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SubjectRe: [RFC/PATCH] Per-device parameter support (13/16)
On Mon, Oct 25, 2004 at 04:08:10PM +1000, Rusty Russell wrote:
> You need __attribute__((aligned(sizeof(void *))): see moduleparam.h.
> The padding between structures is arch-dependent (x86-64 found out the
> hard way when we changed kernel_param previously). Not sure why you're
> using a separate section here anyway.

As the defs array of struct device_paramset_def is assembled by
putting struct device_params continuosly, there shouldn't be any other
intervening data while defining the array. So, the wrapping arguments
(struct kparam_array, struct kparam_flag) should stay out of line.
That's why they're put in different subsections.

> > +/* Bit flag */
> > +#define __DEVICE_PARAM_FLAG(Name, Field, Flag, Dfl, Inv, Perm, Desc) \
> > + param_check_uint(__devparam_data(Name), \
> > + &(((__devparam_type *)0)->Field)); \
> > + static struct kparam_flag __devparam_data(__param_flag_##Name) \
> > + __devparam_extra_section = { 0, Flag, Inv }; \
> > + __DEVICE_PARAM_CALL_RANGED(Name, param_set_flag, param_get_flag,\
> > + &__devparam_data(__param_flag_##Name), \
> > + sizeof(struct kparam_flag), 1, 0, Dfl, 0, Perm, Desc, \
> > + offsetof(struct kparam_flag, pflags), \
> > + offsetof(__devparam_type, Field), -1, -1)
> > +
> > +#define DEVICE_PARAM_FLAG(Name, Field, Flag, Dfl, Perm, Desc) \
> > + __DEVICE_PARAM_FLAG(Name, Field, Flag, Dfl, 0, Perm, Desc)
> Haven't read closely, but why is FLAG special?

Because flag requires wrapping struct kparam_flag structure. As I've
explained in another mail, wrapping structures need to be copied and
fixed up before being used. So, the sizeof and offsetof's.

> I think your macros might be better off always having the range, which
> would reduce the number of functions.. I like including the description
> in the macros: that's an improvement.
> I always disliked the _NAMED versions (if it's a good name for users,
> it's usually a good name for the variable and vice versa), but you might
> want to consider always having it if that's the common case.

Again, I just think it's better to be a bit rich in module/device
parameter handling and provide convenient interface as it can reduce
code in many places. But that's me. :-)

As this is the last mail of this series of replies, I want to add
some more stuff to discuss about.

1. What do you think about merging bus_paramset_def, dev_paramset_def
and aux_paramset_defs into single paramset_defs array. The only
functional difference is that, with the change, a driver should
explicitly specify the corresponding bus paramset_def and pass the
resulting paramset to the bus registration function explicitly if the
bus wants to accept its own paramset.

I think it's better because it's simpler and more consistent, and it
gives drivers a chance to peek or mangle bus parameters as needed.

2. What do you think about adding support for user-controllable
device - driver association? We'll be able to tell the kernel to
'associate device A to driver B with these parameters'. With proper
user-space programs we'll be able to have fine control over device
drivers. Also, user-space tool will be able to set the kernel device
name as it likes from the start so discrepancies between kernel device
names and user-space device names can be removed (udev).

What I was thinking was

- adding a sysctl variable which enables the feature and when it's
enabled the kernel stops automatically associating devices to matching
- User space tool can write to a driver sysfs node (maybe named
attach) to attach a specific device to the driver.
- A device - driver association is represented by a child kobject of
the associated driver or device.
- Detaching can be done by deleting the file representing the
association. Or writing to a detach sysfs file.

As there's no kobject which represents device - driver association,
devparam currently makes a child kobject of the corresponding device
node itself (the params subdirectory).

3. How about adding a mechanism to automatically add min, max and
default value to @Desc. Though I'm not so sure about this one.

Thanks a lot. :-)


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