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

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Message-ID: <ZFwaA403wLnDg3TI@bhelgaas>
Date:   Wed, 10 May 2023 17:26:11 -0500
From:   Bjorn Helgaas <helgaas@...nel.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>,
        Cyril Brulebois <kibi@...ian.org>,
        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

Re subject, what does the patch actually *do*?

It looks like it "asserts PERST# on BCM2711", which I think would be
more informative than "don't assume 2711 bootloader leaves PERST#
asserted".

On Mon, May 08, 2023 at 06:01:24PM -0400, Jim Quinlan wrote:
> 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

Does this link go with something above?  "[1]" isn't mentioned above.
I did look at that message, but the connection to this patch isn't
obvious to me.

> Signed-off-by: Jim Quinlan <jim2101024@...il.com>
> ---
>  drivers/pci/controller/pcie-brcmstb.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
> index fe0415a98c63..7b698a9a851e 100644
> --- a/drivers/pci/controller/pcie-brcmstb.c
> +++ b/drivers/pci/controller/pcie-brcmstb.c
> @@ -884,6 +884,11 @@ static int brcm_pcie_setup(struct brcm_pcie *pcie)
>  
>  	/* Reset the bridge */
>  	pcie->bridge_sw_init_set(pcie, 1);
> +
> +	/* Ensure that PERST# is asserted; some bootloaders may deassert it. */
> +	if (pcie->type == BCM2711)
> +		pcie->perst_set(pcie, 1);
> +
>  	usleep_range(100, 200);
>  
>  	/* Take the bridge out of reset */
> -- 
> 2.17.1
>