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

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Date:   Mon, 27 May 2019 19:15:21 +0200
From:   Niklas Cassel <niklas.cassel@...aro.org>
To:     Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc:     Stanimir Varbanov <svarbanov@...sol.com>,
        Andy Gross <agross@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        PCI <linux-pci@...r.kernel.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: qcom: Ensure that PERST is asserted for at least
 100 ms

On Fri, May 24, 2019 at 02:43:00PM +0200, Marc Gonzalez wrote:
> On 23/05/2019 21:44, 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>
> > ---
> >  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);
> 
> Currently, qcom_ep_reset_assert() and qcom_ep_reset_deassert() both include
> a call to usleep_range() of 1.0 to 1.5 ms
> 
> Can we git rid of both if we sleep 100 ms before qcom_ep_reset_deassert?

These two sleeps after asserting/deasserting reset in qcom_ep_reset_assert()/
qcom_ep_reset_deassert() matches the sleeps in:
https://source.codeaurora.org/quic/la/kernel/msm-4.14/tree/drivers/pci/host/pci-msm.c?h=LA.UM.7.1.r1-14000-sm8150.0#n1942

and

https://source.codeaurora.org/quic/la/kernel/msm-4.14/tree/drivers/pci/host/pci-msm.c?h=LA.UM.7.1.r1-14000-sm8150.0#n1949

I would rather not remove these since that might affect existing devices.


This new sleep matches matches the sleep in:
https://source.codeaurora.org/quic/la/kernel/msm-4.14/tree/drivers/pci/host/pci-msm.c?h=LA.UM.7.1.r1-14000-sm8150.0#n3926

> 
> Should the msleep() call be included in one of the two wrappers?

This new sleep could be moved into qcom_ep_reset_deassert(),
added before the gpiod_set_value_cansleep(pcie->reset, 0) call,
if Stanimir prefers it to be placed there instead.


Kind regards,
Niklas