Re: [Qemu-devel] [PATCH 2/2] Introduce macro for defining qdev properties

[Blue Swirl <blauwirbel@gmail.com>]

On Fri, Jul 17, 2009 at 9:11 PM, Anthony Liguori<aliguori@us.ibm.com> wrote:
> Paul Brook wrote:
>>
>> I think I agree. Using GCC extensions for error checking (e.g. DO_UPCAST)
>> or performance (__builtin_clz) is fine, but I'm a reluctant to rely on it
>> for correct operation.
>>
>
> Practically speaking, we've never supported anything but GCC and I doubt we
> ever will.  In this case, it's an important part of something I'm trying to
> fix about the current property system.
>
> It seems very brittle to me.  You have to specify a type in both the state
> structure and in the property definition.  Those two things are very, very
> far apart in the code.  Right now, the rules about type compatible are ill
> defined which makes it more likely to break beyond simple mistakes.  For
> instance, uint32 is used for uint32_t, int32_t, and int.  That seems odd.
>
> I also don't like the fact that we mix field type information with display
> information.  I haven't thought about the best solution to this but I think
> it's either introducing new struct types or adding an optional decorator
> parameter.
>
> The system I'm aiming for looks like this:
>
> typedef struct {
>  SysBusDevice parent;
>
>  /* public */
>  uint32_t queue_depth;
>  uint32_t tx_mitigation_delay;
>  CharDriverState *chr;
>
>  /* private */
>  ...
> } MyDeviceState;
>
> static Property my_device_properties[] = {
>  QDEV_PROP(MyDeviceState, queue_depth),
>  QDEV_PROP(MyDeviceState, tx_mitigation_delay),
>  QDEV_PROP(MyDeviceState, chr),
>  {}
> };
>
> Where there's a connection between properties and device state fields and
> there's no duplicate type information.  That means that for the most part,
> the rules of type compatible can be ignored by most users.
>
> I'd like to see most uses of QDEV_PROP_NAME eliminated by renaming variables
> and accepting '-' in place of '_'.  We'll always need a way to accept
> default values.
>
> I'm not sure how to do this without GCC extensions.  We could potentially
> add macro decorators and use a sparse-like tool to extract property lists
> automatically from device state.

Then there is the template way:

typedef struct MyDeviceState {
#define PROP(type, name) type name;
#define C_TYPE
#include "mydevice_props.hx"
} MyDeviceState;
#undef PROP
#undef C_TYPE

static Property my_device_properties[] = {
#define PROP(type, name) glue(QDEV_PROP_, type)(MyDeviceState, name),
#include "mydevice_props.hx"
 {}
};

Where mydevice_props.hx contains:
#ifdef C_TYPE
#define I32 uint32_t
#define I64 uint64_t
#define CHRDEV CharDriverState *
#endif

PROP(I32, queue_depth)
PROP(I32, tx_mitigation_delay)
PROP(CHRDEV, chr)