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Date: Thu, 2 May 2024 09:05:53 +0200
From: Maxime Chevallier <maxime.chevallier@bootlin.com>
To: Heiner Kallweit <hkallweit1@gmail.com>
Cc: davem@davemloft.net, netdev@vger.kernel.org,
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Subject: Re: [PATCH net-next] net: phy: Don't conditionally compile the
 phy_link_topology creation
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Hi,

On Tue, 30 Apr 2024 23:29:37 +0200
Heiner Kallweit <hkallweit1@gmail.com> wrote:

> On 30.04.2024 13:57, Maxime Chevallier wrote:
> > Hello Heiner,
> > 
> > On Tue, 30 Apr 2024 10:17:31 +0200
> > Heiner Kallweit <hkallweit1@gmail.com> wrote:
> >   
> >> On 29.04.2024 15:10, Maxime Chevallier wrote:  
> >>> The core of the phy_link_topology isn't directly tied to phylib, and at
> >>> the moment it's initialized, phylib might not be loaded yet. Move the
> >>> initialization of the topology to the phy_link_topology_core header,
> >>> which contains the bare minimum so that we can initialize it at netdev
> >>> creation.
> >>>     
> >>
> >> The change fixes the issue for me, but according to my personal taste
> >> the code isn't intuitive and still error-prone. Also there's no good
> >> reason to inline a function like phy_link_topo_create() and make it
> >> publicly available. Do you expect it to be ever used outside net core?
> >> In general it may make sense to add a config symbol for the topology
> >> extension, there seem to be very few, specialized use cases for it.  
> > 
> > I think I'm missing the point here then. Do you mean adding a Kconfig
> > option to explicitely turn phy_link_topology on ? or build it as a
> > dedicated kernel module ?
> > 
> > Or do you see something such as "if phylib is M or Y, then build the
> > topology stuff and make sure it's allocated when a netdev gets created
> > ?"
> > 
> > Thanks,
> > 
> > Maxime
> >   
> >>  
> >>> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> >>> Closes: https://lore.kernel.org/netdev/2e11b89d-100f-49e7-9c9a-834cc0b82f97@gmail.com/
> >>> Closes: https://lore.kernel.org/netdev/20240409201553.GA4124869@dev-arch.thelio-3990X/
> >>> ---
> >>>  drivers/net/phy/phy_link_topology.c    | 23 --------------------
> >>>  include/linux/phy_link_topology.h      |  5 -----
> >>>  include/linux/phy_link_topology_core.h | 30 +++++++++++++++++---------
> >>>  3 files changed, 20 insertions(+), 38 deletions(-)
> >>>
> >>> diff --git a/drivers/net/phy/phy_link_topology.c b/drivers/net/phy/phy_link_topology.c
> >>> index 985941c5c558..960aedd73308 100644
> >>> --- a/drivers/net/phy/phy_link_topology.c
> >>> +++ b/drivers/net/phy/phy_link_topology.c
> >>> @@ -12,29 +12,6 @@
> >>>  #include <linux/rtnetlink.h>
> >>>  #include <linux/xarray.h>
> >>>  
> >>> -struct phy_link_topology *phy_link_topo_create(struct net_device *dev)
> >>> -{
> >>> -	struct phy_link_topology *topo;
> >>> -
> >>> -	topo = kzalloc(sizeof(*topo), GFP_KERNEL);
> >>> -	if (!topo)
> >>> -		return ERR_PTR(-ENOMEM);
> >>> -
> >>> -	xa_init_flags(&topo->phys, XA_FLAGS_ALLOC1);
> >>> -	topo->next_phy_index = 1;
> >>> -
> >>> -	return topo;
> >>> -}
> >>> -
> >>> -void phy_link_topo_destroy(struct phy_link_topology *topo)
> >>> -{
> >>> -	if (!topo)
> >>> -		return;
> >>> -
> >>> -	xa_destroy(&topo->phys);
> >>> -	kfree(topo);
> >>> -}
> >>> -
> >>>  int phy_link_topo_add_phy(struct phy_link_topology *topo,
> >>>  			  struct phy_device *phy,
> >>>  			  enum phy_upstream upt, void *upstream)
> >>> diff --git a/include/linux/phy_link_topology.h b/include/linux/phy_link_topology.h
> >>> index 6b79feb607e7..ad72d7881257 100644
> >>> --- a/include/linux/phy_link_topology.h
> >>> +++ b/include/linux/phy_link_topology.h
> >>> @@ -32,11 +32,6 @@ struct phy_device_node {
> >>>  	struct phy_device *phy;
> >>>  };
> >>>  
> >>> -struct phy_link_topology {
> >>> -	struct xarray phys;
> >>> -	u32 next_phy_index;
> >>> -};
> >>> -
> >>>  static inline struct phy_device *
> >>>  phy_link_topo_get_phy(struct phy_link_topology *topo, u32 phyindex)
> >>>  {
> >>> diff --git a/include/linux/phy_link_topology_core.h b/include/linux/phy_link_topology_core.h
> >>> index 0a6479055745..0116ec49cd1b 100644
> >>> --- a/include/linux/phy_link_topology_core.h
> >>> +++ b/include/linux/phy_link_topology_core.h
> >>> @@ -2,24 +2,34 @@
> >>>  #ifndef __PHY_LINK_TOPOLOGY_CORE_H
> >>>  #define __PHY_LINK_TOPOLOGY_CORE_H
> >>>  
> >>> -struct phy_link_topology;
> >>> +#include <linux/xarray.h>
> >>>  
> >>> -#if IS_REACHABLE(CONFIG_PHYLIB)
> >>> -
> >>> -struct phy_link_topology *phy_link_topo_create(struct net_device *dev);
> >>> -void phy_link_topo_destroy(struct phy_link_topology *topo);
> >>> -
> >>> -#else
> >>> +struct phy_link_topology {
> >>> +	struct xarray phys;
> >>> +	u32 next_phy_index;
> >>> +};
> >>>  
> >>>  static inline struct phy_link_topology *phy_link_topo_create(struct net_device *dev)
> >>>  {
> >>> -	return NULL;
> >>> +	struct phy_link_topology *topo;
> >>> +
> >>> +	topo = kzalloc(sizeof(*topo), GFP_KERNEL);
> >>> +	if (!topo)
> >>> +		return ERR_PTR(-ENOMEM);
> >>> +
> >>> +	xa_init_flags(&topo->phys, XA_FLAGS_ALLOC1);
> >>> +	topo->next_phy_index = 1;
> >>> +
> >>> +	return topo;
> >>>  }
> >>>  
> >>>  static inline void phy_link_topo_destroy(struct phy_link_topology *topo)
> >>>  {
> >>> -}
> >>> +	if (!topo)
> >>> +		return;
> >>>  
> >>> -#endif
> >>> +	xa_destroy(&topo->phys);
> >>> +	kfree(topo);
> >>> +}
> >>>  
> >>>  #endif /* __PHY_LINK_TOPOLOGY_CORE_H */    
> >>  
> >   
> 
> To go a little bit more into detail:
> 
> phy_link_topo_create() and phy_link_topo_destroy() are used in net/core/dev.c
> only. Do you expect them to be ever used by other callers?
> If not, their functionality could be moved to net/core/dev.c.
> Supposedly guarded by IS_ENABLED(CONFIG_PHYLIB), alternatively a new config
> symbol for link_topo support.
> 
> To get rid of the dependency you could also lazy-inizialize
> netdev->link_topo. For this phy_link_topo_add_phy() would have
> to take the netdev as first argument, not the topo.
> Then the first call to phy_link_topo_add_phy() would initialize
> netdev->link_topo.
> 
> I think functions like phy_link_topo_get_phy() should also check for
> topo != NULL first, maybe combined with a WARN_ON().
> They are exported and you have no control over its use.

Thanks Heiner for the explanations. I'll rework based on that. The
original reason I didn't directly include the netdev as a parameter for
these function, or didn't put any helper in net/core/dev.c is because I
wanted to avoid too strong of a link between the topology and netdev.

There are some PHYs for which we can't assign any netdev (PHYs that
would sit in-between 2 chained DSA switches is the only example I have
in mind though), but TBH the code in its actual shape doesn't address
these either, so it's a useless design constraint.

So, let me indeed do that, it will probably make for a simpler and more
straightforward design.

Thanks for the input,

Maxime