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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 14 Jun 2021 14:49:45 +0200
Message-ID: <CAKfTPtA7p1JMP9g8o_qpZ4-2u4KVNs4VENJtrV4H7EnqrmYqHw@mail.gmail.com>
Subject: Re: [PATCH V3 1/3] gpio: Add virtio-gpio driver
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Viresh Kumar <viresh.kumar@linaro.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@metux.net>,
        Viresh Kumar <vireshk@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bill Mills <bill.mills@linaro.org>,
        =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>,
        Stratos Mailing List <stratos-dev@op-lists.linaro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@redhat.com>, virtualization@lists.linux-foundation.org
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On Mon, 14 Jun 2021 at 14:33, Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Mon, Jun 14, 2021 at 12:23 PM Viresh Kumar <viresh.kumar@linaro.org> 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?
> > >
> > > Would qemu simply add the required DT properties to the device node that
> > > corresponds to the virtio device in this case?
> > >
> > > From what I can tell, both the mmio and pci variants of virtio can have their
> > > dev->of_node populated, but I don't see the logic in register_virtio_device()
> > > that looks up the of_node of the virtio_device that the of_gpio code then
> > > tries to refer to.
> >
> > To be honest, I haven't tried this yet and I was expecting it to be
> > already taken care of. I was relying on the DTB automatically
> > generated by Qemu to get the driver probed and didn't have a look at
> > it as well.
> >
> > I now understand that it won't be that straight forward. The same must
> > be true for adding an i2c device to an i2c bus over virtio (The way I
> > tested that earlier was by using the sysfs file to add a device to a
> > bus).
>
> Yes, correct, we had the same discussion about i2c.  Again, this is
> relatively straightforward when the controller and the device attached
> to it (i2c controller/client or gpio controller/function) are both emulated
> by qemu, but a lot harder when the controller and device are
> implemented in different programs.
>
> > This may be something lacking generally for virtio-pci thing, not
> > sure though.
>
> I think most importantly we need a DT binding to describe what device
> nodes are supposed to look like underneath a virtio-mmio or
> virtio-pci device in order for a hypervisor to pass down the
> information to a guest OS in a generic way. We can probably borrow
> the USB naming, and replace compatible="usbVID,PID" with
> compatible="virtioDID", with the device ID in hexadecimal digits,
> such as "virtio22" for I2C (virtio device ID 34 == 0x22) if we decide
> to have a sub-node under the device, or we just point dev->of_node
> of the virtio device to the platform/pci device that is its parent
> in Linux.
>
> Adding the Linux guest code to the virtio layer should be fairly
> straightforward, and I suppose it could be mostly copied from the
> corresponding code that added this for mmc in commit 25185f3f31c9
> ("mmc: Add SDIO function devicetree subnode parsing") and for USB
> in commit 69bec7259853 ("USB: core: let USB device know device
> node") and 1a7e3948cb9f ("USB: add device-tree support for
> interfaces").

And something similar is also done with SCMI protocols which are
defined in a SCMI node. A  typical example:

    cpu@0 {
        ...
        clocks = <&scmi_dvfs 0>;
        ...
    };

    deviceX: deviceX@YYYYYYY {
        ...
        clocks = <&scmi_clk 0>;
        ...
    };

    scmi: scmi {
        compatible = "arm,scmi-virtio";
        #address-cells = <1>;
        #size-cells = <0>;

        scmi_devpd: protocol@11 {
            reg = <0x11>;
            #power-domain-cells = <1>;
        };

        scmi_clk: protocol@14 {
            reg = <0x14>;
            #clock-cells = <1>;
        };

        scmi_sensors: protocol@15 {
            reg = <0x15>;
            #thermal-sensor-cells = <1>;
        };

        scmi_dvfs: protocol@13 {
            reg = <0x13>;
            #clock-cells = <1>;
        };
    };

>
>         Arnd