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

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Date:   Tue, 22 Jun 2021 23:19:01 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Luca Ceresoli <luca@...aceresoli.net>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        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

On Tuesday 22 June 2021 23:08:07 Luca Ceresoli wrote:
> On 22/06/21 22:52, Pali Rohár wrote:
> > On Tuesday 22 June 2021 19:27:37 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
> > 
> > Ou! This is a problem in DT. It needs to be defined in a way that state
> > is same for every DTS device which uses this driver.
> 
> Why?

I'm starting to be confused by triple or more negations (asserting,
signal inverter, active low)...

In your patch is GPIO set value to 0 and Kishon wrote that GPIO set
value to 0 for those two boards assert PERST# line. Asserting PERST#
line cause endpoint PCIe card to be in reset state. And in pci-dra7xx.c
driver there is no other code which de-asserts PERST# line.

So based on all this information I deduced that your patch will cause
putting PCIe cards into reset state (forever) and therefore they would
not work.

Or do I have here some mistake?

> These are different boards and each specifies its own polarity.
> They are already opposite to each other right now:
> 
> https://elixir.bootlin.com/linux/v5.13-rc7/source/arch/arm/boot/dts/am57xx-beagle-x15-common.dtsi#L602
> 
> https://elixir.bootlin.com/linux/v5.13-rc7/source/arch/arm/boot/dts/am571x-idk.dts#L196
> 
> -- 
> Luca
>