Re: [PATCH v3] platform/x86: Add driver for ACPI WMAA EC-based backlight control

[Daniel Dadap <ddadap@nvidia.com>]

Thanks again:

On 8/25/21 4:05 AM, Andy Shevchenko wrote:
> On Wed, Aug 25, 2021 at 1:09 AM Daniel Dadap <ddadap@nvidia.com> wrote:
>> A number of upcoming notebook computer designs drive the internal
>> display panel's backlight PWM through the Embedded Controller (EC).
>> This EC-based backlight control can be plumbed through to an ACPI
>> "WMAA" method interface, which in turn can be wrapped by WMI with
>> the GUID handle 603E9613-EF25-4338-A3D0-C46177516DB7.
>>
>> Add a new driver, aliased to the WMAA WMI GUID, to expose a sysfs
>> backlight class driver to control backlight levels on systems with
>> EC-driven backlights.
> I tried to avoid repetition of the comments given by others.
>
> So, mine below.
>
> ...
>
>> +config WMAA_BACKLIGHT_WMI
>> +       tristate "ACPI WMAA Backlight Driver"
>> +       depends on ACPI_WMI
>> +       depends on BACKLIGHT_CLASS_DEVICE
>> +       help
>> +         This driver provides a sysfs backlight interface for notebook
>> +         systems which expose the WMAA ACPI method and an associated WMI
>> +         wrapper to drive LCD backlight levels through the system's
>> +         Embedded Controller.
> Please, add a sentence to tell how the module will be called. There
> are plenty of examples in the kernel.
>
> ...
>
>> +struct wmaa_args {
>> +       u32 set;
>> +       u32 val;
>> +       u32 ret;
>> +       u32 ignored[3];
>> +};
> I guess this structure deserves a kernel doc.


Do you have a recommended location? From a quick skim I didn't see any 
document in Documentation/ that seemed most appropriate to add this to.


>
> ...
>
>> +static const struct wmi_device_id wmaa_backlight_wmi_id_table[] = {
>> +       { .guid_string = WMAA_WMI_GUID },
>> +       { },
> No comma for termination.
>
>> +};
> ...
>
>> +static int wmaa_backlight_get_brightness(struct backlight_device *bd)
>> +{
>> +       u32 level;
>> +       int ret;
>> +
>> +       ret = wmaa_get_brightness(&level);
>> +       WARN_ON(ret != 0);
> Why?


To differentiate a 0 because the level is actually 0 versus a 0 because 
there was an error. The backlight device API doesn't seem to have a way 
to report errors.


>> +       return ret == 0 ? level : 0;
>> +}
> ...
>
>> +static int wmaa_backlight_wmi_probe(struct wmi_device *w, const void *ctx)
>> +{
>> +       struct backlight_properties props = {0};
> {} is slightly better.
>
>> +       struct wmi_wmaa_priv *priv;
>> +       u32 source;
>> +       int ret;
>> +
>> +       priv = devm_kmalloc(&w->dev, sizeof(*priv), GFP_KERNEL);
>> +       if(!priv)
>> +               return -ENOMEM;
>> +       wdev = w;
> I'm wondering if it's possible to avoid having a global variable.


It is; this can be stored in the struct backlight_device.


>
>> +       ret = wmaa_get_brightness_source(&source);
>> +       if (ret)
>> +               goto done;
>> +
>> +       if (source != WMAA_SOURCE_EC) {
>> +               ret = -ENODEV;
>> +               goto done;
>> +       }
>> +
>> +       // Register a backlight handler
>> +       props.type = BACKLIGHT_PLATFORM;
>> +       ret = wmaa_get_max_brightness(&props.max_brightness);
>> +       if (ret)
>> +               goto done;
>> +
>> +       ret = wmaa_get_brightness(&props.brightness);
>> +       if (ret)
>> +               goto done;
>> +
>> +       priv->backlight = backlight_device_register("wmaa_backlight",
>> +               NULL, NULL, &wmaa_backlight_ops, &props);
>> +       if (IS_ERR(priv->backlight))
>> +               return PTR_ERR(priv->backlight);
>> +
>> +       dev_set_drvdata(&w->dev, priv);
>> +done:
> Useless. Return directly.


Yeah, most likely there used to be some other cleanup here. Thanks.


>
>> +       return ret;
>> +}
> ...
>
>> +static struct wmi_driver wmaa_backlight_wmi_driver = {
>> +       .driver = {
>> +               .name = "wmaa-backlight",
>> +       },
>> +       .probe = wmaa_backlight_wmi_probe,
>> +       .remove = wmaa_backlight_wmi_remove,
>> +       .id_table = wmaa_backlight_wmi_id_table,
>> +};
>> +
> Redundant blank line.
>
>> +module_wmi_driver(wmaa_backlight_wmi_driver);
> ...
>
>> +
>> +MODULE_ALIAS("wmi:"WMAA_WMI_GUID);
> Can you move this closer to GUID? But I'm not sure what is the
> preferred style. Hans?


I'll do whatever is most stylistically preferred.