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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 27 May 2020 11:53:48 +0200
Message-ID: <CAK8P3a35fjXt1F2hJygup5gWfjPHZTuU+VD69K5uzrNhhgu0Pw@mail.gmail.com>
Subject: Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Zhangfei Gao <zhangfei.gao@linaro.org>,
        Joerg Roedel <joro@8bytes.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Hanjun Guo <guohanjun@huawei.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>,
        jean-philippe <jean-philippe@linaro.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        kenneth-lee-2012@foxmail.com, Wangzhou <wangzhou1@hisilicon.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>,
        "open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-pci <linux-pci@vger.kernel.org>
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On Wed, May 27, 2020 at 11:00 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Tue, May 26, 2020 at 07:49:07PM +0800, Zhangfei Gao wrote:
> > Some platform devices appear as PCI but are actually on the AMBA bus,
>
> Why would these devices not just show up on the AMBA bus and use all of
> that logic instead of being a PCI device and having to go through odd
> fixes like this?

There is a general move to having hardware be discoverable even with
ARM processors. Having on-chip devices be discoverable using PCI config
space is how x86 SoCs usually do it, and that is generally a good thing
as it means we don't need to describe them in DT

I guess as the hardware designers are still learning about it, this is not
always done correctly. In general, we can also describe PCI devices on
DT and do fixups during the probing there, but I suspect that won't work
as easily using ACPI probing, so the fixup is keyed off the hardware ID,
again as is common for x86 on-chip devices.

      Arnd