Re: [PATCH 09/13] cpufreq: acpi-cpufreq: Remove unused ID structs

On Wed, 15 Jul 2020, Lee Jones wrote:

On Wed, 15 Jul 2020, Rafael J. Wysocki wrote:

quoted

On Wed, Jul 15, 2020 at 1:34 PM Lee Jones [off-list ref] wrote:

quoted

On Wed, 15 Jul 2020, Rafael J. Wysocki wrote:

quoted

On Wed, Jul 15, 2020 at 5:27 AM Viresh Kumar [off-list ref] wrote:

quoted

On 15-07-20, 08:54, Viresh Kumar wrote:

quoted

On 14-07-20, 22:03, Lee Jones wrote:

quoted

On Tue, 14 Jul 2020, Rafael J. Wysocki wrote:

quoted

On Tue, Jul 14, 2020 at 4:51 PM Lee Jones [off-list ref] wrote:

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Can't see them being used anywhere and the compiler doesn't complain
that they're missing, so ...

Aren't they needed for automatic module loading in certain configurations?

Any idea how that works, or where the code is for that?

The MODULE_DEVICE_TABLE() thingy creates a map of vendor-id,
product-id that the kernel keeps after boot (and so there is no static
reference of it for the compiler), later when a device is hotplugged
into the kernel it refers to the map to find the related driver for it
and loads it if it isn't already loaded.

This has some of it, search for MODULE_DEVICE_TABLE() in it.
Documentation/driver-api/usb/hotplug.rst

And you just need to add __maybe_unused to them to suppress the
warning.

Wouldn't that cause the compiler to optimize them away if it doesn't
see any users?

It looks like they're only unused when !MODULE,

OK

quoted

in which case optimising them away would be the correct thing to do, no?

It would be good if someone with a little more knowledge could provide
a second opinion though.  I would think (hope) that the compiler would
be smart enough to see when its actually in use.  After all, it is the
compiler that places the information into the device table.

If that is not the case, then the MODULE_DEVICE_TABLE() magic is
broken and will need fixing.  Removing boiler-plate is good, but not
at the expense of obfuscation.

Okay, I'm satisfied.  This test build is without __maybe_unused:

# All configs built as modules (MODULE) - the compiler knows to use the tables

 $ ccache make -f Makefile -j24 KBUILD_OUTPUT=../builds/build-x86 allmodconfig
 $ ccache make -f Makefile -j24 KBUILD_OUTPUT=../builds/build-x86  W=1  drivers/cpufreq/
 [...]
   CC [M]  drivers/cpufreq/pcc-cpufreq.o

# All configs built-in (!MODULE) - the compiler sees that they are unused

 $ ccache make -f Makefile -j24 KBUILD_OUTPUT=../builds/build-x86 allyesconfig
 $ ccache make -f Makefile -j24 KBUILD_OUTPUT=../builds/build-x86  W=1  drivers/cpufreq/
   CC      drivers/cpufreq/pcc-cpufreq.o
  drivers/cpufreq/pcc-cpufreq.c:619:36: warning: ‘processor_device_ids’ defined but not used [-Wunused-const-variable=]
  619 | static const struct acpi_device_id processor_device_ids[] = {
      |                                    ^~~~~~~~~~~~~~~~~~~~

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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