Re: [PATCH net-next 1/4] net: mvneta: Convert to be 64 bits compatible

[Jisheng Zhang]

Hi Marcin, Gregory, Arnd,

On Wed, 23 Nov 2016 17:02:11 +0100 Marcin Wojtas  wrote:

> Hi Gregory,
> 
> 2016-11-23 14:07 GMT+01:00 Gregory CLEMENT:
> > Hi Jisheng, Arnd,
> >
> >
> > Thanks for your feedback.
> >
> >
> >  On mer., nov. 23 2016, Arnd Bergmann wrote:
> >  
> >> On Wednesday, November 23, 2016 5:53:41 PM CET Jisheng Zhang wrote:  
> >>> On Tue, 22 Nov 2016 22:04:12 +0100 Arnd Bergmann wrote:
> >>>  
> >>> > On Tuesday, November 22, 2016 5:48:41 PM CET Gregory CLEMENT wrote:  
> >>> > > +#ifdef CONFIG_64BIT
> >>> > > +       void *data_tmp;
> >>> > > +
> >>> > > +       /* In Neta HW only 32 bits data is supported, so in order to
> >>> > > +        * obtain whole 64 bits address from RX descriptor, we store
> >>> > > +        * the upper 32 bits when allocating buffer, and put it back
> >>> > > +        * when using buffer cookie for accessing packet in memory.
> >>> > > +        * Frags should be allocated from single 'memory' region,
> >>> > > +        * hence common upper address half should be sufficient.
> >>> > > +        */
> >>> > > +       data_tmp = mvneta_frag_alloc(pp->frag_size);
> >>> > > +       if (data_tmp) {
> >>> > > +               pp->data_high = (u64)upper_32_bits((u64)data_tmp) << 32;
> >>> > > +               mvneta_frag_free(pp->frag_size, data_tmp);
> >>> > > +       }
> >>> > >  
> >>> >
> >>> > How does this work when the region spans a n*4GB address boundary?  
> >>>
> >>> indeed. We also make use of this driver on 64bit platforms. We use
> >>> different solution to make the driver 64bit safe.
> >>>
> >>> solA: make use of the reserved field in the mvneta_rx_desc, such
> >>> as reserved2 etc. Yes, the field is marked as "for future use, PnC", but
> >>> now it's not used at all. This is one possible solution however.  
> >>
> >> Right, this sounds like the most straightforward choice.  
> >
> > The PnC (which stands for Parsing and Classification) is not used yet
> > indeed but this field will be needed when we will enable it. It is
> > something we want to do but it is not planned in a near future. However
> > from the datasheets I have it seems only present on the Armada XP. It is
> > not mentioned on datasheets for the Armada 38x or the Armada 3700.
> >  
> 
> It is not mentioned in A38x spec, but this SoC has exactly the same
> PnC as Armada XP (they differ only with used SRAM details). I wouldn't
> be surprised if it was supported on A3700 as well.
> 
> > That would mean it was safe to use on of this field in 64-bits mode on
> > the Armada 3700.
> >
> > So I am going to take this approach.
> >  
> 
> I think for now it's safe and is much easier than handling extra
> software ring for virtual addresses.
> 

solB (a SW shadow cookie) perhaps gives a better performance: in hot path,
such as mvneta_rx(), the driver accesses buf_cookie and buf_phys_addr of
rx_desc which is allocated by dma_alloc_coherent, it's noncacheable if the
device isn't cache-coherent. I didn't measure the performance difference,
because in fact we take solA as well internally. From your experience,
can the performance gain deserve the complex code?

Thanks,
Jisheng