Hi Andy,
On Wed, 14 Jun 2023 14:51:43 +0300
Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:
> On Wed, Jun 14, 2023 at 12:42 PM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
> > On Wed, 14 Jun 2023 12:02:57 +0300
> > Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:
> > > On Wed, Jun 14, 2023 at 10:49 AM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
>
> ...
>
> > > > + typeof(__array[0] + 0) __element = __array[--__len]; \
> > >
> > > Do we need the ' + 0' part?
> >
> > Yes.
> >
> > __array can be an array of const items and it is legitimate to get the
> > minimum value from const items.
> >
> > typeof(__array[0]) keeps the const qualifier but we need to assign __element
> > in the loop.
> > One way to drop the const qualifier is to get the type from a rvalue computed
> > from __array[0]. This rvalue has to have the exact same type with only the const
> > dropped.
> > '__array[0] + 0' was a perfect canditate.
>
> Seems like this also deserves a comment. But if the series is accepted
> as is, it may be done as a follow up.
>
Finally not so simple ...
I did some deeper tests and the macros need to be fixed.
I hope this one (with comments added) is correct:
--- 8 ---
/*
* Do not check the array parameter using __must_be_array().
* In the following legit use-case where the "array" passed is a simple pointer,
* __must_be_array() will return a failure.
* --- 8< ---
* int *buff
* ...
* min = min_array(buff, nb_items);
* --- 8< ---
*
* The first typeof(&(array)[0]) is needed in order to support arrays of both
* 'int *buff' and 'int buf[N]' types.
*
* typeof(__array[0] + 0) used for __element is needed as the array can be an
* array of const items.
* In order to discard the const qualifier use an arithmetic operation (rvalue).
* This arithmetic operation discard the const but also can lead to an integer
* promotion. For instance, a const s8 __array[0] lead to an int __element due
* to the promotion.
* In this case, simple min() or max() operation fails (type mismatch).
* Use min_t() or max_t() (op_t parameter) enforcing the type in order to avoid
* the min() or max() failure.
*/
#define __minmax_array(op_t, array, len) ({ \
typeof(&(array)[0]) __array = (array); \
typeof(len) __len = (len); \
typeof(__array[0] + 0) __element = __array[--__len]; \
while (__len--) \
__element = op_t(typeof(__array[0]), __element, __array[__len]); \
__element; })
/**
* min_array - return minimum of values present in an array
* @array: array
* @len: array length
*
* Note that @len must not be zero (empty array).
*/
#define min_array(array, len) __minmax_array(min_t, array, len)
/**
* max_array - return maximum of values present in an array
* @array: array
* @len: array length
*
* Note that @len must not be zero (empty array).
*/
#define max_array(array, len) __minmax_array(max_t, array, len)
--- 8< ---
Tested ok from user-space on my x86_64 using the following types for *buff
and buff[N]:
- signed/unsigned char
- signed/unsigned short
- signed/unsigned int
- signed/unsigned long
- signed/unsigned long long
- float, double, long double (even if not used in the kernel)
Can you give me your feedback on this last version ?
If you are ok, it will be present in the next iteration of the series.
Otherwise, as a last ressort, I will drop the {min,max}_array() and switch
back to the specific min/max computation in IIO inkern.c
Sorry for this back and forth and this last minute detected bug.
Best regards,
Hervé
--
Hervé Codina, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
