netdev - Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync DMA memory for non-coherent devices

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Date:   Fri, 15 Nov 2019 09:17:03 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net,
        thomas.petazzoni@...tlin.com, brouer@...hat.com,
        matteo.croce@...hat.com
Subject: Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync
 DMA memory for non-coherent devices

Hi Jonathan, 

> 
[...]
> > For the skb reserved queues, this depends on the 'anything'. If the rest
> > of the
> > layers touch (or rather write) into that area, then we'll again gave to
> > sync.
> > If we know that the data has not been altered though, we can hand them
> > back to
> > the device skipping that sync right?
> 
> Sure, but this is also true for eBPF programs.  How would the driver know
> that
> the data has not been altered / compacted by the upper layers?

I haven't looked that up in detail. I was just explaining the reasoning behind
picking a u8 instead of a flag. As Jesper pointed we can get the same result by
using len = 0, so i am fine with the proposed change.

Thanks
/Ilias
> -- 
> Jonathan