linux-kernel - Re: [PATCH v5 4/5] PCI: brcmstb: Don't assume 2711 bootloader leaves PERST# asserted

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Date:   Tue, 9 May 2023 09:51:13 +0200
From:   Cyril Brulebois <kibi@...ian.org>
To:     Jim Quinlan <jim2101024@...il.com>
Cc:     linux-pci@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com, james.quinlan@...adcom.com,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/5] PCI: brcmstb: Don't assume 2711 bootloader leaves
 PERST# asserted

Hi,

Jim Quinlan <jim2101024@...il.com> (2023-05-08):
> The current PCIe driver assumes PERST# is asserted when probe() is invoked.
> The reasons are as follows:
> 
> (1) One Broadcom SOC (7278) causes a panic if the PERST# register is
>     written during this time window.
> 
> (2) If PERST# is deasserted at Linux probe() time, experience and QA
>     suspend/resume tests have shown that some endpoint devices fail if the
>     PERST# is pulsed (deasserted => asserted => deasserted) quickly in this
>     fashion, even though the timing is in accordance with their datasheets.
> 
> (3) Keeping things in reset tends to save power, if for some reason the
>     PCIe driver is not yet present.
> 
> Broadcom STB and CM SOCs bootloaders always have PERST# asserted at
> probe().  This is not necessarily the case for the 2711/RPi bootloader,
> so, for 2711/RPi SOCs, do what Raspian OS does and assert PERST#.
> 
> [1] https://lore.kernel.org/linux-pci/20230411165919.23955-1-jim2101024@gmail.com/T/#m39ebab8bc2827b2304aeeff470a6c6a58f46f987

It would probably make sense to remove that [1] link entirely, to match
the reference removal between v4 and v5.


Cheers,
-- 
Cyril Brulebois (kibi@...ian.org)            <https://debamax.com/>
D-I release manager -- Release team member -- Freelance Consultant

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