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

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Date:   Thu, 10 Jun 2021 19:03:33 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Viresh Kumar <vireshk@...nel.org>,
        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>,
        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 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?

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).

Thanks,
Jean

[1] Documentation/devicetree/bindings/virtio/iommu.txt

> 
> I've made a few attempts, but nothing practically useful, which would be
> accepted by the corresponding maintainers, yet. We'd either need some
> very special logic in DT probing or pseudo-bus'es for the mapping.
> (DT wants to do those connections via phandle's, which in turn need the
> referenced nodes to be present in the DT).
> 
> >  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.
> 
> Have you ever successfully bound a virtio device via DT ?
> 
> 
> --mtx
> 
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> Enrico Weigelt, metux IT consult
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