Re: [PATCH 2/2] PCI: imx: Initial imx7d pm support

[Leonard Crestez <leonard.crestez@nxp.com>]

On Fri, 2018-06-08 at 16:33 +0200, Lucas Stach wrote:
> Am Dienstag, den 29.05.2018, 22:39 +0300 schrieb Leonard Crestez:
> > On imx7d the phy is turned off in suspend and must be reset on resume.
> > Right now lspci -v fails after a suspend/resume cycle, fix this by
> > adding minimal suspend/resume code from the nxp vendor tree.
> > 
> > This is currently only enabled for imx7 but the same sequence can be
> > applied to other imx pcie variants.

> > +#ifdef CONFIG_PM_SLEEP
> > +static int imx6_pcie_suspend_noirq(struct device *dev)
> > +{
> > +	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
> > +
> > +	if (imx6_pcie->variant == IMX7D) {
> 
> Instead of this large indented block, please just have an early return
> at the start of this function, like:
> 
> if (imx6_pcie->variant != IMX7D)
> 	return 0;
> 
> Same for the resume function.

OK. The resume sequence is mostly the same for all SOCs where it
applies.

> > +static int imx6_pcie_resume_noirq(struct device *dev)
> > +{
> > +	int ret = 0;
> > +	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
> > +	struct pcie_port *pp = &imx6_pcie->pci->pp;
> > 
> > +
> > +	if (imx6_pcie->variant == IMX7D) {
> > +		imx6_pcie_ltssm_disable(dev);
> 
> The LTSSM disable seems misplaced here. I would have expected it to be
> in the suspend function.

This is a requirement for reinitializing the core: LTSSM needs to be
turned off during initialization.

> > +		/*
> > +		 * controller maybe turn off, re-configure again
> > +		 */
> > +		dw_pcie_setup_rc(pp);
> > +
> > +		imx6_pcie_ltssm_enable(dev);
> > +
> > +		ret = imx6_pcie_wait_for_link(imx6_pcie);
> > +		if (ret < 0)
> > +			pr_info("pcie link is down after resume.\n");
> 
> If the PHY has been powered down and LTSSM stopped I guess we need to
> do the full imx6_pcie_establish_link() dance again here to reliably re-
> establish the link. The above seems unsafe.

What imx6_pcie_establish_link does additionally is some workaround for
link gen detection. I agree that it should be included.

This would make resume mostly the same as imx_pcie_host_init except for
dw_pcie_msi_init; that needs to be saved/restored separately.


Another issue that should be discussed here is that on 6sx and 7d the
gpc PCIE power domain is not defined correctly: the PCIE block is in
the DISPMIX domain on both SOCs and it is only PCIE_PHY which has a
separate power domain.

This matters: enabling power-gating for displays will break pcie if
this relationship is not taken into account. Here are some options:

1) Make &pd_pcie a child of &pd_disp by hacking into gpc probe
functions and calling pm_genpd_add_subdomain. Not very nice.

2) Support nesting PGCs in GPC code? Lots of code and still an
incorrect representation of hardware.

3) Set power-domains = <&pd_disp> and enable runtime PM on &pd_pcie?

4) Add separate devices for the pcie-phy. These would be mostly empty
but still different, for example on imx6sx the PHY is not even
accessible on the bus but only though PCIE registers.

Solutions 1-3 seem like workarounds while 4 seems excessive, would
appreciate some feedback.

--
Regards,
Leonard