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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Thu, 10 Jun 2021 21:41:01 +0200
Message-ID: <CAK8P3a1E7WzhX4NvUPn6iDzsUqTzAEMcsow7feTGRWTLe9+FFQ@mail.gmail.com>
Subject: Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@metux.net>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Viresh Kumar <vireshk@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        virtualization@lists.linux-foundation.org,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@redhat.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Stratos Mailing List <stratos-dev@op-lists.linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@metux.net>,
        Jason Wang <jasowang@redhat.com>
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Precedence: bulk

On Thu, Jun 10, 2021 at 7:03 PM Jean-Philippe Brucker
<jean-philippe@linaro.org> wrote:
>
> On Thu, Jun 10, 2021 at 04:00:39PM +0000, Enrico Weigelt, metux IT consult via Stratos-dev wrote:
> > On 10.06.21 15:22, Arnd Bergmann wrote:
> >
> > > Can you give an example of how this would be hooked up to other drivers
> > > using those gpios. Can you give an example of how using the "gpio-keys" or
> > > "gpio-leds" drivers in combination with virtio-gpio looks like in the DT?
> >
> > Connecting between self-probing bus'es and DT is generally tricky. IMHO
> > we don't have any generic mechanism for that.
>
> DT does have a generic description of PCI endpoints, which virtio-iommu
> relies on to express the relation between IOMMU and endpoint nodes [1].
> I think the problem here is similar: the client node needs a phandle to
> the GPIO controller which may use virtio-pci transport?

Right, the code to set dev->of_node is fairly simple, the device probe
just needs to scan for child nodes. Aside from PCI, similar code exists
for USB and MMC/SDIO, which are usually discoverable but sometimes
need additional properties.

> Note that it mostly works if the device is on the root PCI bus. Behind a
> bridge the OS may change the device's bus number as needed, so the BDF
> reference in DT is only valid if the software providing the DT description
> (VMM or firmware) initializes bus numbers accordingly (and I don't
> remember if Linux supports this case well).

I think you can mark the host bridge as "probe-only" to prevent the OS
(at least Linux) from renumbering the buses.

The part I did not find though is assigning dev->of_node in the virtio_device
to a child of the PCI device node.

      Arnd