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

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Message-ID: <b20bae1d-cd6b-864b-8357-0da365c8406f@lucaceresoli.net>
Date:   Tue, 22 Jun 2021 23:04:33 +0200
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Pali Rohár <pali@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     linus.walleij@...aro.org, 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 Kishon,

On 22/06/21 15:57, Kishon Vijay Abraham I wrote:
> Hi Luca, Pali,
> 
> On 22/06/21 7:01 pm, Luca Ceresoli wrote:
>> 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.
>>
> 
> While the patch itself is correct, this kind-of changes the behavior on
> already upstreamed platforms. Previously the driver expected #PERST to
> be asserted be external means (or default power-up state) and only takes
> care of de-asserting the #PERST line.
> 
> There are 2 platforms that will be impacted due to this change
> 1) arch/arm/boot/dts/am57xx-beagle-x15-common.dtsi (has an inverter on
> GPIO line)
> 2) arch/arm/boot/dts/am571x-idk.dts (directly connected to #PERST)
> For 1), gpiod_set_value(reset, 0) will assert the PERST line due to the
> inverter (and GPIO_ACTIVE_LOW)
> For 2), gpiod_set_value(reset, 0) will assert the PERST line because we
> have GPIO_ACTIVE_HIGH
> 
> So this patch should have to be accompanied with DT changes (and this
> patch also breaks old DT compatibility).

Thanks for researching and reporting which platforms are affected. I can
certainly take care of changing these two DTs in the next patch
iteration but I have no way to test them: I have access to an X15 but
without any expansion boards, and no IDK.

-- 
Luca