Re: [PATCH v2] platform/x86: Add Driver to set up lid GPEs on MS Surface device

From: Maximilian Luz
Date: Wed Sep 16 2020 - 13:55:48 EST

On 9/16/20 7:13 PM, Barnabás Pőcze wrote:
+ }s
+ return 0;
+static int surface_gpe_probe(struct platform_device *pdev)
+ struct surface_lid_device *lid;
+ u32 gpe_number;
+ int status;
+ status = device_property_read_u32(&pdev->dev, "gpe", &gpe_number);
+ if (status)
+ return -ENODEV;

'device_property_read_u32()' returns an error code, you could simply return that
instead of hiding it.

My thought there was that if the "gpe" property isn't present or of a
different type, this is not a device that we want to/can handle. Thus
the -ENODEV. Although I think a debug print statement may be useful

I see, I just wanted to bring to your attention that 'device_property_read_u32()'
returns various standard error codes and you could simply return those.

I think one could also argue that module-loading should have taken care
of filtering out devices that we don't load on, so -ENODEV would be
redundant here. At least if one neglects that a user could try to
manually bind the driver to a device. Following that thought, I guess it
makes more sense to return the actual value here.

+ lid->gpe_number = gpe_number;
+ platform_set_drvdata(pdev, lid);
+ status = surface_lid_enable_wakeup(&pdev->dev, false);
+ if (status) {
+ acpi_disable_gpe(NULL, gpe_number);
+ platform_set_drvdata(pdev, NULL);

Why is 'platform_set_drvdata(pdev, NULL)' needed?

Is this not required for clean-up once the driver data has been set? Or
does the driver-base take care of that for us when the driver is
removed/fails to probe? My reasoning was that I don't want to leave
stuff around for any other driver to trip on (and rather have that
driver oops on a NULL-pointer). If the driver-core already takes care of
NULL-ing that, that line is not needed. Unfortunately that behavior
doesn't seem to be explained in the documentation.

I'm not aware that it would be required. As a matter of fact, only two x86
platform drivers (intel_pmc_core, ideapad-laptop) do any cleanup of driver data.
There are much more hits (536) for "set_drvdata(.* NULL" when scanning all drivers.
There are 4864 hits for "set_drvdata(.*" that have no 'NULL' in them.

There is drivers/base/dd.c:really_probe(), which seems to be the place where driver
probes are actually called. And it calls 'dev_set_drvdata(dev, NULL)' if the probe
fails. And it also sets the driver data to NULL in '__device_release_driver()',
so I'm pretty sure the driver core takes care of it.

I see, thanks! Would make sense that the core takes care of that.

+ return status;
+ }
+ return 0;
+static int __init surface_gpe_init(void)
+ const struct dmi_system_id *match;
+ const struct property_entry *props;
+ struct platform_device *pdev;
+ struct fwnode_handle *fwnode;
+ int status;
+ match = dmi_first_match(dmi_lid_device_table);
+ if (!match) {
+ pr_info(KBUILD_MODNAME": no device detected, exiting\n");

If you put
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
before including any headers, you can simply write 'pr_info("no device...")' and it'll
be prefixed by the module name. This is the "usual" way of achieving what you want.

Right, thanks!

+ return 0;

Shouldn't it return -ENODEV?

How does module auto-loading behave with a -ENODEV return value in init?
I know that in the driver's probe callback it signals that the driver
isn't intended for the device. Is this the same for modules or would a
user get an error message in the kernel log? As I couldn't find any
documentation on this, I assumed it didn't behave the same and would
emit an error message.

The reason I don't want to emit an error message here is that the module
can be loaded for devices that it's not intended (and that's not
something we can fix with a better MODULE_ALIAS as Microsoft cleverly
named their 5th generation Surface Pro "Surface Pro", without any
version number). Mainly, I don't want users to get a random error
message that doesn't indicate an actual error.

I wasn't sure, so I tested it. It prints the "no device" message, but that's it,
no more indication of the -ENODEV error in the kernel log during automatic
module loading at boot.

You could print "no compatible Microsoft Surface device found, exitig" (or something
similar). I think this provides enough information for any user to decide if
they should be concerned or not.

I ran the same test (with same results) earlier today and also did some
digging: From what I can tell, udev is responsible for auto-loading and
the code doing that can be found at [1]. This code seems to, by default,
log any errors as debug output. Only in verbose mode it logs them as
error, with the exception of -ENODEV, which then is specifically logged
only as notice.

It also seems to be used by a couple of other modules this way. So I
guess that's the expected use-case for -ENODEV in module-init and pretty
much guarantees the behavior I've wanted.


Thanks again. If there are no other comments, I'll likely submit a v3
addressing the issues tomorrow.