Re: [PATCH v2 2/5] PCI: stm32: Add PCIe host support for STM32MP25

[Christian Bruel]

On 12/18/24 10:46, Manivannan Sadhasivam wrote:
> On Mon, Dec 16, 2024 at 10:00:27AM +0100, Christian Bruel wrote:
>
> [...]
>
> > > > +		msleep(PCIE_T_RRS_READY_MS);
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +static void stm32_pcie_stop_link(struct dw_pcie *pci)
> > > > +{
> > > > +	struct stm32_pcie *stm32_pcie = to_stm32_pcie(pci);
> > > > +
> > > > +	regmap_update_bits(stm32_pcie->regmap, SYSCFG_PCIECR,
> > > > +			   STM32MP25_PCIECR_LTSSM_EN, 0);
> > > > +
> > > > +	/* Assert PERST# */
> > > > +	if (stm32_pcie->perst_gpio)
> > > > +		gpiod_set_value(stm32_pcie->perst_gpio, 1);
> > >
> > > I don't like tying PERST# handling with start/stop link. PERST# should be
> > > handled based on the power/clock state.
> >
> > I don't understand your point: We turn off the PHY in suspend_noirq(), so
> > that seems a logical place to de-assert in resume_noirq after the refclk is
> > ready. PERST# should be kept active until the PHY stablilizes the clock in
> > resume. From the PCIe electromechanical specs, PERST# goes active while the
> > refclk is not stable/
>
> While your understanding about PERST# is correct, your implementation is not.
> You are toggling PERST# from start/stop link callbacks which are supposed to
> control the LTSSM state only. I don't have issues with toggling PERST# in
> stm32_pcie_{suspend/resume}_noirq().

Ah OK. this function is split now into 2 functional blocks in the 
upcoming version

> > > > +}
> > > > +
> > > > +static int stm32_pcie_suspend(struct device *dev)
> > > > +{
> > > > +	struct stm32_pcie *stm32_pcie = dev_get_drvdata(dev);
> > > > +
> > > > +	if (device_may_wakeup(dev) || device_wakeup_path(dev))
> > > > +		enable_irq_wake(stm32_pcie->wake_irq);
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int stm32_pcie_resume(struct device *dev)
> > > > +{
> > > > +	struct stm32_pcie *stm32_pcie = dev_get_drvdata(dev);
> > > > +
> > > > +	if (device_may_wakeup(dev) || device_wakeup_path(dev))
> > > > +		disable_irq_wake(stm32_pcie->wake_irq);
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int stm32_pcie_suspend_noirq(struct device *dev)
> > > > +{
> > > > +	struct stm32_pcie *stm32_pcie = dev_get_drvdata(dev);
> > > > +
> > > > +	stm32_pcie->link_is_up = dw_pcie_link_up(stm32_pcie->pci);
> > > > +
> > > > +	stm32_pcie_stop_link(stm32_pcie->pci);
> > >
> > > I don't understand how endpoint can wakeup the host if PERST# gets asserted.
> >
> > The stm32 PCIe doesn't support L2, we don't expect an in-band beacon for the
> > wakeup. We support wakeup only from sideband WAKE#, that will restart the
> > link from IRQ
>
> I don't understand how WAKE# is supported without L2. Only in L2 state, endpoint
> will make use of Vaux and it will wakeup the host using either beacon or WAKE#.
> If you don't support L2, then the endpoint will reach L3 (link off) state.

I think this is what happens, my understanding is that the device is 
still powered to get the wakeup event (eg WoL magic packet), triggers 
the PCIe wake_IRQ from the WAKE#.

> > > > +	clk_disable_unprepare(stm32_pcie->clk);
> > > > +
> > > > +	if (!device_may_wakeup(dev) && !device_wakeup_path(dev))
> > > > +		phy_exit(stm32_pcie->phy);
> > > > +
> > > > +	return pinctrl_pm_select_sleep_state(dev);
> > > > +}
> > > > +
> > > > +static int stm32_pcie_resume_noirq(struct device *dev)
> > > > +{
> > > > +	struct stm32_pcie *stm32_pcie = dev_get_drvdata(dev);
> > > > +	struct dw_pcie *pci = stm32_pcie->pci;
> > > > +	struct dw_pcie_rp *pp = &pci->pp;
> > > > +	int ret;
> > > > +
> > > > +	/* init_state must be called first to force clk_req# gpio when no
> > >
> > > CLKREQ#
> > >
> > > Why RC should control CLKREQ#?
> >
> > REFCLK is gated with CLKREQ#, So we cannot access the core
> > without CLKREQ# if no device is present. So force it with a init pinmux
> > the time to init the PHY and the core DBI registers
>
> Ok. You should add a comment to clarify it in the code as this is not a standard
> behavior.

OK

> > > Also please use preferred style for multi-line comments:
> > >
> > > 	/*
> > > 	 * ...
> > > 	 */
> > >
> > > > +	 * device is plugged.
> > > > +	 */
> > > > +	if (!IS_ERR(dev->pins->init_state))
> > > > +		ret = pinctrl_select_state(dev->pins->p, dev->pins->init_state);
> > > > +	else
> > > > +		ret = pinctrl_pm_select_default_state(dev);
> > > > +
> > > > +	if (ret) {
> > > > +		dev_err(dev, "Failed to activate pinctrl pm state: %d\n", ret);
> > > > +		return ret;
> > > > +	}
> > > > +
> > > > +	if (!device_may_wakeup(dev) && !device_wakeup_path(dev)) {
> > > > +		ret = phy_init(stm32_pcie->phy);
> > > > +		if (ret) {
> > > > +			pinctrl_pm_select_default_state(dev);
> > > > +			return ret;
> > > > +		}
> > > > +	}
> > > > +
> > > > +	ret = clk_prepare_enable(stm32_pcie->clk);
> > > > +	if (ret)
> > > > +		goto clk_err;
> > >
> > > Please name the goto labels of their purpose. Like err_phy_exit.
> >
> > OK
> >
> > > > +
> > > > +	ret = dw_pcie_setup_rc(pp);
> > > > +	if (ret)
> > > > +		goto pcie_err;
> > >
> > > This should be, 'err_disable_clk'.
> > >
> > > > +
> > > > +	if (stm32_pcie->link_is_up) {
> > >
> > > Why do you need this check? You cannot start the link in the absence of an
> > > endpoint?
> >
> > It is an optimization to avoid unnecessary "dw_pcie_wait_for_link" if no
> > device is present during suspend
>
> In that case you'll not trigger LTSSM if there was no endpoint connected before
> suspend. What if users connect an endpoint after resume?

Yes, exactly. We don't support hotplug, and plugging a device while the 
system is in stand-by is something that we don't expect. The imx6 
platform does this also.

thanks,

Christian

> - Mani