linux-kernel - Re: [PATCH v3 1/2] ACPI: IPMI: Add helper to wait for when SMI is selected

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Date: Thu, 4 Jan 2024 18:11:29 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: minyard@....org
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Kai-Heng Feng <kai.heng.feng@...onical.com>, jdelvare@...e.com, 
	linux@...ck-us.net, Len Brown <lenb@...nel.org>, Robert Moore <robert.moore@...el.com>, 
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	acpica-devel@...ts.linux.dev
Subject: Re: [PATCH v3 1/2] ACPI: IPMI: Add helper to wait for when SMI is selected

On Thu, Jan 4, 2024 at 4:42 PM Corey Minyard <minyard@....org> wrote:
>
> On Thu, Jan 04, 2024 at 02:34:52PM +0100, Rafael J. Wysocki wrote:
> > On Thu, Jan 4, 2024 at 3:48 AM Kai-Heng Feng
> > <kai.heng.feng@...onical.com> wrote:
> > >
> > > The function of acpi_power_meter module on Dell system requires IPMI
> > > handler is installed and SMI is selected.
> >
> > Does the firmware use _DEP to let the OS know about this dependency?
> >
> > > So add a helper to let acpi_power_meter know when IPMI handler and SMI
> > > are ready.
> > >
> > > Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> > > ---
> > > v3:
> > >  - New patch.
> > >
> > >  drivers/acpi/acpi_ipmi.c | 17 ++++++++++++++++-
> > >  include/acpi/acpi_bus.h  |  5 +++++
> > >  2 files changed, 21 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/acpi/acpi_ipmi.c b/drivers/acpi/acpi_ipmi.c
> > > index 0555f68c2dfd..54862cab7171 100644
> > > --- a/drivers/acpi/acpi_ipmi.c
> > > +++ b/drivers/acpi/acpi_ipmi.c
> > > @@ -23,6 +23,8 @@ MODULE_LICENSE("GPL");
> > >  #define IPMI_TIMEOUT                   (5000)
> > >  #define ACPI_IPMI_MAX_MSG_LENGTH       64
> > >
> > > +static struct completion smi_selected;
> > > +
> > >  struct acpi_ipmi_device {
> > >         /* the device list attached to driver_data.ipmi_devices */
> > >         struct list_head head;
> > > @@ -463,8 +465,10 @@ static void ipmi_register_bmc(int iface, struct device *dev)
> > >                 if (temp->handle == handle)
> > >                         goto err_lock;
> > >         }
> > > -       if (!driver_data.selected_smi)
> > > +       if (!driver_data.selected_smi) {
> > >                 driver_data.selected_smi = ipmi_device;
> > > +               complete(&smi_selected);
> > > +       }
> > >         list_add_tail(&ipmi_device->head, &driver_data.ipmi_devices);
> > >         mutex_unlock(&driver_data.ipmi_lock);
> > >
> > > @@ -578,10 +582,21 @@ acpi_ipmi_space_handler(u32 function, acpi_physical_address address,
> > >         return status;
> > >  }
> > >
> > > +int acpi_wait_for_acpi_ipmi(void)
> > > +{
> > > +       long ret;
> > > +
> > > +       ret = wait_for_completion_interruptible_timeout(&smi_selected, 2 * HZ);
> > > +
> > > +       return ret > 0 ? 0 : -ETIMEDOUT;
> >
> > What will happen if the IPMI driver is unloaded after this has returned 0?
>
> The IPMI driver can't be unloaded if it has a user.

Because of the use of the exported symbol, right?

> I've been following this, but I know little about ACPI.  Beyond this
> solution, the only other solution I could come up with was to start the
> IPMI driver earlier.  But then you are in a chicken-and-egg situation
> (https://dictionary.cambridge.org/dictionary/english/chicken-and-egg-situation).
> Which was the reason for the SPMI table, but that's really kind of
> useless for this, even if the SPMI table existed.

Fine.

Let me reply to the patch with some more comments then.