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

From: Robin Murphy <robin.murphy@arm.com>
To: Marc Zyngier <maz@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org
Cc: "Pali Rohár" <pali@kernel.org>,
	"Alyssa Rosenzweig" <alyssa@rosenzweig.io>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Mark Kettenis" <mark.kettenis@xs4all.nl>,
	"Luca Ceresoli" <luca@lucaceresoli.net>,
	kernel-team@android.com
Subject: Re: [PATCH v3 1/3] PCI: apple: Follow the PCIe specifications when resetting the port
Date: Wed, 24 Nov 2021 12:56:06 +0000	[thread overview]
Message-ID: <fa1d802c-43a0-cbec-c16b-c662e3fdf7fa@arm.com> (raw)
In-Reply-To: <20211123180636.80558-2-maz@kernel.org>

On 2021-11-23 18:06, 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
> 
> Implementing this results in a booting system.
> 
> Fixes: 1e33888fbe44 ("PCI: apple: Add initial hardware bring-up")
> Acked-by: Pali Rohár <pali@kernel.org>
> 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 | 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);

FWIW, given that getting the GPIO with GPIOD_OUT_LOW should have had 
this effect in the first place, if this isn't a no-op at this point then 
it would hint at something being more significantly wrong down at the 
GPIO/pinctrl end :/

Once you fix the polarity in the later patch, though, adding the 
explicit reset assertion here does seem a far nicer option than fiddling 
the flags to preserve the implicit assertion earlier.

Cheers,
Robin.

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

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