linux-kernel - Re: [PATCH] PCI: qcom: Ensure that PERST is asserted for at least 100 ms

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Message-ID: <20190526054918.GK2085@tuxbook-pro>
Date:   Sat, 25 May 2019 22:49:18 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Niklas Cassel <niklas.cassel@...aro.org>
Cc:     Stanimir Varbanov <svarbanov@...sol.com>,
        Andy Gross <agross@...nel.org>,
        David Brown <david.brown@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: qcom: Ensure that PERST is asserted for at least
 100 ms

On Thu 23 May 12:44 PDT 2019, Niklas Cassel wrote:

> Currently, there is only a 1 ms sleep after asserting PERST.
> 
> Reading the datasheets for different endpoints, some require PERST to be
> asserted for 10 ms in order for the endpoint to perform a reset, others
> require it to be asserted for 50 ms.
> 
> Several SoCs using this driver uses PCIe Mini Card, where we don't know
> what endpoint will be plugged in.
> 
> The PCI Express Card Electromechanical Specification specifies:
> "On power up, the deassertion of PERST# is delayed 100 ms (TPVPERL) from
> the power rails achieving specified operating limits."
> 
> Add a sleep of 100 ms before deasserting PERST, in order to ensure that
> we are compliant with the spec.
> 
> Signed-off-by: Niklas Cassel <niklas.cassel@...aro.org>

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

> ---
>  drivers/pci/controller/dwc/pcie-qcom.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> index 0ed235d560e3..cae24376237c 100644
> --- a/drivers/pci/controller/dwc/pcie-qcom.c
> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> @@ -1110,6 +1110,8 @@ static int qcom_pcie_host_init(struct pcie_port *pp)
>  	if (IS_ENABLED(CONFIG_PCI_MSI))
>  		dw_pcie_msi_init(pp);
>  
> +	/* Ensure that PERST has been asserted for at least 100 ms */
> +	msleep(100);
>  	qcom_ep_reset_deassert(pcie);
>  
>  	ret = qcom_pcie_establish_link(pcie);
> -- 
> 2.21.0
>