:
https://lore.kernel.org/linux-pci/20210310110535.zh4pnn4vpmvzwl5q@pali/

So I would like to know, why was chosen 100ms for msleep() in this
patch?

> > Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Cc: Alyssa Rosenzweig <alyssa@rosenzweig.io>
> > Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > ---
> >  drivers/pci/controller/pcie-apple.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/pcie-apple.c b/drivers/pci/controller/pcie-apple.c
> > index 1bf4d75b61be..bbea5f6e0a68 100644
> > --- a/drivers/pci/controller/pcie-apple.c
> > +++ b/drivers/pci/controller/pcie-apple.c
> > @@ -543,6 +543,9 @@ static int apple_pcie_setup_port(struct apple_pcie *pcie,
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	/* Hold #PERST for 100ms as per the spec */
> > +	gpiod_set_value(reset, 0);
> > +	msleep(100);
> >  	rmw_set(PORT_PERST_OFF, port->base + PORT_PERST);
> >  	gpiod_set_value(reset, 1);
> >  
> > -- 
> > 2.30.2
> > 