Messages in this thread | | | Date | Wed, 19 Jan 2022 14:27:49 -0600 | From | Bjorn Helgaas <> | Subject | Re: Convert type of 'struct dmi_system_id -> driver_data' from 'void *' to kernel_ulong_t? |
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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
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