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Date: Mon, 3 Jul 2023 13:32:07 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Paolo Abeni
 <pabeni@redhat.com>, Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
 Larysa Zaremba <larysa.zaremba@intel.com>, Yunsheng Lin
 <linyunsheng@huawei.com>, Alexander Duyck <alexanderduyck@fb.com>, "Jesper
 Dangaard Brouer" <hawk@kernel.org>, Ilias Apalodimas
 <ilias.apalodimas@linaro.org>, <netdev@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC net-next 2/4] net: page_pool: avoid calling no-op
 externals when possible
Message-ID: <20230703133207.4f0c54ce@kernel.org>
In-Reply-To: <413e3e21-e941-46d0-bc36-fd9715a55fc4@intel.com>
References: <20230629152305.905962-1-aleksander.lobakin@intel.com>
	<20230629152305.905962-3-aleksander.lobakin@intel.com>
	<69e827e239dab9fd7986ee43cef599d024c8535f.camel@gmail.com>
	<ac4a8761-410e-e8cc-d6b2-d56b820a7888@intel.com>
	<CAKgT0UfZCGnWgOH96E4GV3ZP6LLbROHM7SHE8NKwq+exX+Gk_Q@mail.gmail.com>
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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?