linux-kernel - Re: [PATCH v3 1/3] PCI: apple: Follow the PCIe specifications when resetting the port

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Message-ID: <20211207162257.GA44468@bhelgaas>
Date:   Tue, 7 Dec 2021 10:22:57 -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,
        Pali Rohár <pali@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Luca Ceresoli <luca@...aceresoli.net>, kernel-team@...roid.com
Subject: Re: [PATCH v3 1/3] PCI: apple: Follow the PCIe specifications when
 resetting the port

On Tue, Nov 23, 2021 at 06:06:34PM +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).
> 
> The PCIe spec requirements are two-fold:
> 
> - #PERST must be asserted before setting up the clocks, and
>   stay asserted for at least 100us (Tperst-clk).
> 
> - Once #PERST is deasserted, the OS must wait for at least 100ms
>   "from the end of a Conventional Reset" before we can start talking
>   to the devices

Unless somebody objects, I'll s/#PERST/PERST#/ to match the spec
usage, both here and in the comments below.

I also notice gpiod_get_from_of_node(..., "#PERST") earlier in
apple_pcie_setup_port().  If it wouldn't break anything, I'd like to
change that, too.

> Implementing this results in a booting system.
> 
> Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
> Acked-by: Pali Rohár <pali@...nel.org>
> 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 | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pci/controller/pcie-apple.c b/drivers/pci/controller/pcie-apple.c
> index 1bf4d75b61be..957960a733c4 100644
> --- a/drivers/pci/controller/pcie-apple.c
> +++ b/drivers/pci/controller/pcie-apple.c
> @@ -539,13 +539,23 @@ static int apple_pcie_setup_port(struct apple_pcie *pcie,
>  
>  	rmw_set(PORT_APPCLK_EN, port->base + PORT_APPCLK);
>  
> +	/* Engage #PERST before setting up the clock */
> +	gpiod_set_value(reset, 0);
> +
>  	ret = apple_pcie_setup_refclk(pcie, port);
>  	if (ret < 0)
>  		return ret;
>  
> +	/* The minimal Tperst-clk value is 100us (PCIe CMS r2.0, 2.6.2) */
> +	usleep_range(100, 200);

Spec says "min 100us from REFCLK stable before PERST# inactive".  So I
guess when apple_pcie_setup_refclk() returns, we know REFCLK is
already stable?

> +	/* Deassert #PERST */
>  	rmw_set(PORT_PERST_OFF, port->base + PORT_PERST);
>  	gpiod_set_value(reset, 1);
>  
> +	/* Wait for 100ms after #PERST deassertion (PCIe r2.0, 6.6.1) */
> +	msleep(100);

Does this port support speeds greater than 5 GT/s?  If so, 6.6.1 says
we need "100ms after Link training completes," not just after
deasserting PERST#.

I'll update this citation to "PCIe r5.0, 6.6.1" to reference the
current spec.

>  	ret = readl_relaxed_poll_timeout(port->base + PORT_STATUS, stat,
>  					 stat & PORT_STATUS_READY, 100, 250000);
>  	if (ret < 0) {
> -- 
> 2.30.2
>