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

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Date:   Wed, 17 Nov 2021 14:12:45 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pci@...r.kernel.org, kernel-team@...roid.com,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pali Rohár <pali@...nel.org>
Subject: Re: [PATCH] PCI: apple: Reset the port for 100ms on probe

[+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.

> Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Cc: Alyssa Rosenzweig <alyssa@...enzweig.io>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Cc: Bjorn Helgaas <bhelgaas@...gle.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
>