netdev - Re: [PATCH net-next 1/5] net: mvpp2: use the same buffer pool for all ports

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Date:   Mon, 5 Mar 2018 13:56:04 +0100
From:   Antoine Tenart <antoine.tenart@...tlin.com>
To:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        Stefan Chulski <stefanc@...vell.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, maxime.chevallier@...tlin.com,
        gregory.clement@...tlin.com, miquel.raynal@...tlin.com,
        nadavh@...vell.com, ymarkman@...vell.com, mw@...ihalf.com
Subject: Re: [PATCH net-next 1/5] net: mvpp2: use the same buffer pool for
 all ports

Hi Thomas,

On Mon, Mar 05, 2018 at 01:41:48PM +0100, Thomas Petazzoni wrote:
> On Mon, 5 Mar 2018 11:48:13 +0100, Antoine Tenart wrote:
> 
> > > > +static void mvpp2_setup_bm_pool(void)
> > > > +{
> > > > +	/* Short pool */
> > > > +	mvpp2_pools[MVPP2_BM_SHORT].buf_num  = MVPP2_BM_SHORT_BUF_NUM;
> > > > +	mvpp2_pools[MVPP2_BM_SHORT].pkt_size = MVPP2_BM_SHORT_PKT_SIZE;
> > > > +
> > > > +	/* Long pool */
> > > > +	mvpp2_pools[MVPP2_BM_LONG].buf_num  = MVPP2_BM_LONG_BUF_NUM;
> > > > +	mvpp2_pools[MVPP2_BM_LONG].pkt_size = MVPP2_BM_LONG_PKT_SIZE;
> > > > +}  
> > > 
> > >  ?  
> > 
> > I wanted to do this, but it's no possible as MVPP2_BM_SHORT_PKT_SIZE and
> > MVPP2_BM_LONG_PKT_SIZE use a core definition which expands at some point
> > to __max(...) which has to be called from within a function.
> 
> Hum, weird:
> 
> #define MVPP2_BM_LONG_PKT_SIZE	MVPP2_RX_MAX_PKT_SIZE(MVPP2_BM_LONG_FRAME_SIZE)
> #define MVPP2_BM_LONG_FRAME_SIZE		2048
> 
> #define MVPP2_RX_MAX_PKT_SIZE(total_size) \
>         ((total_size) - NET_SKB_PAD - MVPP2_SKB_SHINFO_SIZE)
> 
> #define MVPP2_SKB_SHINFO_SIZE \
>         SKB_DATA_ALIGN(sizeof(struct skb_shared_info))
> 
> #define SKB_DATA_ALIGN(X)        ALIGN(X, SMP_CACHE_BYTES)
> 
> I don't really see a __max(...) call.

NET_SKB_PAD expends to __max(...).

> And if this value really expands depending on other values, then it
> isn't really a constant, and should be considered as a constant, no?

Well, the padding isn't truly a constant for some reasons, you can see
an explanation here:
https://elixir.bootlin.com/linux/latest/source/include/linux/skbuff.h#L2446

But this value can only change at compile time, so it really depends on
what is the definition of a constant is :)

Antoine

-- 
Antoine Ténart, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com