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

From: Prasad Malisetty
Date: Mon Aug 09 2021 - 13:11:36 EST

On 2021-07-17 02:08, Bjorn Andersson wrote:
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@xxxxxxxxxxxxxx>
> ---
> 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.


Some drivers that use this strategy:


> + clk_set_parent(res->gcc_pcie_1_pipe_clk_src, res->phy_pipe_clk);
> return clk_prepare_enable(res->pipe_clk);
> }
> --

Hi Bjorn,

Thanks for your review and inputs.

I would like to add more details on this requirement. The hardware ver. of SM8250 & SC7280 platforms is the same but
where as only for SC7280, we need to switch gcc_pcie_1_pipe_clk source between TXCO and gcc_pcie_1_pipe_clk hence this is SoC level
specific requirement. all existing platforms doesn't need this pipe clk handling.

I will use boolean flag entry in SoC dtsi such as "pipe-clk-source-switch" to differentiate between SM8250 and SC7280 instead of compatible

Please provide any other better approach for the above case.


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