Re: [PATCH] power: sequencing: qcom-wcn: explain why we need the WLAN_EN GPIO hack

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Dec 17, 2024 at 02:07:14PM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> 
> With the recent rework of the PCI power control code, the workaround for
> the wlan-enable GPIO - where we don't set a default (low) state in the
> power sequencing driver, but instead request the pin as-is - should no
> longer be needed but some platforms still fail to probe the WLAN
> controller. This is caused by the Qcom PCIe controller and needs a
> workaround in the controller driver so add a FIXME to eventually remove
> the hack from this driver once this is done.
> 
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> ---
>  drivers/power/sequencing/pwrseq-qcom-wcn.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/power/sequencing/pwrseq-qcom-wcn.c b/drivers/power/sequencing/pwrseq-qcom-wcn.c
> index cc03b5aaa8f2..9d6a68ac719f 100644
> --- a/drivers/power/sequencing/pwrseq-qcom-wcn.c
> +++ b/drivers/power/sequencing/pwrseq-qcom-wcn.c
> @@ -396,6 +396,14 @@ static int pwrseq_qcom_wcn_probe(struct platform_device *pdev)
>  		return dev_err_probe(dev, PTR_ERR(ctx->bt_gpio),
>  				     "Failed to get the Bluetooth enable GPIO\n");
>  
> +	/*
> +	 * FIXME: This should actually be GPIOD_OUT_LOW. The driver model can
> +	 * correctly handle provider <-> consumer dependencies but there is a
> +	 * known issue with Qcom PCIe controllers where, if the device is
> +	 * powered off abrubtly (without controller driver noticing), the PCIe
> +	 * link moves to link down state. Until the link-down handling is
> +	 * addressed in the controller driver, we need to keep this workaround.

Maybe we should add some info on how GPIOD_OUT_LOW causes link down. Like,

	/*
	 * FIXME: This should actually be GPIOD_OUT_LOW. But doing so would
	 * cause the WLAN power to be toggled, resulting in PCIe link down.
	 * Since the PCIe controller driver is not handling link down currently,
	 * the device becomes unusable. So we need to keep this workaround until
	 * the link down handling is implemented in the controller driver.
	 */

But the comment applies to gpiod_direction_output() call as well, right?

- Mani

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