Re: [PATCH net-next 3/8] net: ethernet: mtk_eth_soc: move MAX_DEVS in mtk_soc_data

[Lorenzo Bianconi]

> On Sun, Jun 11, 2023 at 01:35:17AM +0100, Daniel Golle wrote:
> > @@ -1106,14 +1105,14 @@ struct mtk_eth {
> >  	spinlock_t			tx_irq_lock;
> >  	spinlock_t			rx_irq_lock;
> >  	struct net_device		dummy_dev;
> > -	struct net_device		*netdev[MTK_MAX_DEVS];
> > -	struct mtk_mac			*mac[MTK_MAX_DEVS];
> > +	struct net_device		**netdev;
> > +	struct mtk_mac			**mac;
> >  	int				irq[3];
> >  	u32				msg_enable;
> >  	unsigned long			sysclk;
> >  	struct regmap			*ethsys;
> >  	struct regmap			*infra;
> > -	struct phylink_pcs		*sgmii_pcs[MTK_MAX_DEVS];
> > +	struct phylink_pcs		**sgmii_pcs;
> >  	struct regmap			*pctl;
> >  	bool				hwlro;
> >  	refcount_t			dma_refcnt;
> 
> Is it really worth the extra allocations?
> 
> There's three pointers here per device. Let's talk about modern systems,
> so that's 8 bytes each, and if MTK_MAX_DEVS was two, that's 48 bytes in
> all. If we expanded the array to allow three, that would be 72 bytes.
> 
> If we allocate separately, then we're allocating 16 or 24 bytes three
> times depending on whether we want two or three of them.
> 
> On arm64, I'm seeing the minimum slab size as 128 bytes, which means
> that's the minimum memory allocation. So, allocating three arrays will
> be 384 bytes in all, irrespective of whether we want two or three
> entries.
> 
> That's a waste of about 5x the memory over just expanding the arrays!

ack, I agree. I will fix it.

Regards,
Lorenzo

> 
> If you want to go down the route of dynamically allocating these, it
> would make better sense to combine them into a single structure that
> itself is an array, and thus requiring only one allocation. That
> reduces the wastage to about 56 bytes for three ports or 80 bytes
> for two.
> 
> Thanks.
> 
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