Subject: Re: [PATCH 2/2] ath10k: support MAC address randomization in scan
To: Brian Norris <briannorris@chromium.org>
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Cc: Kalle Valo <kvalo@codeaurora.org>, cjhuang@codeaurora.org,
        linux-wireless@vger.kernel.org, ath10k@lists.infradead.org
From: Arend van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <5AD66BD9.4000706@broadcom.com> (sfid-20180417_234927_986247_705E1FDB)
Date: Tue, 17 Apr 2018 23:49:13 +0200
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On 4/17/2018 6:07 PM, Brian Norris wrote:
> On Tue, Apr 17, 2018 at 10:22:13AM +0200, Arend van Spriel wrote:
>> On 4/17/2018 2:28 AM, Brian Norris wrote:
>>> It looks like the status quo for looking for SCHED_SCAN support is to
>>> check if NL80211_CMD_START_SCHED_SCAN shows up in the command support
>>> list. (IOW, that's what wpa_supplicant does.) We'll probably need to
>>> imitate that.
>>
>> I believe checking command support is not really recommended. Instead, you
>> better check NL80211_ATTR_SCHED_SCAN_MAX_REQS being non-zero (since kernel
>> 4.12 that is).
>
> Why not? Command support checking is what wpa_supplicant is doing.

That's not really a good argument. A couple (or more) years ago 
wpa_supplicant was not doing nl80211 but wext and some other using 
driver private ioctls, but that did not make it the best approach.

The START_SCHED_SCAN command is indeed still provided to user-space:

@@ -1376,7 +1377,7 @@ static int nl80211_add_commands_unsplit(struct 
cfg80211_r
                 CMD(tdls_mgmt, TDLS_MGMT);
                 CMD(tdls_oper, TDLS_OPER);
         }
-       if (rdev->wiphy.flags & WIPHY_FLAG_SUPPORTS_SCHED_SCAN)
+       if (rdev->wiphy.max_sched_scan_reqs)
                 CMD(sched_scan_start, START_SCHED_SCAN);
         CMD(probe_client, PROBE_CLIENT);
         CMD(set_noack_map, SET_NOACK_MAP);

It was left in because existing user-space apps might depend on it.

> I noticed NL80211_ATTR_SCHED_SCAN_MAX_REQS, but unfortunately, we have
> to support older kernels. It looks like randomization was added in
> v3.19, and as you point out, that's only available in v4.12.

I figured that would be the issue so I dug up the kernel version.

> I welcome other alternatives if you have them to offer.

Nope. Not if you want to be kernel version agnostic.

Regards,
Arend