Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

From: "Pali Rohár" <pali@kernel.org>
To: Luca Ceresoli <luca@lucaceresoli.net>
Cc: linux-pci@vger.kernel.org, linux-omap@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Kishon Vijay Abraham I <kishon@ti.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour
Date: Mon, 31 May 2021 15:32:11 +0200	[thread overview]
Message-ID: <20210531133211.llyiq3jcfy25tmz4@pali> (raw)
In-Reply-To: <20210531090540.2663171-1-luca@lucaceresoli.net>

On Monday 31 May 2021 11:05:40 Luca Ceresoli wrote:
> The PCIe PERSTn reset pin is active low and should be asserted, then
> deasserted.
> 
> The current implementation only drives the pin once in "HIGH" position,
> thus presumably it was intended to deassert the pin. This has two problems:
> 
>   1) it assumes the pin was asserted by other means before loading the
>      driver
>   2) it has the wrong polarity, since "HIGH" means "active", and the pin is
>      presumably configured as active low coherently with the PCIe
>      convention, thus it is driven physically to 0, keeping the device
>      under reset unless the pin is configured as active high.
> 
> Fix both problems by:
> 
>   1) keeping devm_gpiod_get_optional(dev, NULL, GPIOD_OUT_HIGH) as is, but
>      assuming the pin is correctly configured as "active low" this now
>      becomes a reset assertion
>   2) adding gpiod_set_value(reset, 0) after a delay to deassert reset
> 
> Fixes: 78bdcad05ea1 ("PCI: dra7xx: Add support to make GPIO drive PERST# line")
> Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
> 
> ---
> 
> Changes v1 -> v2:
>  - No changes to the patch
>  - Reword commit message according to suggestions from Bjorn Helgaas (from
>    another patchset)
>  - Add Fixes: tag
> ---
>  drivers/pci/controller/dwc/pci-dra7xx.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pci/controller/dwc/pci-dra7xx.c b/drivers/pci/controller/dwc/pci-dra7xx.c
> index cb5d4c245ff6..11f392b7a9a2 100644
> --- a/drivers/pci/controller/dwc/pci-dra7xx.c
> +++ b/drivers/pci/controller/dwc/pci-dra7xx.c
> @@ -801,6 +801,8 @@ static int dra7xx_pcie_probe(struct platform_device *pdev)
>  		dev_err(&pdev->dev, "gpio request failed, ret %d\n", ret);
>  		goto err_gpio;
>  	}
> +	usleep_range(1000, 2000);

Hello! Just a note that this is again a new code pattern in another
driver for different wait value of PCIe Warm Reset timeout. I sent email
about these issues:
https://lore.kernel.org/linux-pci/20210310110535.zh4pnn4vpmvzwl5q@pali/

Luca, how did you choose value 1000-2000 us? Do you have some reference
or specification which says that this value needs to be used?

> +	gpiod_set_value(reset, 0);
>  
>  	reg = dra7xx_pcie_readl(dra7xx, PCIECTRL_DRA7XX_CONF_DEVICE_CMD);
>  	reg &= ~LTSSM_EN;
> -- 
> 2.25.1
> 