Re: Convert type of 'struct dmi_system_id -> driver_data' from 'void *' to kernel_ulong_t?

[Kai-Heng Feng]

Hi Hans an Bjorn,

On Thu, Jan 20, 2022 at 4:45 AM Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>
> Hi,
>
> On 1/19/22 21:27, Bjorn Helgaas wrote:
> > On Wed, Jan 19, 2022 at 11:20:36AM +0100, Hans de Goede wrote:
> >> On 1/19/22 02:22, Kai-Heng Feng wrote:
> >>> I wonder if there's any reason to use 'void *' instead of
> >>> kernel_ulong_t for 'driver_data' in 'struct dmi_system_id'?
> >>>
> >>> I'd like to use the driver_data for applying quirk flags, and I found
> >>> out unlike most other struct *_id, the dmi variant is using 'void *'
> >>> for driver_data. Is there any technical reason for this?
> >>> ...
> >
> >> You are asking for a technical reason why "void *" was used,
> >> but lets turn that around, why do you believe that "unsigned long"
> >> is inherently a better type here ?
> >>
> >> driver_data in most places in the kernel (like data for
> >> all sort of callback functions) actually typically is a void *
> >> already, because often people want to pass more data then what
> >> fits in a single long and this also applies to driver-id attached
> >> data.
> >
> > FWIW, "egrep "context;|data;|info;" include/linux/mod_devicetable.h"
> > says 4 of the ~40 instances use a void *; the others use
> > kernel_ulong_t.
>
> Right inside mod_devicetable.h kernel_ulong_t is the norm, but outside
> e.g. inside struct device and with dev_set_drvdata/dev_get_drvdata
> and all their many derratives using void * is the norm.
>
> So looking at the kernel as a whole using kernel_ulong_t seems
> to be the exception. But maybe that indeed has something to
> do with:
>
> > f45d069a5628 ("PCI dynids - documentation fixes, id_table NULL check")
> > [1] (from the tglx history tree) added the original hint for
> > pci_device_id that:
> >
> >   Best practice for use of driver_data is to use it as an index into a
> >   static list of equivalant device types, not to use it as a pointer.
> >
> > I don't know the background of that, but I could imagine that using an
> > index rather than a pointer makes things like /sys/bus/pci/.../new_id
> > easier and safer.
>
> Right, interesting.
>
> OTOH we have:
>
> const void *device_get_match_data(struct device *dev);
>
> Which is a wrapper to easily get the driver_data for popular firmware
> based matches (ACPI/of), which also returns a void *...
>
> Actually the rule seems to be that firmware-id matching,
> including WMI GUID matching uses void * where as hw-id
> (e.g prod:vend matching) uses kernel_ulong_t with acpi_device_id
> being the exception since it is a fwid using kernel_ulong_t,
> which then gets "fixed" by acpi_device_get_match_data turning
> it into a void * for the caller.
>
> As DMI matching is closer to firmware compatible/id matching then
> to actual hw-id matching, it seems that it actually follows
> the pattern of fw-id matches using void * where as hw-id
> matches using void * .
>
> TBH I don't care much either way, but I also really don't see
> strong reasons to spend a lot of time on changing any of this.

Thanks for all the info.
I think they are great justifications that I should avoid all the
hustles to convert the type, and stick to the pointer/integer casting.

Kai-Heng

>
> Regards,
>
> Hans
>