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

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

On Fri 16 Jul 14:37 CDT 2021, Stephen Boyd wrote:

> Quoting Prasad Malisetty (2021-07-16 06:58:47)
> > This is a new requirement for sc7280 SoC.
> > To enable gdsc gcc_pcie_1_pipe_clk_src should be TCXO.
> 
> Why? Can you add that detail here? Presumably it's something like the
> GDSC needs a running clk to send a reset through the flops or something
> like that.
> 

Which presumably means that we need to "park" gcc_pcie_N_pipe_clk_src
whenever the PHY pipe is paused due to a suspend or runtime suspend.

I find this part of the commit message to primarily describing the next
patch (that is yet to be posted).

> > after PHY initialization gcc_pcie_1_pipe_clk_src needs
> > to switch from TCXO to gcc_pcie_1_pipe_clk.
> >
> > 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
> > @@ -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);
> > +       }
> > +
> >         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"))
> > +               clk_set_parent(res->gcc_pcie_1_pipe_clk_src, res->phy_pipe_clk);
> 
> Is anything wrong if we call clk_set_parent() here when this driver is
> running on previous SoCs where the parent is assigned via DT?

We don't assign the parent on previous platforms, we apparently just
rely on the reset value (afaict).

So I think it makes sense for all platforms to explicitly mux
pipe_clk_src to phy::pipe_clk, one the PHY is up and running.

But I was under the impression that we have the BRANCH_HALT_SKIP on the
pipe_clk because there was some sort of feedback loop to the PHY's
calibration... What this patch indicates is that we should park
pipe_clk_src onto XO at boot time, then after the PHY starts ticking we
should enable and reparent the clk_src - at which point I don't see why
we need the HALT_SKIP.

> Also, shouldn't we make sure the parent is XO at driver probe time so
> that powering on the GDSC works properly?
> 
> It all feels like a kludge though given that the GDSC is the one that
> requires the clock to be running at XO and we're working around that in
> the pcie driver instead of sticking that logic into the GDSC. What do we
> do if the GDSC is already enabled out of boot instead of being the power
> on reset (POR) configuration?
> 

What happens if we boot the device out of NVME...


PS. Are we certain that it's the PCIe driver and not the PHY that should
do this dance? I really would like to see the continuation of this patch
to see the full picture...

Regards,
Bjorn

> >
> >         return clk_prepare_enable(res->pipe_clk);
> >  }