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Date: Fri, 12 Apr 2024 07:08:14 -0700
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Subject: Re: [PATCH 01/13] wifi: cfg80211: Add provision to advertise multiple
 radio in one wiphy
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To: Vasanthakumar Thiagarajan <quic_vthiagar@quicinc.com>,
 Johannes Berg <johannes@sipsolutions.net>,
 Karthikeyan Periyasamy <quic_periyasa@quicinc.com>,
 ath12k@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
References: <20240328072916.1164195-1-quic_periyasa@quicinc.com>
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 <cb991a00-fce2-5a3d-c03f-579b44f922f5@quicinc.com>
From: Ben Greear <greearb@candelatech.com>
Organization: Candela Technologies
In-Reply-To: <cb991a00-fce2-5a3d-c03f-579b44f922f5@quicinc.com>
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On 4/11/24 21:11, Vasanthakumar Thiagarajan wrote:
> 
> 
> On 4/11/2024 2:33 AM, Ben Greear wrote:
>> On 4/10/24 08:42, Johannes Berg wrote:
>>> On Wed, 2024-04-10 at 07:37 -0700, Ben Greear wrote:
>>>> On 4/10/24 00:56, Johannes Berg wrote:
>>>>> On Fri, 2024-03-29 at 07:47 -0700, Ben Greear wrote:
>>>>>> On 3/29/24 07:30, Johannes Berg wrote:
>>>>>>> On Fri, 2024-03-29 at 19:41 +0530, Vasanthakumar Thiagarajan wrote:
>>>>>>>>>
>>>>>>>>>> + * @hw_chans: list of the channels supported by every constituent underlying
>>>>>>>>>> + *    hardware. Drivers abstracting multiple discrete hardware (radio) under
>>>>>>>>>> + *    one wiphy can advertise the list of channels supported by each physical
>>>>>>>>>> + *    hardware in this list. Underlying hardware specific channel list can be
>>>>>>>>>> + *    used while describing interface combination for each of them.
>>>>>>>>>
>>>>>>>>> I'd expect there to be a limit on channels being within a single band on
>>>>>>>>> a single "hardware"?
>>>>>>>>>
>>>>>>>>
>>>>>>>> There are ath12k hardware supporting multiple band which need to be
>>>>>>>> registered under one mac80211_hw/wiphy. This design is to support such
>>>>>>>> hardware.
>>>>>>>
>>>>>>> Oh OK, that was something that I didn't have in mind any more, or never
>>>>>>> knew or paid attention to.
>>>>>>
>>>>>> Would it work to leave the phy reporting pretty much as it is now, but add
>>>>>> a 'associated_peer_radios' list section, so that each phy could report the phys
>>>>>> associated with it?  Then user-space, driver, mac80211 etc could look up the
>>>>>> other phys as needed to get a full picture?
>>>>>>
>>>>>
>>>>> There's not really a good way to _do_ that short of creating multiple
>>>>> wiphys, but that causes _massive_ complexity in the stack (both cfg80211
>>>>> and mac80211) so we rejected it years ago.
>>>>
>>>> I thought the problem ath12k is trying to fix is that there are currently multiple phys (radios) that needed to be made to
>>>> look like a single phy?
>>>
>>> Correct.
>>>
>>>> For dual and tri-concurrent radios, I think we will need them to look like 3 individual radios for non-MLO use
>>>> cases
>>>
>>> No, I don't see why, and if you want that we wouldn't support it anyway,
>>> you'd have to have a module option or something to decide which way to
>>> go.
>>>
>>> But it really ought to not be needed - the point of these patches is to
>>> give userspace enough information to know how to (and where) to create
>>> separate BSSes, with or without MLO between them.
>>>
>>>> For instance, mt7996 currently reports 3 single-band wiphys, and each can be used independently.
>>>> But assuming it starts supporting MLO, then those 3 single band wiphys will need to start acting
>>>> at least somewhat like a single entity
>>>
>>> Yes.
>>>
>>>> (while also concurrently being able to act as individual
>>>> wiphys so that one can do a mix of MLO and non MLO sta/AP.)
>>>
>>> No.
>>
>> Hello Johannes,
>>
>> Is there any design document for the combined phy approach somewhere publicly available?
>>
>> It is hard to understand the over all goals by just reading patches as they show up on
>> the public mailing lists...
>>
> 
> Hi Ben,
> 
> I dont think there is a document for this composite phy approach. But we try to clarify
> as much as possible in the commit log and kernel-doc. Pls let us know the area which
> is more appropriate to be clarified in the path.
> 
> Vasanth

I am worried that the whole approach has problems that would be better solved with different
architecture.  I understand that someone has made a decision to go with the combined approach,
and I am sure they have reasons.  It would be good to see some details about how this combined
approach can work in lots of different use cases, including with un-modified user-space, and
including what changes *are* required in user-space to keep current features and control working
with combined wiphy approach.

My over-all concerns are that I feel user-space is still going to need to understand the individual
underlying phys and be able to read/modify them individually.  Older radios will continue to have single phy
mappings, so that must be supported pretty much forever.  So it seems there is going to be a huge amount
of duplicated code up and down the stack and user-space.

Having your team grind on a large patch set that turns out to have fundamental flaws would be
a huge waste of time for all involved.

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com