linux-kernel - Re: [PATCH v4 2/3] dwc: PCI: intel: PCIe RC controller driver

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Message-ID: <20191022130905.GA133961@google.com>
Date:   Tue, 22 Oct 2019 08:09:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Dilip Kota <eswara.kota@...ux.intel.com>
Cc:     jingoohan1@...il.com, gustavo.pimentel@...opsys.com,
        lorenzo.pieralisi@....com, andrew.murray@....com, robh@...nel.org,
        martin.blumenstingl@...glemail.com, linux-pci@...r.kernel.org,
        hch@...radead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, andriy.shevchenko@...el.com,
        cheol.yong.kim@...el.com, chuanhua.lei@...ux.intel.com,
        qi-ming.wu@...el.com
Subject: Re: [PATCH v4 2/3] dwc: PCI: intel: PCIe RC controller driver

On Tue, Oct 22, 2019 at 05:07:47PM +0800, Dilip Kota wrote:
> On 10/22/2019 1:17 AM, Bjorn Helgaas wrote:
> > On Mon, Oct 21, 2019 at 02:39:19PM +0800, Dilip Kota wrote:
> > > Add support to PCIe RC controller on Intel Gateway SoCs.
> > > PCIe controller is based of Synopsys DesignWare pci core.
> > > 
> > > Intel PCIe driver requires Upconfig support, fast training
> > > sequence configuration and link speed change. So adding the
> > > respective helper functions in the pcie DesignWare framework.
> > > It also programs hardware autonomous speed during speed
> > > configuration so defining it in pci_regs.h.
> > > 
> > > +static void intel_pcie_link_setup(struct intel_pcie_port *lpp)
> > > +{
> > > +	u32 val;
> > > +
> > > +	val = pcie_rc_cfg_rd(lpp, PCIE_CAP_OFST + PCI_EXP_LNKCAP);
> > > +	lpp->max_speed = FIELD_GET(PCI_EXP_LNKCAP_SLS, val);
> > > +	lpp->max_width = FIELD_GET(PCI_EXP_LNKCAP_MLW, val);
> > > +
> > > +	val = pcie_rc_cfg_rd(lpp, PCIE_CAP_OFST + PCI_EXP_LNKCTL);
> > > +
> > > +	val &= ~(PCI_EXP_LNKCTL_LD | PCI_EXP_LNKCTL_ASPMC);
> > > +	val |= (PCI_EXP_LNKSTA_SLC << 16) | PCI_EXP_LNKCTL_CCC |
> > > +	       PCI_EXP_LNKCTL_RCB;

> > PCI_EXP_LNKCTL_CCC is RW.  But doesn't it depend on the components on
> > both ends of the link?  Do you know what device is at the other end?
> > I would have assumed that you'd have to start with CCC==0, which
> > should be most conservative, then set CCC=1 only if you know both ends
> > have a common clock.
> PCIe RC and endpoint device are having the common clock so set the CCC=1.

How do you know what the endpoint device is?  Is this driver only for
a specific embedded configuration where the endpoint is always
soldered down?  There's no possibility of this RC being used with a
connector?

Shouldn't this be either discoverable or configurable via DT or
something?  pcie_aspm_configure_common_clock() seems to do something
similar, but I can't really vouch for its correctness.

Bjorn