On Sun, Oct 14, 2007 at 09:06:20PM +0200, Ivo van Doorn wrote:

On Sunday 14 October 2007, Adrian Bunk wrote:

quoted

drivers/net/wireless/rt2x00/rt2x00ring.h contains the following:

<--  snip  -->

...
/*
 * data_desc
 * Each data entry also contains a descriptor which is used by the
 * device to determine what should be done with the packet and
 * what the current status is.
 * This structure is greatly simplified, but the descriptors
 * are basically a list of little endian 32 bit values.
 * Make the array by default 1 word big, this will allow us
 * to use sizeof() correctly.
 */
struct data_desc {
        __le32 word[1];
};
...
/*
 * TX/RX Descriptor access functions.
 */
static inline void rt2x00_desc_read(struct data_desc *desc,
                                    const u8 word, u32 *value)
{
        *value = le32_to_cpu(desc->word[word]);
}

static inline void rt2x00_desc_write(struct data_desc *desc,
                                     const u8 word, const u32 value)
{
        desc->word[word] = cpu_to_le32(value);
}
...

<--  snip  -->

I haven't checked whether it might work in all cases, but passing 
non-zero values as second parameter to rt2x00_desc_{read,write}()
(as is done in many cases) is even in the best case bad coding style.

Access to the array is correct, even with non-zero values the code that is
reading/writing to the array knows the exact size of the descriptor. Within
rt2x00 are however 5 drivers who have different descriptor sizes. That means
I can't create a structure which has the correct array length.

The structure itself is just a simple map over preallocated memory
(skb->data in case of USB or dma in case of PCI).

So possible solutions would be:
 - remove struct data_desc and make it a void* or __le32*
   This is at the cost of code readibility since the above pointers
   have less meaning then a pointer to a structure which can be nicely
   documented.
...

The worst is a wrong meaning.
__le32 word[1] is an array with _one_ element.

And an __le32* can be used as an array.

Ivo

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed