Subject: Re: [PATCH net-next 18/20] net: ethernet: qualcomm: Add PPE MAC support for phylink

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 22 Jan 2024 17:36:44 +0000

On Mon, Jan 22, 2024 at 11:01:26PM +0800, Lei Wei wrote:
> 
> 
> On 1/10/2024 8:18 PM, Russell King (Oracle) wrote:
> > On Wed, Jan 10, 2024 at 07:40:30PM +0800, Luo Jie wrote:
> > > @@ -352,6 +1230,12 @@ static int ppe_port_maxframe_set(struct ppe_device *ppe_dev,
> > >   }
> > >   static struct ppe_device_ops qcom_ppe_ops = {
> > > +	.phylink_setup = ppe_phylink_setup,
> > > +	.phylink_destroy = ppe_phylink_destroy,
> > > +	.phylink_mac_config = ppe_phylink_mac_config,
> > > +	.phylink_mac_link_up = ppe_phylink_mac_link_up,
> > > +	.phylink_mac_link_down = ppe_phylink_mac_link_down,
> > > +	.phylink_mac_select_pcs = ppe_phylink_mac_select_pcs,
> > >   	.set_maxframe = ppe_port_maxframe_set,
> > >   };
> > 
> > Why this extra layer of abstraction? If you need separate phylink
> > operations, why not implement separate phylink_mac_ops structures?
> > 
> 
> This PPE driver will serve as the base driver for higher level drivers
> such as the ethernet DMA (EDMA) driver and the DSA switch driver.

Why not have the higher level drivers provide a pointer to the
appropriate phylink_mac_ops structure? Having extra levels of
indirection makes my future maintenance of phylink harder (I'm already
bugged by DSA doing this, and it's a right pain.)

For example, if one of your higher level drivers needs the mac_prepare
or mac_finish functionality, you have to add a shim, extra function
pointers and so on.

If I need to add an extra parameter to a method, then I have to fix
up your shim layer _as well_ as all the called methods - in other
words, it adds extra maintenance burden.

It also makes detecting whether an implementation provides something
or not harder - see the problems when mac_select_pcs() was introduced
and rather than testing to see whether the method is populated, we
have to call the method with a dummy value to discover whether the
sub-driver implements it or not. Honestly, I would really like to get
rid of DSA's phylink_mac_ops shim layer.

-- 
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