Message-ID: <CAJZ5v0gecxs3fMi7tHz9bLBEjTRqv_Fh0iTC4BM30C8zvRMRog@mail.gmail.com>
Date: Tue, 15 Oct 2024 15:37:38 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Greg KH <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, linuxarm@...wei.com, 
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Subject: Re: [PATCH v13 12/18] platform: Add __free() based cleanup function
 for platform_device_put

On Tue, Oct 15, 2024 at 3:34 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> On Tue, 15 Oct 2024 12:17:25 +0200
> Greg KH <gregkh@...uxfoundation.org> wrote:
>
> > On Tue, Oct 15, 2024 at 10:40:54AM +0100, Jonathan Cameron wrote:
> > > On Tue, 15 Oct 2024 10:10:25 +0100
> > > Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> > >
> > > > On Mon, 14 Oct 2024 20:06:40 +0200
> > > > "Rafael J. Wysocki" <rafael@...nel.org> wrote:
> > > >
> > > > > On Mon, Oct 14, 2024 at 7:17 PM Jonathan Cameron
> > > > > <Jonathan.Cameron@...wei.com> wrote:
> > > > > >
> > > > > > On Mon, 14 Oct 2024 18:04:37 +0200
> > > > > > Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > > >
> > > > > > > On Mon, Oct 14, 2024 at 06:00:51PM +0200, Greg KH wrote:
> > > > > > > > On Mon, Oct 14, 2024 at 04:43:39PM +0100, Jonathan Cameron wrote:
> > > > > > > > > On Wed, 9 Oct 2024 13:41:13 +0100
> > > > > > > > > <shiju.jose@...wei.com> wrote:
> > > > > > > > >
> > > > > > > > > > From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > > > > > > > > >
> > > > > > > > > > Add __free() based cleanup function for platform_device_put().
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > > > > > > > > > Signed-off-by: Shiju Jose <shiju.jose@...wei.com>
> > > > > > > > > > ---
> > > > > > > > > >  include/linux/platform_device.h | 1 +
> > > > > > > > > >  1 file changed, 1 insertion(+)
> > > > > > > > > >
> > > > > > > > > > diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
> > > > > > > > > > index d422db6eec63..606533b88f44 100644
> > > > > > > > > > --- a/include/linux/platform_device.h
> > > > > > > > > > +++ b/include/linux/platform_device.h
> > > > > > > > > > @@ -232,6 +232,7 @@ extern int platform_device_add_data(struct platform_device *pdev,
> > > > > > > > > >  extern int platform_device_add(struct platform_device *pdev);
> > > > > > > > > >  extern void platform_device_del(struct platform_device *pdev);
> > > > > > > > > >  extern void platform_device_put(struct platform_device *pdev);
> > > > > > > > > > +DEFINE_FREE(platform_device_put, struct platform_device *, if (_T) platform_device_put(_T))
> > > > > > > > > >
> > > > > > > > > >  struct platform_driver {
> > > > > > > > > >         int (*probe)(struct platform_device *);
> > > > > > > > >
> > > > > > > > > +CC Greg KH and Rafael.
> > > > > > > > >
> > > > > > > > > Makes sure to include them on v14 as this needs review from a driver core point
> > > > > > > > > of view I think.
> > > > > > > >
> > > > > > > > Why is this needed for a platform device?  This feels like you will have
> > > > > > > > to do more work to "keep" the reference on the normal path than you to
> > > > > > > > today to release the reference on the error path, right?  Have a pointer
> > > > > > > > to a patch that uses this?
> > > > > > >
> > > > > > > Ah, is it this one:
> > > > > > >       https://lore.kernel.org/all/20241014164955.00003439@Huawei.com/
> > > > > > > ?
> > > > > > >
> > > > > > > If so, no, that's an abuse of a platform device, don't do that, make a
> > > > > > > REAL device on the bus that this device lives on.  If it doesn't live on
> > > > > > > a real bus, then put it on the virtual bus but do NOT abuse the platform
> > > > > > > device layer for something like this.
> > > > > >
> > > > > > Ok.  Probably virtual bus it is then.  Rafael, what do you think makes sense
> > > > > > for a 'feature' that is described only by an ACPI table (here RAS2)?
> > > > > > Kind of similar(ish) to say IORT.
> > > > >
> > > > > Good question.
> > > > >
> > > > > I guess it depends on whether or not there are any registers to access
> > > > > or AML to interact with.  If so, I think that a platform device makes
> > > > > sense.
> > > >
> > > > Unfortunately still a gray area I think.
> > > >
> > > > This does access mailbox memory addresses, but they are provided
> > > > by an existing platform device, so maybe platform device for this
> > > > device is still inappropriate :(
> > > >
> > > > What this uses is:
> > > > PCC channel (mailbox in memory + doorbells, etc but that is indirectly
> > > > provided as a service via reference in ACPI to the PCCT table entry
> > > > allowing this to find the mailbox device - which is a platform
> > > > device drivers/mailbox/pcc.c).
> > > > Because it's all spec defined content in the mailbox messages, we don't
> > > > have the more flexible (and newer I think) 'register' via operation region
> > > > stuff in AML.
> > > >
> > > > A wrinkle though.  The mailbox data is mapped into this driver via
> > > > an acpi_os_ioremap() call.
> > > >
> > > > So I'm thinking we don't have a strong reason for a platform device
> > > > other than 'similarity' to other examples.  Never the strongest reason!
> > > >
> > > > We'll explore alternatives and see what they end up looking like.
> > > >
> > > > Jonathan
> > > >
> > >
> > > Greg,
> > >
> > > I'm struggling a little to figure out how you envision the virtual bus
> > > working here.  So before we spend too much time implementing the wrong thing
> > > as it feels non trivial, let me check my understanding.
> > >
> > > Would this mean registering a ras2 bus via subsys_virtual_register().
> > > (Similar to done for memory tiers)
> >
> > It should show up under /sys/devices/virtual/ is what I mean.
> >
> > > On that we'd then add all the devices: one per RAS2 PCC descriptor (these
> > > are one per independent feature). Each feature has its own mailbox sub
> > > channel (via a reference to the PCC mailbox devices .
> > > Typically you have one of these per feature type per numa node, but
> > > that isn't guaranteed.  That will then need wiring up with bus->probe() etc
> > > so that the RAS2 edac feature drivers can find this later and bind to it to
> > > register with edac etc.
> > >
> > > So spinning up a full new bus, to support this?  I'm not against that.
> >
> > No, again, see how the stuff that shows up in /sys/devices/virtual
> > works, that should be much simpler.
> >
> > But really, as this is a "bus", just make a new one.  I don't understand
> > why ACPI isn't creating your devices for you, as this is ACPI code,
> > perhaps just fix that up instead?
>
> Filling in that registration gap was the purpose of the next patch.
>
> In all the similar cases that I can find where a static ACPI table is parsed
> (maybe Rafael can point at another) one of two things happen:
> 1) The table is parsed and platform devices are created for the entries found
>   (IORT etc) to which drivers later bind.
> 2) There is a 'fake' device in DSDT. E.g. ACPI0012 for NFIT, ACPI0017 for CEDT
> (the CXL table) that are there to trigger enumeration of the static table and
> those are done via device on the acpi bus.  That allows late enumeration and
> registration of devices with the appropriate classes etc.
>
> We'll explore the options but right now I'm thinking a new bus/acpi_ras2 is probably
> the way to go.

Have you looked at auxiliary devices?

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