linux-kernel - Re: [PATCH v3 net-next] net: vlan: Use vlan_prio instead of vlan

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Message-ID: <4275a6d14b8e209331251fa7a3a1f3094ee60915@linux.dev>
Date: Tue, 22 Oct 2024 01:48:03 +0000
From: "Yajun Deng" <yajun.deng@...ux.dev>
To: "Guillaume Nault" <gnault@...hat.com>, "Ido Schimmel" <idosch@...sch.org>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net-next] net: vlan: Use vlan_prio instead of vlan_qos
 in mapping

October 22, 2024 at 12:27 AM, "Guillaume Nault" <gnault@...hat.com> wrote:



> 
> On Sun, Oct 20, 2024 at 03:29:21PM +0300, Ido Schimmel wrote:
> 
> > 
> > On Fri, Oct 18, 2024 at 10:12:33PM +0800, Yajun Deng wrote:
> > 
> >  The vlan_qos member is used to save the vlan qos, but we only save the
> > 
> >  priority. Also, we will get the priority in vlan netlink and proc.
> > 
> >  We can just save the vlan priority using vlan_prio, so we can use vlan_prio
> > 
> >  to get the priority directly.
> > 
> >  
> > 
> >  For flexibility, we introduced vlan_dev_get_egress_priority() helper
> > 
> >  function. After this patch, we will call vlan_dev_get_egress_priority()
> > 
> >  instead of vlan_dev_get_egress_qos_mask() in irdma.ko and rdma_cm.ko.
> > 
> >  Because we don't need the shift and mask operations anymore.
> > 
> >  
> > 
> >  There is no functional changes.
> > 
> >  
> > 
> >  Not sure I understand the motivation.
> > 
> >  
> > 
> >  IIUC, currently, struct vlan_priority_tci_mapping::vlan_qos is shifted
> > 
> >  and masked in the control path (vlan_dev_set_egress_priority) so that
> > 
> >  these calculations would not need to be performed in the data path where
> > 
> >  the VLAN header is constructed (vlan_dev_hard_header /
> > 
> >  vlan_dev_hard_start_xmit).
> > 
> >  
> > 
> >  This patch seems to move these calculations to the data path so that
> > 
> >  they would not need to be performed in the control path when dumping the
> > 
> >  priority mapping via netlink / proc.
> > 
> >  
> > 
> >  Why is it a good trade-off?
> > 
> 
> I agree with Ido. The commit description doesn't explain why these
> 
> changes are made and I also can't see how this patch can improve
> 
> performances.
> 
> If it's about code readability, why not just add a helper that gets a
> 
> struct vlan_priority_tci_mapping pointer as input and returns a __u8
> 
> corresponding to the priority? This way, the /proc and netlink handlers
> 
> (and other potential users) wouldn't have to do the bit shifting and
> 
> masking manually.
>

Okay, that's a better way.