Re: [PATCH v2] PCI: apple: Follow the PCIe specifications when resetting the port

From: "Pali Rohár" <pali@kernel.org>
To: Marc Zyngier <maz@kernel.org>, Luca Ceresoli <luca@lucaceresoli.net>
Cc: 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 v2] PCI: apple: Follow the PCIe specifications when resetting the port
Date: Mon, 22 Nov 2021 13:03:47 +0100	[thread overview]
Message-ID: <20211122120347.6qyiycqqjkgqvtta@pali> (raw)
In-Reply-To: <20211122104156.518063-1-maz@kernel.org>

On Monday 22 November 2021 10:41:56 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")
> 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>
> Cc: Pali Rohár <pali@kernel.org>

Looks good, but see comment below.

Acked-by: Pali Rohár <pali@kernel.org>

> ---
>  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);
> +
> +	/* Deassert #PERST */
>  	rmw_set(PORT_PERST_OFF, port->base + PORT_PERST);
>  	gpiod_set_value(reset, 1);

+ Luca

Just one comment. PERST# (PCIe Reset) is active-low signal. De-asserting
means to really set value to 1.

But there was a discussion that de-asserting should be done by call:
  gpiod_set_value(reset, 0);

https://lore.kernel.org/linux-pci/51be082a-ff10-8a19-5648-f279aabcac51@lucaceresoli.net/

Could we make this new pcie-apple.c driver to use gpiod_set_value(reset, 0)
for de-asserting, like in other drivers?

>  
> +	/* 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) {
> -- 
> 2.30.2
> 