On Sat, Mar 16, 2019 at 09:42:40PM +0100, Felix Fietkau wrote:
> Avoids expensive 64-bit atomic access in the data path
<snip>
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7603/mac.c
> b/drivers/net/wireless/mediatek/mt76/mt7603/mac.c
> index d6e260ca1423..fb1961ac9dc6 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7603/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7603/mac.c
> @@ -924,7 +924,11 @@ mt7603_mac_write_txwi(struct mt7603_dev *dev, __le32
> *txwi,
> txwi[3] = cpu_to_le32(val);
>
> if (key) {
> - u64 pn = atomic64_inc_return(&key->tx_pn);
> + u64 pn;
> +
> + spin_lock(&dev->mt76.lock);
> + pn = ++wcid->tx_pn;
> + spin_unlock(&dev->mt76.lock);
It's interesting that atomic op is more expensive that taking spinlock
and do the operation under spinlock protection. This should not be
the case, atomic ops are provided exactly to avoid locking and should
be faster than spin_lock. Perhaps on architecture where this happen
(presumably MIPS 32bit) either atomic ops assembly code should be fixed
or arch should switch generic lib/atomic64.c .
Stanislaw
