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 10:03:52 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jonathan Lemon <jonathan.lemon@...il.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

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

Cheers
/Ilias
> 
> > 
> > Cheers
> > /Ilias
> > > 
> > > -- 
> > > Best regards,
> > >   Jesper Dangaard Brouer
> > >   MSc.CS, Principal Kernel Engineer at Red Hat
> > >   LinkedIn: http://www.linkedin.com/in/brouer
> > > 
> >