Re: [PATCH 2/2] mac80211: expose txq queue depth and size to drivers

[Felix Fietkau <nbd@openwrt.org>]

On 2016-01-26 12:56, Michal Kazior wrote:
> On 26 January 2016 at 11:45, Felix Fietkau <nbd@openwrt.org> wrote:
>> On 2016-01-21 14:23, Michal Kazior wrote:
>>> This will allow drivers to make more educated
>>> decisions whether to defer transmission or not.
>>>
>>> Relying on wake_tx_queue() call count implicitly
>>> was not possible because it could be called
>>> without queued frame count actually changing on
>>> software tx aggregation start/stop code paths.
>>>
>>> It was also not possible to know how long
>>> byte-wise queue was without dequeueing.
>>>
>>> Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
>> Instead of exposing these in the struct to the driver directly, please
>> make a function to get them. Since the number of frames is already
>> tracked in txqi->queue, you can avoid counter duplication that way.
> 
> Hmm, so you suggest to have something like:
> 
>  void
>  ieee80211_get_txq_depth(struct ieee80211_txq *txq,
>                          unsigned int *frame_cnt,
>                          unsigned int *byte_count) {
>    struct txq_info *txqi = txq_to_info(txq);
> 
>    if (frame_cnt)
>      *frame_cnt = txqi->queue.qlen;
> 
>    if (byte_count)
>      *byte_cnt = txqi->byte_cnt;
>  }
> 
> Correct?
> 
> Sounds reasonable.
Right.

>> Also, that way you can fix a race condition between accessing the number
>> of frames counter and the bytes counter.
> 
> I don't see a point in maintaining coherency between the two counters
> with regard to each other alone. Do you have a use-case that would
> actually make use of that property?
> 
> I'd like to avoid any unnecessary spinlocks.
OK. I guess we can leave them out for now. How frequently are you going
to call this function?

- Felix