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:   Thu, 14 Nov 2019 12:27:40 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Ilias Apalodimas" <ilias.apalodimas@...aro.org>
Cc:     "Lorenzo Bianconi" <lorenzo@...nel.org>, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net,
        thomas.petazzoni@...tlin.com, brouer@...hat.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 14 Nov 2019, at 10:53, Ilias Apalodimas wrote:

> [...]
>>> index 2cbcdbdec254..defbfd90ab46 100644
>>> --- a/include/net/page_pool.h
>>> +++ b/include/net/page_pool.h
>>> @@ -65,6 +65,9 @@ struct page_pool_params {
>>>  	int		nid;  /* Numa node id to allocate from pages from */
>>>  	struct device	*dev; /* device, for DMA pre-mapping purposes */
>>>  	enum dma_data_direction dma_dir; /* DMA mapping direction */
>>> +	unsigned int	max_len; /* max DMA sync memory size */
>>> +	unsigned int	offset;  /* DMA addr offset */
>>> +	u8 sync;
>>>  };
>>
>> 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

Since .sync alone doesn't make sense if the page pool isn't performing 
any
DMA mapping, right?  Then existing drivers, if they're converted, can 
just
add the SYNC flag.

I did see the initial case where only the RX_BUF_SIZE (1536) is sync'd
instead of the full page.

Could you expand on your 'skb-only queue' comment?  I'm currently 
running
a variant of your patch where iommu mapped pages are attached to skb's 
and
sent up the stack, then reclaimed on release.  I imagine that with this
change, they would have the full RX_BUF_SIZE sync'd before returning to 
the
driver, since the upper layers could basically do anything with the 
buffer
area.
-- 
Jonathan