Re: [RFC] PCI: imx6: add support for internal oscillator on i.MX7D

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Sep 24, 2021 at 10:05:15AM +0200, Matthias Schiffer wrote:
> Adds support for a DT property fsl,internal-osc to select the internal
> oscillator for the PCIe PHY.

If you repost this, please update the subject to match the conventions
(use "git log --oneline drivers/pci/controller/dwc/pci-imx6.c" and
note capitalization).

> Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Cc: Markus Niebel <Markus.Niebel@ew.tq-group.com>
> ---
> 
> Okay, so while this patch is nice and short, I'm note sure if it's a good
> solution, hence I submit it as an RFC. It is roughly based on code from
> older linux-imx versions [1] - although it seems this feature was never
> supported on i.MX7D even by linux-imx (possibly because of compliance
> issues with the internal clock, however I haven't found a definitive
> erratum backing this), but only on other SoC like i.MX6QP.
> 
> The device tree binding docs of the driver are somewhat lacking, but
> looking at [1] it seems that an external reference clock takes the place of
> the "pcie_bus" clock - various pieces of the driver skip enabling/disabling
> this clock when an external clock is configured.
> 
> From this I've come to the conclusion that the clock settings in
> imx7d.dtsi do not really make sense: The pcie_bus clock is configured to
> PLL_ENET_MAIN_100M_CLK, but this seems wrong for both internal and
> external reference clocks:
> 
> - For the internal clock, the correct clock should be PCIE_PHY_ROOT_CLK
>   according to the reference manual
> - The external clocks, this should refer to an actual external clock, or
>   possibly a fixed-clock node
> 
> I would be great if someone with more insight into this could chime in
> and tell me if my reasoning here is correct or not.
> 
> Unfortunately I only have our MBa7x at my disposal for further
> experimentation. This board does not have an external reference clock for
> the PCIe PHY, so I cannot test the behaviour for settings that use an
> external clock. Without this patch (and adding the new flag to the MBa7x
> DTS), the boot will hang while waiting for the PCIe link to come up.
> 
> So, for the actual question (given that my thoughts above make any sense):
> How do we want to implement this?
> 
> 1. A simple boolean flag, like this patch provides
> 2. Allow Device Trees not to specify a "pcie_bus" clock at all, meaning
>    it should use the internal clock
> 3. Special handling when the "pcie_bus" clock is configured to
>    PCIE_PHY_ROOT_CLK - is such a thing even possible, or is this
>    breaking the clock driver's abstraction too much?
> 4. Something more involved, with a proper clock sel as the source for
>    "pcie_bus"
> 
> Solution 4. seems difficult to implement nicely, as the PCIe driver
> also fiddles with IMX7D_GPR12_PCIE_PHY_REFCLK_SEL for power management:
> the clock selection is switched back to the internal clock in
> imx6_pcie_clk_disable(), which also disables its source PCIE_PHY_ROOT_CLK,
> effectively gating the clock.
> 
> Regards,
> Matthias
> 
> 
> [1] https://source.codeaurora.org/external/imx/linux-imx/tree/drivers/pci/host/pci-imx6.c?h=imx_4.1.15_2.0.0_ga
> 
>  drivers/pci/controller/dwc/pci-imx6.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
> index 80fc98acf097..021499b9ee7c 100644
> --- a/drivers/pci/controller/dwc/pci-imx6.c
> +++ b/drivers/pci/controller/dwc/pci-imx6.c
> @@ -83,6 +83,7 @@ struct imx6_pcie {
>  	struct regulator	*vpcie;
>  	struct regulator	*vph;
>  	void __iomem		*phy_base;
> +	bool			internal_osc;

Prefer bitfield ("unsigned int internal_osc:1") over bool in structs.

>  	/* power domain for pcie */
>  	struct device		*pd_pcie;
> @@ -637,7 +638,9 @@ static void imx6_pcie_init_phy(struct imx6_pcie *imx6_pcie)
>  		break;
>  	case IMX7D:
>  		regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR12,
> -				   IMX7D_GPR12_PCIE_PHY_REFCLK_SEL, 0);
> +				   IMX7D_GPR12_PCIE_PHY_REFCLK_SEL,
> +				   imx6_pcie->internal_osc ?
> +					IMX7D_GPR12_PCIE_PHY_REFCLK_SEL : 0);
>  		break;
>  	case IMX6SX:
>  		regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR12,
> @@ -1130,6 +1133,9 @@ static int imx6_pcie_probe(struct platform_device *pdev)
>  				 &imx6_pcie->tx_swing_low))
>  		imx6_pcie->tx_swing_low = 127;
>  
> +	if (of_property_read_bool(node, "fsl,internal-osc"))
> +		imx6_pcie->internal_osc = true;
> +
>  	/* Limit link speed */
>  	pci->link_gen = 1;
>  	ret = of_property_read_u32(node, "fsl,max-link-speed", &pci->link_gen);
> -- 
> 2.17.1
>