linux-kernel - Re: [PATCH v14 05/11] PCI: kirin: give more time for PERST# reset to finish

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Message-ID: <20211023145552.3ly4kukfyrark7c2@pali>
Date:   Sat, 23 Oct 2021 16:55:52 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>, linuxarm@...wei.com,
        mauro.chehab@...wei.com,
        Krzysztof Wilczyński <kw@...ux.com>,
        Songxiaowei <songxiaowei@...ilicon.com>,
        Binghui Wang <wangbinghui@...ilicon.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh@...nel.org>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v14 05/11] PCI: kirin: give more time for PERST# reset to
 finish

On Saturday 23 October 2021 14:45:34 Mauro Carvalho Chehab wrote:
> Em Sat, 23 Oct 2021 12:40:11 +0200
> Pali Rohár <pali@...nel.org> escreveu:
> > It is classic setup for boards with just one PCIe port.
> > 
> > > with 3 elements connected to the bus: an Ethernet card, a M.2 slot and
> > > a mini PCIe slot. It seems HiKey 970 is unique with regards to PERST# signal,
> > > as there are 4 independent PERST# signals there:
> > > 
> > > 	- one for PEX 8606 (the PCIe root port);
> > > 	- one for Ethernet;
> > > 	- one for M.2;
> > > 	- one for mini-PCIe.  
> > 
> > This is not unique setup, its pretty normal. Every PCIe card has (own)
> > PERST# pin and obviously you want to control each pin separately via SW.
> > And because PCIe switch is also (upstream) PCIe device it has also
> > PERST# pin.
> 
> Based on the discussions we had to add per-port DT PERST# gpios, it
> sounded to me that this is was not a typical setup ;-)
> 
> It seems that the typical setup is to have a single PERST# connected
> to all devices inside the bus.

Hello!

I'm sure it is not unique :) Just seems that these boards either do not
use device tree (x86-based) or do not have specified reset-gpios in DTS
at all.

Sometimes there is no need to touch PERST# gpio as either firmware
during boot handles it (x86 BIOS/UEFI case, or U-Boot for arm case) or
because board/cpu reset toggle PERST# in a way that is compatible for
cards init.

Looks like you have non-x86 board, which does not have PCIe init code in
firmware, needs special handling of PERST# and you are doing it with
upstream kernel :-) So maybe all these conditions are unique... But HW
design not.

As this setup with reset-gpios per card in DTS nodes is something which
I will need too, I sent email to Rob with proposal how to universally
declare it in DTS, independently of PCIe controller (you are on CC):
https://lore.kernel.org/linux-pci/20211023144252.z7ou2l2tvm6cvtf7@pali/

Due to how PCIe cards are broken, PERST# signal is sometimes the only
one option how to reset card at runtime. So requirement for separate
PERST# per card configurable at runtime by OS will be requirement for
more and more boards.