linux-kernel - How long should be PCIe card in Warm Reset state?

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Date:   Wed, 10 Mar 2021 12:05:35 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Amey Narkhede <ameynarkhede02@...il.com>,
        Marek Behún <kabel@...nel.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: How long should be PCIe card in Warm Reset state?

Hello!

I would like to open a question about PCIe Warm Reset. Warm Reset of
PCIe card is triggered by asserting PERST# signal and in most cases
PERST# signal is controlled by GPIO.

Basically every native Linux PCIe controller driver is doing this Warm
Reset of connected PCIe card during native driver initialization
procedure.

And now the important question is: How long should be PCIe card in Warm
Reset state? After which timeout can be PERST# signal de-asserted by
Linux controller driver?

Lorenzo and Rob already expressed concerns [1] [2] that this Warm Reset
timeout should not be driver specific and I agree with them.

I have done investigation which timeout is using which native PCIe
driver [3] and basically every driver is using different timeout.

I have tried to find timeouts in PCIe specifications, I was not able to
understand and deduce correct timeout value for Warm Reset from PCIe
specifications. What I have found is written in my email [4].

Alex (as a "reset expert"), could you look at this issue?

Or is there somebody else who understand PCIe specifications and PCIe
diagrams to figure out what is the minimal timeout for de-asserting
PERST# signal?

There are still some issues with WiFi cards (e.g. Compex one) which
sometimes do not appear on PCIe bus. And based on these "reset timeout
differences" in Linux PCIe controller drivers, I suspect that it is not
(only) the problems in WiFi cards but also in Linux PCIe controller
drivers. In my email [3] I have written that I figured out that WLE1216
card needs to be in Warm Reset state for at least 10ms, otherwise card
is not detected.

[1] - https://lore.kernel.org/linux-pci/20200513115940.fiemtnxfqcyqo6ik@pali/
[2] - https://lore.kernel.org/linux-pci/20200507212002.GA32182@bogus/
[3] - https://lore.kernel.org/linux-pci/20200424092546.25p3hdtkehohe3xw@pali/
[4] - https://lore.kernel.org/linux-pci/20200430082245.xblvb7xeamm4e336@pali/