linux-kernel - Re: Query regarding the use of pcie-designware-plat.c file

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Message-ID: <2970bdf2-bef2-bdcb-6ee3-ac1181d97b78@nvidia.com>
Date:   Wed, 9 Jun 2021 17:54:38 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>, Vidya Sagar <vidyas@...dia.com>
CC:     <lorenzo.pieralisi@....com>, <bhelgaas@...gle.com>,
        <robh+dt@...nel.org>, <amurray@...goodpenguin.co.uk>,
        <gustavo.pimentel@...opsys.com>, <jingoohan1@...il.com>,
        <Joao.Pinto@...opsys.com>, Thierry Reding <treding@...dia.com>,
        Krishna Thota <kthota@...dia.com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: Query regarding the use of pcie-designware-plat.c file


On 09/06/2021 17:30, Bjorn Helgaas wrote:
> On Wed, Jun 09, 2021 at 12:52:37AM +0530, Vidya Sagar wrote:
>> Hi,
>> I would like to know what is the use of pcie-designware-plat.c file. This
>> looks like a skeleton file and can't really work with any specific hardware
>> as such.
>> Some context for this mail thread is, if the config CONFIG_PCIE_DW_PLAT is
>> enabled in a system where a Synopsys DesignWare IP based PCIe controller is
>> present and its configuration is enabled (Ex:- Tegra194 system with
>> CONFIG_PCIE_TEGRA194_HOST enabled), then, it can so happen that the probe of
>> pcie-designware-plat.c called first (because all DWC based PCIe controller
>> nodes have "snps,dw-pcie" compatibility string) and can crash the system.
> 
> What's the crash?  If a device claims to be compatible with
> "snps,dw-pcie" and pcie-designware-plat.c claims to know how to
> operate "snps,dw-pcie" devices, it seems like something is wrong.
> 
> "snps,dw-pcie" is a generic device type, so pcie-designware-plat.c
> might not know how to operate device-specific details of some of those
> devices, but basic functionality should work and it certainly
> shouldn't crash.

It is not really a crash but a hang when trying to access the hardware
before it has been properly initialised.

The scenario I saw was that if the Tegra194 PCIe driver was built as a
module but the pcie-designware-plat.c was built into the kernel, then on
boot we would attempt to probe the pcie-designware-plat.c driver because
module was not available yet and this would hang. Hence, I removed the
"snps,dw-pcie" compatible string for Tegra194 to avoid this and ONLY
probe the Tegra194 PCIe driver.

Sagar is wondering why this hang is only seen/reported for Tegra and
could this happen to other platforms? I think that is potentially could.

Jon

-- 
nvpublic