linux-kernel - Re: Convert type of 'struct dmi_system_id -> driver_data' from 'void *' to kernel_ulong

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Date:   Wed, 19 Jan 2022 14:27:49 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>, jkosina@...e.cz,
        Jason Gunthorpe <jgg@...pe.ca>,
        Bjorn Helgaas <bhelgaas@...gle.com>, mgurtovoy@...dia.com,
        linux@...ssschuh.net, Arnd Bergmann <arnd@...db.de>,
        stephan@...hold.net, "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: Convert type of 'struct dmi_system_id -> driver_data' from 'void
 *' to kernel_ulong_t?

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.

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.

Bjorn

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=f45d069a5628