Re: [PATCH] introduce WEXT scan capabilities

[Dave <kilroyd@googlemail.com>]

Dan Williams wrote:
> On Fri, 2007-12-07 at 18:12 -0800, David Miller wrote:
>> From: Dan Williams <dcbw-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>> Date: Fri, 07 Dec 2007 19:22:46 -0500
>>
>>> @@ -1040,6 +1049,16 @@ struct	iw_range
>>>  	 * because each entry contain its channel index */
>>>  
>>>  	__u32		enc_capa;	/* IW_ENC_CAPA_* bit field */
>>> +
>>> +	/* Do *NOT* use those fields, they are just used as padding to get
>>> +	 * proper alignement with user space */
>>> +	__s32		reserved1;
>>> +	__s32		reserved2;
>>> +	__u16		reserved3;
>>> +	__s32		reserved4;
>>> +	__u32		reserved5;
>>> +
>>> +	__u32		scan_capa;	/* IW_SCAN_CAPA_* bit field */
>>>  };
>>>  
>>>  /*
>> Major NACK.  These datastructure usages are complete wrong, and
>> we have to stop spreading this problem instead of continuing on
>> with it as if it's OK.
> 
> There's not too much we can do here.  We need a better way to support
> driver/card capabilities in WEXT right _now_, in parallel with
> cfg80211/nl80211.  The other alternative here is to have a 64-bit
> generic capabilities field-to-end-all-fields and add more bitfield
> position constants to that without extending the structure any more.
> 
> Is there a better way you'd propose to do this _in_WEXT_?

Since iw_range is not packed, there are a few locations where there is some padding. You could quite easily shoehorn an 8 bit bitmask into the existing structure without impacting backward compatibility (unless userspace is using the padding for something). For example:

--- a/include/linux/wireless.h
+++ b/include/linux/wireless.h
@@ -1035,6 +1035,7 @@ struct    iw_range
        /* Frequency */
        __u16           num_channels;   /* Number of channels [0; num - 1] */
        __u8            num_frequency;  /* Number of entry in the list */
+       __u8            scan_capa;      /* scan capabilities */
        struct iw_freq  freq[IW_MAX_FREQUENCIES];       /* list */
        /* Note : this frequency list doesn't need to fit channel numbers,
         * because each entry contain its channel index */

Other candidate blocks are Old Frequency, Rates, Encoder stuff, Transmit power.

Dave.

> I don't really forsee any more extending of this structure, since I
> think scan capabilities are the last thing we really need to know about.