linux-kernel - Re: [PATCH 10/11] acpi: Export acpi_bus

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Date:	Mon, 18 Jan 2016 19:26:01 +0100
From:	Lukas Wunner <lukas@...ner.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Ankitprasad Sharma <ankitprasad.r.sharma@...el.com>,
	intel-gfx@...ts.freedesktop.org, akash.goel@...el.com,
	shashidhar.hiremath@...el.com, tvrtko.ursulin@...ux.intel.com,
	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/11] acpi: Export acpi_bus_type

Hi,

On Mon, Jan 18, 2016 at 03:57:29PM +0100, Rafael J. Wysocki wrote:
> On Monday, January 18, 2016 02:31:00 PM Ankitprasad Sharma wrote:
> > On Fri, 2016-01-15 at 15:51 +0100, Rafael J. Wysocki wrote:
> > > On Thursday, January 14, 2016 11:46:46 AM ankitprasad.r.sharma@...el.com wrote:
> > > > From: Ankitprasad Sharma <ankitprasad.r.sharma@...el.com>
> > > > 
> > > > Some modules, like i915.ko, needs to detect when certain ACPI features
> > > > are active inorder to prevent corruption on contended resources.
> > > > In particular, use of BIOS RapidStart Technology may corrupt the contents
> > > > of the reserved graphics memory, due to unalarmed hibernation. In which
> > > > case i915.ko cannot assume that it (reserved gfx memory) remains
> > > > unmodified and must recreate teh contents and importantly not use it to
> > > > store unrecoverable user data.
> > > > 
> > > > Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@...el.com>
> > > > Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> > > > Cc: Len Brown <lenb@...nel.org>
> > > > Cc: linux-acpi@...r.kernel.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > ---
> > > >  drivers/acpi/bus.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> > > > index a212cef..69509c7 100644
> > > > --- a/drivers/acpi/bus.c
> > > > +++ b/drivers/acpi/bus.c
> > > > @@ -814,6 +814,7 @@ struct bus_type acpi_bus_type = {
> > > >  	.remove		= acpi_device_remove,
> > > >  	.uevent		= acpi_device_uevent,
> > > >  };
> > > > +EXPORT_SYMBOL_GPL(acpi_bus_type);
> > > >  
> > > >  /* --------------------------------------------------------------------------
> > > >                               Initialization/Cleanup
> > > > 
> > > 
> > > No.
> > > 
> > > I see no reason whatsoever for doing this.
> > > 
> > > Thanks,
> > > Rafael
> > Hi Rafael,
> > 
> > Thanks for the response.
> > 
> > Can you please help me with, how to detect the presence of a certain
> > acpi device using its id (for example, INT3392 for Intel RST device)? 
> 
> If you want to check if the device ir present at all, you cen use
> acpi_device_is_present() introduced recently (although that would need
> to be exported if you want to use it from a driver).

acpi_dev_present() is exported, so can be used in drivers just fine:
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=2d12b6b381ba059d5f92798f5ea739672a2f5fcf


> > As you might have seen (in the next patch in this series), that we use
> > this symbol (acpi_bus_type) to iterate over all the devices registered
> > on acpi bus, to check if there is a device with id INT3392 present or
> > not.

Ankitprasad, just change your patch [11/11] thusly:

-       if (intel_detect_acpi_rst()) {
+       if (acpi_dev_present("INT3392")) {


Using bus_for_each_dev() was the wrong approach, most drivers call
acpi_get_devices() to detect the presence of a particular HID,
however that necessitates the definition of a callback in each driver,
leading to lots of duplicate code. Hence the introduction of
acpi_dev_present() which is also faster because it just iterates over
a list instead of walking the namespace.

This new API landed in Linus' tree last Tuesday (PST), so you need
to merge Linus' tree back into yours or wait until it gets merged
into drm-intel-nightly.

Best regards,

Lukas