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

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Message-ID: <4d4c0d4d-41b4-4756-5189-bffa15f88406@ti.com>
Date:   Tue, 22 Jun 2021 19:27:37 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Luca Ceresoli <luca@...aceresoli.net>,
        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 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
Kishon