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 08:47:25 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Ilias Apalodimas" <ilias.apalodimas@...aro.org>
Cc:     "Lorenzo Bianconi" <lorenzo.bianconi@...hat.com>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>,
        "Lorenzo Bianconi" <lorenzo@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net, thomas.petazzoni@...tlin.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

On 15 Nov 2019, at 0:03, Ilias Apalodimas wrote:

> Hi Lorenzo,
>
>>>>>>>>> How about using PP_FLAG_DMA_SYNC instead of another flag word?
>>>>>>>>> (then it can also be gated on having DMA_MAP enabled)
>>>>>>>>
>>>>>>>> You mean instead of the u8?
>>>>>>>> As you pointed out on your V2 comment of the mail, some cards 
>>>>>>>> don't
>>>>>>>> sync back to device.
>>>>>>>>
>>>>>>>> As the API tries to be generic a u8 was choosen instead of a 
>>>>>>>> flag
>>>>>>>> to cover these use cases. So in time we'll change the semantics 
>>>>>>>> of
>>>>>>>> this to 'always sync', 'dont sync if it's an skb-only queue' 
>>>>>>>> etc.
>>>>>>>>
>>>>>>>> The first case Lorenzo covered is sync the required len only 
>>>>>>>> instead
>>>>>>>> of the full buffer
>>>>>>>
>>>>>>> Yes, I meant instead of:
>>>>>>> +		.sync = 1,
>>>>>>>
>>>>>>> Something like:
>>>>>>>         .flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC
>>>>>>>
>>>>
>>>> I actually agree and think we could use a flag. I suggest
>>>> PP_FLAG_DMA_SYNC_DEV to indicate that this DMA-sync-for-device.
>>>>
>>>> Ilias notice that the change I requested to Lorenzo, that 
>>>> dma_sync_size
>>>> default value is 0xFFFFFFFF (-1).  That makes dma_sync_size==0 a 
>>>> valid
>>>> value, which you can use in the cases, where you know that nobody 
>>>> have
>>>> written into the data-area.  This allow us to selectively choose it 
>>>> for
>>>> these cases.
>>>
>>> Okay, then i guess the flag is a better fit for this.
>>> The only difference would be that the sync semantics will be done on 
>>> 'per
>>> packet' basis,  instead of 'per pool', but that should be fine for 
>>> our cases.
>>
>> Ack, fine for me.
>> Do you think when checking for PP_FLAG_DMA_SYNC_DEV we should even 
>> verify
>> PP_FLAG_DMA_MAP? Something like:
>>
>> if ((pool->p.flags & (PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV)) ==
>>     (PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV))
>> 	page_pool_dma_sync_for_device();
>>
>> Regards,
>> Lorenzo
>
> I think it's better to do the check once on the pool registration and 
> maybe
> refuse to allocate the pool? Syncing without mapping doesn't really 
> make sense

+1.
-- 
Jonathan