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

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Date: Wed, 5 Jul 2023 16:41:11 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
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

From: Jakub Kicinski <kuba@...nel.org>
Date: Mon, 3 Jul 2023 13:32:07 -0700

> 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?

PP currently doesn't have a field for internal flags or so, so I reused
the existing one :s But you're probably right, that would make it more
obvious.

1. Driver sets PP_SYNC_DEV.
2. PP doesn't set its internal one until dma_need_sync() returns false.
3. PP-sync-for-dev checks for the internal flag.

Although needs more lines to be changed :D

Thanks,
Olek