linux-kernel - Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

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Date:   Mon, 14 Jun 2021 15:24:37 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "Stefano Garzarella --cc virtualization @ lists . linux-foundation . org" 
        <sgarzare@...hat.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Stratos Mailing List <stratos-dev@...lists.linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

On Mon, 14 Jun 2021 at 15:00, Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Mon, Jun 14, 2021 at 2:50 PM Vincent Guittot via Stratos-dev
> <stratos-dev@...lists.linaro.org> wrote:>
> > On Mon, 14 Jun 2021 at 14:33, Arnd Bergmann <arnd@...nel.org> wrote:
> > > On Mon, Jun 14, 2021 at 12:23 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > 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@...YYYY {
> >         ...
> >         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>;
> >         };
> >     };
>
> But this example seem to be completely different from the ones I mentioned:
> The scmi node that you have here looks like it shows up under the root of the
> device tree, not below the virtio device that implements the scmi transport.

I was thinking of something like below:

    deviceX: deviceX@...YYYY {
        ...
        gpio = <&virtio_gpio 0>;
        ...
    };

    virtio_mmio@...0000 {
        dma-coherent;
        interrupts = <0x0 0x10 0x1>;
        reg = <0x0 0xa000000 0x0 0x200>;
        compatible = "virtio,mmio";

        virtio_gpio: protocol@22 {
            reg = <0x22>;
        };
    };

>
>          Arnd