Re: Multi-Advertising: implementation options, timing questions

[Marcel Holtmann <marcel@holtmann.org>]

Hi Florian,

> > Now that most of the logic for a single instance is in place it should
>> be straightforward to extend it for multiple instances. I would start
>> by turning the adv_instance field of hci_dev into a list of
>> adv_instances and a way to determine and get a pointer to the
>> currently active instance.
> 
> Yes, agreed. That's exactly where I started. Happy to hear that you got the same procedure in mind. I experimented with a dynamically extended array and a linked list. The latter seems to be the better choice as it allows us to easily remove entries in the middle of the list.
> 
>> You would then modify the code in
>> net/bluetooth/mgmt.c that accesses the instance 1 parameters to use
>> the new getter instead. Once all of that is done, the second step
>> would be a matter of inserting new elements into the advertising list
>> and handling the round-robin logic and the duration parameter.
> 
> Agreed. That's what I had in mind, too.
> 
>> If it makes things easier I can start tackling the first step which is
>> mostly refactoring my recent code to enable multiple instances. You
>> can then cleanly build the multiple instance logic on top of that.
> 
> It's up to you. Just let me know so that I don't interfere with your work. I don't want to block you as I'll certainly take much longer to get it right (newbie + evenings/weekends only). Today I just set up my dev env and made the build + e2e-tests run on a vm with a minimal kernel. So not much done in the code, yet.
> 
> Apart from that I got a few thoughts while familiarizing myself with the source that you might be able to comment on:
> 
> - Why do we have two flags to distinguish between multi- and single-instance advertising (HCI_ADVERTISING[_INSTANCE])? Doesn't that allow for inconsistencies (=both true)? Wouldn't it be better to interpret one as "advertising generally en-/disabled" and the other as a switch between single and multi adv mode? That would also allow us to keep track of the adv mode when advertising is temporarily disabled and then re-enabled. As Marcel commented this might be necessary for some controllers in multi-adv mode when adv data cannot be changed on-the-fly.

the HCI_ADVERTISING maps to the advertising setting. And it always takes precedence. It is essentially instance 0. This fact is actually documented. So if both flags are true, then instance 0 is used and all the other ones will be disabled.

> - What do you think of the idea to handle "set adv" and multi-adv more uniformly? I have the following in mind:
> 1) whenever an adv mode switch occurs, all current adv instances will be canceled and destroyed
> 2) "set adv" will add/replace a single instance to the list
> 3) "add adv" will add instances up to max_instances
> This would probably dry up the code and duplicate memory structures quite a bit and also remove some logic quirks.

See comment above, we can not really change legacy API. It has to stay around for backwards compatibility. And that is why set advertising takes precedence over anything added by add advertising.

> - Instances are currently being identified by an integer "adv_info.instance". When we add instances more dynamically would it not be better to pass pointers around and get rid of that integer? That would remove the necessity to keep track of, synchronize and verify instance ids.

What are you planning to verify here. The instance id is coming from userspace.

> - The mgmt_rp_read_adv_features structure contains an unused instance[0]. Seems to be redundant and could be removed, right?

Read up on what instance[0] actually does in a struct. We have used these constructs before. I am sensing that you misunderstood what this is for.

Regards

Marcel