Re: [RFC/RFT 1/2] mac80211: Add NEED_ALIGNED4_SKBS hw flag

[Felix Fietkau <nbd@openwrt.org>]

On 2015-12-17 11:29, Johannes Berg wrote:
> On Thu, 2015-12-17 at 10:20 +0100, Janusz Dziedzic wrote:
>> HW/driver should set NEED_ALIGNED4_SKBS flag in case require
>> aligned skbs to four-byte boundaries.
>> 
>> Before we have to do memmove() in the driver before
>> pass this to HW and memmove() back in tx completion.
>> This patch allow to save CPU and skip such memmoves.
> 
> Can you quantify that btw? It shouldn't be that expensive since it's
> all in the cache for read, so just has to be written out to RAM before
> the DMA can happen...
On many devices that I'm using, the data path is definitely too bloated
for the packet to still be in cache, so I do think this will help.
Remember, having only 32 KiB Dcache (no L2) is not uncommon on devices
running ath9k.

> And on status it will be pulled into the cache
> anyway, and you don't even need the write to happen before you can
> continue.
This doesn't make sense to me. The write needs to happen before the
device can do DMA...

- Felix