Re: [PATCH v4 4/4] PCIe: qcom: Add support to control pipe clk src

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Fri 16 Jul 10:06 CDT 2021, Bjorn Helgaas wrote:

> Run this:
> 
>   $ git log --oneline drivers/pci/controller/dwc/pcie-qcom.c
> 
> and make your subject match the style and structure (in particular,
> s/PCIe/PCI/).  In this case, maybe something like this?
> 
>   PCI: qcom: Switch sc7280 gcc_pcie_1_pipe_clk_src after PHY init
> 
> On Fri, Jul 16, 2021 at 07:28:47PM +0530, Prasad Malisetty wrote:
> > This is a new requirement for sc7280 SoC.
> > To enable gdsc gcc_pcie_1_pipe_clk_src should be TCXO.
> > after PHY initialization gcc_pcie_1_pipe_clk_src needs
> > to switch from TCXO to gcc_pcie_1_pipe_clk.
> 
> This says what *needs* to happen, but it doesn't actually say what
> this patch *does*.  I think it's something like:
> 
>   On the sc7280 SoC, the clock source for pcie_1_pipe must be the TCXO
>   while gdsc is enabled.  But after the PHY is initialized, the clock
>   source must be switched to gcc_pcie_1_pipe_clk.
> 
>   On sc7280, switch gcc_pcie_1_pipe_clk_src from TCXO to
>   gcc_pcie_1_pipe_clk after the PHY has been initialized.
> 
> Nits: Rewrap to fill 75 columns or so.  Add blank lines between
> paragraphs.  Start sentences with capital letter.
> 
> > Signed-off-by: Prasad Malisetty <pmaliset@codeaurora.org>
> > ---
> >  drivers/pci/controller/dwc/pcie-qcom.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> > index 8a7a300..9e0e4ab 100644
> > --- a/drivers/pci/controller/dwc/pcie-qcom.c
> > +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> > @@ -166,6 +166,9 @@ struct qcom_pcie_resources_2_7_0 {
> >  	struct regulator_bulk_data supplies[2];
> >  	struct reset_control *pci_reset;
> >  	struct clk *pipe_clk;
> > +	struct clk *gcc_pcie_1_pipe_clk_src;
> > +	struct clk *phy_pipe_clk;
> > +	struct clk *ref_clk_src;
> >  };
> >  
> >  union qcom_pcie_resources {
> > @@ -1167,6 +1170,20 @@ static int qcom_pcie_get_resources_2_7_0(struct qcom_pcie *pcie)
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	if (of_device_is_compatible(dev->of_node, "qcom,pcie-sc7280")) {
> > +		res->gcc_pcie_1_pipe_clk_src = devm_clk_get(dev, "pipe_mux");
> > +		if (IS_ERR(res->gcc_pcie_1_pipe_clk_src))
> > +			return PTR_ERR(res->gcc_pcie_1_pipe_clk_src);
> > +
> > +		res->phy_pipe_clk = devm_clk_get(dev, "phy_pipe");
> > +		if (IS_ERR(res->phy_pipe_clk))
> > +			return PTR_ERR(res->phy_pipe_clk);
> > +
> > +		res->ref_clk_src = devm_clk_get(dev, "ref");
> > +		if (IS_ERR(res->ref_clk_src))
> > +			return PTR_ERR(res->ref_clk_src);
> 
> Not clear why ref_clk_src is here, since it's not used anywhere.  If
> it's not necessary here, drop it and add it in a future patch that
> uses it.
> 
> > +	}
> > +
> >  	res->pipe_clk = devm_clk_get(dev, "pipe");
> >  	return PTR_ERR_OR_ZERO(res->pipe_clk);
> >  }
> > @@ -1255,6 +1272,11 @@ static void qcom_pcie_deinit_2_7_0(struct qcom_pcie *pcie)
> >  static int qcom_pcie_post_init_2_7_0(struct qcom_pcie *pcie)
> >  {
> >  	struct qcom_pcie_resources_2_7_0 *res = &pcie->res.v2_7_0;
> > +	struct dw_pcie *pci = pcie->pci;
> > +	struct device *dev = pci->dev;
> > +
> > +	if (of_device_is_compatible(dev->of_node, "qcom,pcie-sc7280"))
> 
> Using of_device_is_compatible() follows existing style in the driver,
> which is good.  But I'm not sure that's good style in general because
> it's a little repetitious and wasteful.
> 

Following the style is good, but up until the recent sm8250 addition it
was just a hack to deal with legacy platforms that we don't know the
exact details about.

But, all platforms I know of has the pipe_clk from the PHY fed into the
pipe_clk_src mux in the gcc block and then ends up in the PCIe
controller. As such, I suspect that the pipe_clk handling should be moved
to the common code path of the driver and there's definitely no harm in
making sure that the pipe_clk_src mux is explicitly configured on
existing platforms (at least all 2.7.0 based ones).

> qcom_pcie_probe() already calls of_device_get_match_data(), which does
> basically the same thing as of_device_is_compatible(), so I think we
> could take better advantage of that by augmenting struct qcom_pcie_ops
> with these device-specific details.
> 

I agree.

Regards,
Bjorn

> Some drivers that use this strategy:
> 
>   drivers/pci/controller/cadence/pci-j721e.c
>   drivers/pci/controller/dwc/pci-imx6.c
>   drivers/pci/controller/dwc/pci-layerscape.c
>   drivers/pci/controller/dwc/pci-layerscape-ep.c
>   drivers/pci/controller/dwc/pcie-tegra194.c
>   drivers/pci/controller/pci-ftpci100.c
>   drivers/pci/controller/pcie-brcmstb.c
>   drivers/pci/controller/pcie-mediatek.c
> 
> > +		clk_set_parent(res->gcc_pcie_1_pipe_clk_src, res->phy_pipe_clk);
> >  
> >  	return clk_prepare_enable(res->pipe_clk);
> >  }
> > -- 
> > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > a Linux Foundation Collaborative Project
> >