Re: [PATCH v4 05/12] PCI: aardvark: Issue PERST via GPIO

["Pali Rohár" <pali@kernel.org>]

On Thursday 30 April 2020 10:06:18 Pali Rohár wrote:
> +static void advk_pcie_issue_perst(struct advk_pcie *pcie)
> +{
> +	u32 reg;
> +
> +	if (!pcie->reset_gpio)
> +		return;
> +
> +	/* PERST does not work for some cards when link training is enabled */
> +	reg = advk_readl(pcie, PCIE_CORE_CTRL0_REG);
> +	reg &= ~LINK_TRAINING_EN;
> +	advk_writel(pcie, reg, PCIE_CORE_CTRL0_REG);
> +
> +	/* 10ms delay is needed for some cards */
> +	dev_info(&pcie->pdev->dev, "issuing PERST via reset GPIO for 10ms\n");
> +	gpiod_set_value_cansleep(pcie->reset_gpio, 1);
> +	usleep_range(10000, 11000);
> +	gpiod_set_value_cansleep(pcie->reset_gpio, 0);
> +}

Just note about delay between changing GPIO reset:

In V2 there as only 1ms, but be figured out that it is not enough for
WLE900VX cards when they were already initialized in u-boot.

I tried to find in PCI specs if there is a defined timeout for this
operation. I found following 3 delay definitions which could be related:

TPVPERL - PERST# must remain active at least this long after power becomes valid
TPERST - When asserted, PERST# must remain asserted at least this long
TPERSTCLK - PERST# must remain active at least this long after any supplied reference clock is stable

In another spec they have defined also minimal values:

TPVPERL - Power stable to PERST# inactive - Min 100 ms
TPERST - PERST# active time - Min 100 us
TPERSTCLK - REFCLK stable before PERST# inactive - Min 100 us

After experimenting with those Compex WLE900VX cards, I know that 100us
delay is not enough. And I'm not sure if TPVPERL is really relevant for
this case. I understood that TPVPERL is needed when initializing power
again. And because delaying boot by another 100ms is does not have to be
acceptable if there is not strict reason for it, I rather decided to
stay with just 10ms delay.

If you know what is the correct timeout between changing GPIO reset,
please let me know and in future I can fix/reimplement it.