linux-kernel - Re: [PATCH net-next 5/9] page_pool: don't use driver-set flags field directly

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Date:   Tue, 1 Aug 2023 15:36:33 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
        Simon Horman <simon.horman@...igine.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 5/9] page_pool: don't use driver-set flags field
 directly

From: Yunsheng Lin <linyunsheng@...wei.com>
Date: Sat, 29 Jul 2023 19:40:32 +0800

> On 2023/7/28 22:03, Alexander Lobakin wrote:
>> From: Yunsheng Lin <linyunsheng@...wei.com>
>> Date: Fri, 28 Jul 2023 20:36:50 +0800
>>
>>> On 2023/7/27 22:43, Alexander Lobakin wrote:
>>>
>>>>  
>>>>  struct page_pool {
>>>>  	struct page_pool_params p;
>>>> -	long pad;
>>>> +
>>>> +	bool dma_map:1;				/* Perform DMA mapping */
>>>> +	enum {
>>>> +		PP_DMA_SYNC_ACT_DISABLED = 0,	/* Driver didn't ask to sync */
>>>> +		PP_DMA_SYNC_ACT_DO,		/* Perform DMA sync ops */
>>>> +	} dma_sync_act:1;
>>>> +	bool page_frag:1;			/* Allow page fragments */
>>>>  
>>>
>>> Isn't it more common or better to just remove the flags field in
>>> 'struct page_pool_params' and pass the flags by parameter like
>>> below, so that patch 4 is not needed?
>>>
>>> struct page_pool *page_pool_create(const struct page_pool_params *params,
>>> 				   unsigned int	flags);
>>
>> You would need a separate patch to convert all the page_pool_create()
>> users then either way.
>> And it doesn't look really natural to me to pass both driver-set params
>> and driver-set flags as separate function arguments. Someone may then
>> think "why aren't flags just put in the params itself". The fact that
>> Page Pool copies the whole params in the page_pool struct after
>> allocating it is internals, page_pool_create() prototype however isn't.
>> Thoughts?
> 
> It just seems odd to me that dma_map and page_frag is duplicated as we
> seems to have the same info in the page_pool->p.flags.

It's just because we copy the whole &page_pool_params passed by the
driver. It doesn't look good to me to define a new structure and copy
the values field-by-field just to avoid duplicating 3 bits :s

> 
> What about:
> In [PATCH net-next 4/9] page_pool: shrink &page_pool_params a tiny bit,
> 'flags' is bit-field'ed with 'dma_dir', what about changing 'dma_dir'
> to be bit-field'ed with 'dma_sync_act', so that page_pool->p.flags stays
> the same as before, and 'dma_map' & 'page_frag' do not seems be really
> needed as we have the same info in page_pool->p.flags?

Not sure I follow :z ::dma_dir is also passed by the driver, so we can't
drop it from the params struct.

> 
> 
>>
>> Thanks,
>> Olek
>>
>> .
>>

Thanks,
Olek