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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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