linux-kernel - Re: [PATCH] mfd: Add Simple PCI MFD driver

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Date:   Mon, 23 Jan 2023 17:02:35 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Lee Jones <lee@...nel.org>
CC:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] mfd: Add Simple PCI MFD driver

On Mon, Jan 23, 2023 at 04:32:55PM +0100, Lee Jones wrote:
> On Mon, 23 Jan 2023, Vincent Whitchurch wrote:
> > Add a PCI driver which registers all child nodes specified in the
> > devicetree.  It will allow platform devices to be used on virtual
> > systems which already support PCI and devicetree, such as UML with
> > virt-pci.
> > 
> > The driver has no id_table by default; user space needs to provide one
> > using the new_id mechanism in sysfs.
> 
> This feels wrong for several reasons.
> 
> Firstly, I think Greg (Cc:ed) will have something to say about this.
> 
> Secondly, this driver does literally nothing.  

Well, it does do what the commit message says.  If there's another way
of accomplishing that, I'm all ears.

> Why can't you use of of the other, pre-existing "also register my
> children" compatibles?
> 
> See: drivers/bus/simple-pm-bus.c
>      drivers/of/platform.c

simple-pm-bus registers a platform driver, and drivers/of/platform.c
works on the platform bus.  The driver added by this patch is a PCI
driver.  So I don't understand how the files you mention could be used
here?

In case it helps, the relevant nodes in my UML devicetree look something
like this:

    virtio@2 {
        compatible = "virtio,uml";
        virtio-device-id = <1234>;
        ranges;

        pci {
                #address-cells = <3>;
                #size-cells = <2>;
                ranges = <0x0000000 0 0 0 0xf0000000 0 0x20000>;
                compatible = "virtio,device4d2", "pci";
                device_type = "pci";
                bus-range = <0 0>;

                platform_parent: device@0,0 {
                        compatible = "pci494f,dc8";
                        reg = <0x00000 0 0 0x0 0x10000>;
                        ranges;

        		uart@...00 {
        		        compatible = "google,goldfish-tty";
        		        reg = <0x00000 0 0x10000 0 0x10000>;
        		};
                };
        };
    };