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@xxxxxxxxxxxxx> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx> --- 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) -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
