Re: [PATCH] PCI: stm32: Fix LTSSM EP race with start link.

[Christian Bruel]

On 11/14/25 19:59, Bjorn Helgaas wrote:
> On Fri, Nov 14, 2025 at 08:45:52AM +0100, Christian Bruel wrote:
> > If the host has deasserted PERST# and started link training before the link
> > is started on EP side, enabling LTSSM before the endpoint registers are
> > initialized in the perst_irq handler results in probing incorrect values.
> >
> > Thus, wait for the PERST# level-triggered interrupt to start link training
> > at the end of initialization and cleanup the stm32_pcie_[start stop]_link
> > functions.
>
> I've seen this kind of thing in other drivers, and I wondered whether
> it was safe because the host asserts and deasserts PERST#
> asynchronously, independent of anything the endpoint is doing.
>
> I assume it's possible that the host deasserts PERST# before this
> driver has the stm32_pcie_ep_perst_irq_thread() thread set up.  If
> that happens and the driver doesn't see the PERST# interrupt, does
> everything still work correctly?

yes it does. the PERST# interrupt is level-triggered and, if already 
de-asserted, fires only when enabled (it is NOAUTOEN) with start_link.

At that point, the host can enumerate by performing a manual rescan or 
rebind the PCIe driver, restarting the entire probe sequence.

Tested the pcie_epf_test driver with various power-up sequences: full 
power-up the host or device first, and stop or standby PM suspend/resume.


> > Signed-off-by: Christian Bruel <christian.bruel@xxxxxxxxxxx>
> > ---
> >   drivers/pci/controller/dwc/pcie-stm32-ep.c | 38 ++++++------------------------
> >   1 file changed, 7 insertions(+), 31 deletions(-)
> >
> > diff --git a/drivers/pci/controller/dwc/pcie-stm32-ep.c b/drivers/pci/controller/dwc/pcie-stm32-ep.c
> > index 3400c7cd2d88a279c49ef36a99fc7537c381c384..d0654bb43759bb8d0f0d7badbf7bdae839241fcf 100644
> > --- a/drivers/pci/controller/dwc/pcie-stm32-ep.c
> > +++ b/drivers/pci/controller/dwc/pcie-stm32-ep.c
> > @@ -37,36 +37,9 @@ static void stm32_pcie_ep_init(struct dw_pcie_ep *ep)
> >   		dw_pcie_ep_reset_bar(pci, bar);
> >   }
> >   
> > -static int stm32_pcie_enable_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,
> > -			   STM32MP25_PCIECR_LTSSM_EN);
> > -
> > -	return dw_pcie_wait_for_link(pci);
> > -}
> > -
> > -static void stm32_pcie_disable_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);
> > -}
> > -
> >   static int stm32_pcie_start_link(struct dw_pcie *pci)
> >   {
> >   	struct stm32_pcie *stm32_pcie = to_stm32_pcie(pci);
> > -	int ret;
> > -
> > -	dev_dbg(pci->dev, "Enable link\n");
> > -
> > -	ret = stm32_pcie_enable_link(pci);
> > -	if (ret) {
> > -		dev_err(pci->dev, "PCIe cannot establish link: %d\n", ret);
> > -		return ret;
> > -	}
> >   
> >   	enable_irq(stm32_pcie->perst_irq);
> >   
> > @@ -77,11 +50,7 @@ static void stm32_pcie_stop_link(struct dw_pcie *pci)
> >   {
> >   	struct stm32_pcie *stm32_pcie = to_stm32_pcie(pci);
> >   
> > -	dev_dbg(pci->dev, "Disable link\n");
> > -
> >   	disable_irq(stm32_pcie->perst_irq);
> > -
> > -	stm32_pcie_disable_link(pci);
> >   }
> >   
> >   static int stm32_pcie_raise_irq(struct dw_pcie_ep *ep, u8 func_no,
> > @@ -152,6 +121,8 @@ static void stm32_pcie_perst_assert(struct dw_pcie *pci)
> >   
> >   	dev_dbg(dev, "PERST asserted by host\n");
> >   
> > +	regmap_update_bits(stm32_pcie->regmap, SYSCFG_PCIECR, STM32MP25_PCIECR_LTSSM_EN, 0);
> > +
> >   	pci_epc_deinit_notify(ep->epc);
> >   
> >   	stm32_pcie_disable_resources(stm32_pcie);
> > @@ -192,6 +163,11 @@ static void stm32_pcie_perst_deassert(struct dw_pcie *pci)
> >   
> >   	pci_epc_init_notify(ep->epc);
> >   
> > +	/* Enable link training */
> > +	regmap_update_bits(stm32_pcie->regmap, SYSCFG_PCIECR,
> > +			   STM32MP25_PCIECR_LTSSM_EN,
> > +			   STM32MP25_PCIECR_LTSSM_EN);
> > +
> >   	return;
> >   
> >   err_disable_resources:
> >
> > ---
> > base-commit: 31115ecec74fe5c679a149d7037009f26b3aa8a9
> > change-id: 20251113-perst_ep-0b57b9679cf9
> >
> > Best regards,
> > --
> > Christian Bruel <christian.bruel@xxxxxxxxxxx>