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

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

[Gowtham Kudupudi]

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@ferryfair.com>
---
 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);
 
 	return 0;
 }
-- 
2.49.0

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

[Ronald Claveau]

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@ferryfair.com>
> ---
>  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);
>  
>  	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

-- 
Best regards,
Ronald

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

[neil.armstrong]

On 6/15/26 12:34, 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@ferryfair.com>
>> ---
>>   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);

I think this change is valid, other controllers resets PERST
in the host init callback, so either this or move to the
init callback.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>

>>   
>>   	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 ?

Ronald, This fix is valid, it's definitely better to probe the
driver with PERST asserted, please send it.

Neil

> 
> [1] https://github.com/rclaveau-tech/linux-khadas/commit/bee0a02d9756
> 

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

[gowtham]

Hi,

As Neil suggested I moved the PCIe reset to init; I've updated the tag
at
https://github.com/GowthamKudupudi/linux/tree/meson-pcie-warm-reset-linux-7.0.y

Ronald, yes, I do think its proper to initialize PCIe with RESET Active.
Anyway we need the reset *cycle*. Please submit your patch as well.

Thank you!

...Gowtham

On Tue, 16 Jun 2026 08:06:02 +0200
neil.armstrong@linaro.org wrote:

> On 6/15/26 12:34, 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@ferryfair.com>
> >> ---
> >>   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);  
> 
> I think this change is valid, other controllers resets PERST
> in the host init callback, so either this or move to the
> init callback.
> 
> Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
> 
> >>   
> >>   	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 ?  
> 
> Ronald, This fix is valid, it's definitely better to probe the
> driver with PERST asserted, please send it.
> 
> Neil
> 
> > 
> > [1]
> > https://github.com/rclaveau-tech/linux-khadas/commit/bee0a02d9756 
>