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

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Message-ID: <20160119163104.GA9469@wunner.de>
Date:	Tue, 19 Jan 2016 17:31:04 +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 Rafael,

On Tue, Jan 19, 2016 at 12:59:13AM +0100, Rafael J. Wysocki wrote:
> On Tuesday, January 19, 2016 12:00:47 AM Lukas Wunner wrote:
> > Hi,
> > 
> > On Mon, Jan 18, 2016 at 11:46:18PM +0100, Rafael J. Wysocki wrote:
> > > On Monday, January 18, 2016 11:39:07 PM Lukas Wunner wrote:
> 
> [cut]
> 
> > > > > > 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).
> > > > > 
> > > > > I meant acpi_dev_present(), sorry about the mistake.
> > > > > 
> > > > > I guess we should rename it to acpi_device_found() or something similar
> > > > > to avoid such confusion in the future.
> > > > 
> > > > The name was chosen because the PCI equivalent is called pci_dev_present()
> > > > and I assumed that name already stuck in developers' heads, so if they're
> > > > looking for an ACPI presence detection function, that's what they'd look
> > > > for first.
> > > 
> > > But "present" in ACPI really means something different.  There may be ACPI
> > > device objects in the namespace for devices that are not *actually* present.
> > 
> > You mean synthesized devices like LNXSYBUS?
> > Don't think anyone is going to test for the presence of that.
> 
> No, I mean real devices, where the corresponding ACPI object has _STA that
> returns 0.
> 
> There may be a couple of reasons for that.  The device the ACPI object
> corresponds to may not be physically present (eg. it may possible to
> hot-add it) or the device may depend on something else for functionality
> and that thing hasn't been set up yet etc.
> 
> The presence of an ACPI device object in the namespace means that the
> platform firmware knows about the device, but it need not mean that
> the device is really there.  _STA returns that piece of information.

Thank you for the clarification, these are very good points.

The drivers in question use acpi_get_devices() merely to probe for
presence of a device in the namespace. They do not invoke _STA,
nor do they even hold a pointer to the acpi_device or acpi_handle
when detecting presence. Mostly this is about activating quirks
if a certain ACPI device is detected.

Currently about 50% of the calls to acpi_get_devices() in the drivers
fit this pattern and the point of acpi_dev_present() is to give
developers a simple, lightweight tool as an alternative.

However the kernel-doc should be amended to clarify that _STA is not
invoked. The patch below is a suggestion, feel free to rephrase.

Thanks & best regards,

Lukas

-- >8 --
Subject: [PATCH] ACPI / utils: Clarify appropriate usage of acpi_dev_present()

Rafael J. Wysocki pointed out that even though a device is present
in the namespace, its _STA control method might still return 0 in the
"device is present" bit. Amend the documentation accordingly.

Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
Signed-off-by: Lukas Wunner <lukas@...ner.de>
---
 drivers/acpi/utils.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/acpi/utils.c b/drivers/acpi/utils.c
index f2f9873..99af3bc 100644
--- a/drivers/acpi/utils.c
+++ b/drivers/acpi/utils.c
@@ -716,6 +716,8 @@ EXPORT_SYMBOL(acpi_check_dsm);
  *
  * Return %true if the device was present at the moment of invocation.
  * Note that if the device is pluggable, it may since have disappeared.
+ * Also, this merely checks presence in the namespace but does not
+ * invoke the _STA control method.
  *
  * For this function to work, acpi_bus_scan() must have been executed
  * which happens in the subsys_initcall() subsection. Hence, do not
-- 
1.8.5.2 (Apple Git-48)