linux-kernel - Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

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Date:   Tue, 22 Jun 2021 15:31:12 +0200
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     Pali Rohár <pali@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     linus.walleij@...aro.org, kishon@...com, linux-pci@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

Hi,

On 22/06/21 14:16, Pali Rohár wrote:
> On Tuesday 22 June 2021 12:56:04 Lorenzo Pieralisi wrote:
>> [Adding Linus for GPIO discussion, thread:
>> https://lore.kernel.org/linux-pci/20210531090540.2663171-1-luca@lucaceresoli.net]
>>
>> On Tue, Jun 22, 2021 at 01:06:27PM +0200, Pali Rohár wrote:
>>> Hello!
>>>
>>> On Tuesday 22 June 2021 12:57:22 Luca Ceresoli wrote:
>>>> Nothing happened after a few weeks... I understand that knowing the
>>>> correct reset timings is relevant, but unfortunately I cannot help much
>>>> in finding out the correct values.
>>>>
>>>> However I'm wondering what should happen to this patch. It *does* fix a
>>>> real bug, but potentially with an incorrect or non-optimal usleep range.
>>>> Do we really want to ignore a bugfix because we are not sure about how
>>>> long this delay should be?
>>>
>>> As there is no better solution right now, I'm fine with your patch. But
>>> patch needs to be approved by Lorenzo, so please wait for his final
>>> answer.
>>
>> I am not a GPIO expert and I have a feeling this is platform specific
>> beyond what the PCI specification can actually define architecturally.
> 
> In my opinion timeout is not platform specific as I wrote in email:
> https://lore.kernel.org/linux-pci/20210310110535.zh4pnn4vpmvzwl5q@pali/
> 
> My experiments already proved that some PCIe cards needs to be in reset
> state for some minimal time otherwise they cannot be enumerated. And it
> does not matter to which platform you connect those (endpoint) cards.
> 
> I do not think that timeout itself is platform specific. GPIO controls
> PERST# pin and therefore specified sleep value directly drives how long
> is card on the other end of PCIe slot in Warm Reset state. PCIe CEM spec
> directly says that PERST# signal controls PCIe Warm Reset.
> 
> What is here platform specific thing is that PERST# signal is controlled
> by GPIO. But value of signal (high / low) and how long is in signal in
> which state for me sounds like not an platform specific thing, but as
> PCIe / CEM related.

That's exactly my understanding of this matter. At least for the dra7xx
controller it works exactly like this, PERSTn# is nothing but a GPIO
output from the SoC that drives the PERSTn# input of the external chip
without affecting the controller directly.

-- 
Luca