netdev - Re: [PATCH RFC net-next 2/4] net: page_pool: avoid calling no-op externals when possible

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Date: Mon, 3 Jul 2023 13:32:07 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Alexander Duyck <alexander.duyck@...il.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Larysa Zaremba <larysa.zaremba@...el.com>, Yunsheng Lin
 <linyunsheng@...wei.com>, Alexander Duyck <alexanderduyck@...com>, "Jesper
 Dangaard Brouer" <hawk@...nel.org>, Ilias Apalodimas
 <ilias.apalodimas@...aro.org>, <netdev@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC net-next 2/4] net: page_pool: avoid calling no-op
 externals when possible

On Fri, 30 Jun 2023 17:34:02 +0200 Alexander Lobakin wrote:
> > I am not a fan of having the page pool force the syncing either. Last
> > I knew I thought the PP_FLAG_DMA_SYNC_DEV was meant to be set by the  
> 
> Please follow the logics of the patch.
> 
> 1. The driver sets DMA_SYNC_DEV.
> 2. PP tries to shortcut and replaces it with MAYBE_SYNC.
> 3. If dma_need_sync() returns true for some page, it gets replaced back
>    to DMA_SYNC_DEV, no further dma_need_sync() calls for that pool.
> 
> OR
> 
> 1. The driver doesn't set DMA_SYNC_DEV.
> 2. PP doesn't turn on MAYBE_SYNC.
> 3. No dma_need_sync() tests.
> 
> Where does PP force syncs for drivers which don't need them?

I think both Alex and I got confused about what's going on here.

Could you reshuffle the code somehow to make it more obvious?
Rename the flag, perhaps put it in a different field than 
the driver-set PP flags?