Re: [PATCH] PCI: meson: Fix PERST# timing by asserting reset before LTSSM enable

[Manivannan Sadhasivam]

On Mon, Jun 15, 2026 at 12:34:11PM +0200, Ronald Claveau wrote:
> On 6/14/26 3:56 AM, Gowtham Kudupudi wrote:
> > On warm reboot, the PCIe controller's LTSSM starts link training
> > immediately if PERST# is already deasserted from the previous boot.
> > The driver then pulses PERST# for only 500us, which is too short to
> > properly reset the endpoint device that has already started training.
> > 
> > Fix by moving the PERST# assert/deassert pulse BEFORE enabling LTSSM,
> > so the endpoint gets a clean reset cycle before link training begins.
> > 
> > This was found on Amlogic G12B (A311D) with NVMe on an M.2 slot.
> > Cold boot worked because POR held PERST# low; warm reboot did not.
> > The fix was confirmed on a Banana Pi CM4 with Waveshare IO base board.
> > 
> > Signed-off-by: Gowtham Kudupudi <gowtham@xxxxxxxxxxxxx>
> > ---
> >  drivers/pci/controller/dwc/pci-meson.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pci-meson.c b/drivers/pci/controller/dwc/pci-meson.c
> > index 5f8e2f4b3c12..3a7e9f1d5b8c 100644
> > --- a/drivers/pci/controller/dwc/pci-meson.c
> > +++ b/drivers/pci/controller/dwc/pci-meson.c
> > @@ -310,8 +310,8 @@ static int meson_pcie_start_link(struct dw_pcie *pci)
> >  {
> >  	struct meson_pcie *mp = to_meson_pcie(pci);
> >  
> > +	meson_pcie_assert_reset(mp);
> >  	meson_pcie_ltssm_enable(mp);
> > -	meson_pcie_assert_reset(mp);

meson_pcie_assert_reset() itself is wrong as it toggles PERST#. You'd just need:

msleep(PCIE_T_PERST_CLK_US);
gpiod_set_value_cansleep(mp->reset_gpio, GPIOD_OUT_LOW)

but in probe(), after meson_pcie_probe_clocks() as that's when both power and
REFCLK becomes stable.

> >  
> >  	return 0;
> >  }
> 
> Hi Gowtham,
> 
> I have a patch [1] that I haven't submitted yet.
> This might be related to your issue, what do you think ?
> 
> [1] https://github.com/rclaveau-tech/linux-khadas/commit/bee0a02d9756
> 

This patch is also needed as it correctly fixes the PERST# polarity when
requesting the GPIO. Since the devicetree is defining the PERST# GPIO as
ACIVE_LOW, the driver should request it as GPIOD_OUT_HIGH to make sure that
PERST# is asserted.

So please submit the referenced patch.

- Mani

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