Message-ID: <20150921171357.GA11812@ketchup.mtl.sfl>
Date:	Mon, 21 Sep 2015 13:13:58 -0400
From:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
To:	Jiri Pirko <jiri@...nulli.us>
Cc:	Scott Feldman <sfeldma@...il.com>, Netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Ido Schimmel <idosch@...lanox.com>, eladr@...lanox.com,
	Florian Fainelli <f.fainelli@...il.com>,
	Guenter Roeck <linux@...ck-us.net>
Subject: Re: [patch net-next RFC 0/6] switchdev: introduce tranction enfra
 and for pre-commit split

Hi Jiri, Scott,

On Sep. Monday 21 (39) 10:09 AM, Jiri Pirko wrote:
> Mon, Sep 21, 2015 at 09:23:24AM CEST, sfeldma@...il.com wrote:
> >On Sat, Sep 19, 2015 at 5:29 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> >> Jiri Pirko (6):
> >>   switchdev: rename "trans" to "trans_ph".
> >>   switchdev: introduce transaction infrastructure for attr_set and
> >>     obj_add
> >>   rocker: switch to local transaction phase enum
> >>   switchdev: move transaction phase enum under transaction structure
> >>   rocker: use switchdev transaction queue for allocated memory
> >>   switchdev: split commit and prepare phase into two callbacks
> >
> >Patches compile, but first test bombs.  Cut-and-paste of dump at end
> >of this email.
> 
> Told you :)
> 
> 
> >
> >I'm not sure I'm liking this patchset because it looks like a way for
> >switchdev drivers to easily opt-out of the prepare-commit transaction
> >model by simply not implementing the *_pre op.  I would rather drivers
> >explicitly handle the PREPARE phase in code, even if that means
> >skipping it gracefully (in code) with a comment (in code) explaining
> >why it does not matter for this device/operation.  That's what DSA had
> >done, mostly because it was a retro-fit.
> 
> Each driver should handle this inside it. If it does not need prepare
> state, it simply does not implement it. That is the same for all cb,
> ndos, netdev notifiers, etc. It is much cleaner and nicer to have these as
> separate callbacks. Implementing multiple callback in one is just ugly,
> sorry.

This is true, (in DSA) we don't have to implement the prepare phase if
we fully support the feature in hardware.

To give a real example, Marvell switch drivers currently implement all
add/del/dump calls for VLAN FDB (where VID 0 means the port itself). No
prepare phase needed.

Now, I have local patches to enable strict 802.1Q mode in these switches
(all the logic is based on the hardware VLAN table). But it does not use
per-port FDB, so fdb_add with VID 0 doesn't make sense anymore. That's
why we need to push the feature checking down to the drivers in DSA.

I have another pending patch to add .port_fdb_pre_add, where mv88e6xxx
code will return -EOPNOTSUPP if the given VID is 0.

Another example: mv88e6xxx support tagged VLANs, so no hardware check
needed. But the Broadcom Starfighter 2 only supports port-based VLANs
(which is today wrongly implemented through "bridge_join/leave"). By
implementing .port_vlan_pre_add (another pending patch for DSA), the
driver will be able to return -EOPNOTSUPP if !BRIDGE_VLAN_INFO_PVID.

Also, having logic in switchdev drivers to check SWITCHDEV_TRANS_NONE
and SWITCHDEV_TRANS_ABORT is not really nice. Having switchdev handle
the abort phase (calling each destructor) and getting rid of the
SWITCHDEV_TRANS_* flags sounds better to me.

> >Also, the patchset removes the ABORT callback in case of a rollback
> >due to a failed PREPARE.  We can't make the assumption that it's just
> >a memory list to destroy on ABORT.  The driver, on PREPARE, may have
> >reserved device space or staged an operation on the device which we'll
> >need to undo on ABORT.
> 
> Yep, just register an item with custom destructor, there you can do
> whatever. Also, I believe much nicer comparing to current code.
> 
> 
> >
> >So we need ABORT back, and we need PREPARE to not be optional, so
> >what's left list enqueue/dequeue helpers, which I'm not seeing much
> >value in up-leveling as the driver can do list_add/del itself.
> 
> Why would every driver do it itself, over and over when there can be a
> clean infrastructure to do that. Including abort phase. Without the driver
> needed to be involved.

Maybe the term ".destructor" has a too strong meaning to deallocation,
but you can indeed do whatever you need in this function.

> >Am I missing something?  I didn't see a motivation statement for the
> >RFC so I'm not sure where you wanted to take this.
> 
> I want to make current code much nicer, easier to read and implement in
> other drivers. Look at rocker.c and how often there is == PREPARE there.
> It's nearly impossible to followthe code, sorry.
> 
> My next patchset is to un-mess rocker.c (that freaking ofdpa stuff is
> everywhere)

I'm basically for this patchset, but I do have some concerns about the
general switchdev transaction design.

I certainly don't have the perspective that you guys have, so please
tell me if I'm totally wrong. From my point of view (with DSA drivers),
the emphase should be done on asking the hardware if it can handle or
not a given transaction (not simply an hardware feature!).

The driver can handle any allocation and preparation itself, and also
errors can actually occur *during* a commit phase.

Being able to return -EOPNOTSUPP from any add/del/dump function would be
just perfect.

I ideally imagine the following implementation in switchdev drivers:

    int foo_port_obj_add(struct net_device *dev, int id, void *obj)
    {
        struct foo *foo = netdev_priv(dev);
	struct switchdev_obj_port_fdb *fdb;
	struct switchdev_obj_port_vlan *vlan;
        int err;

        switch(id) {
	case SWITCHDEV_OBJ_PORT_FDB:
	    fdb = obj;
	    err = foo_port_fdb_add(foo, fdb);
	    break;
        case SWITCHDEV_OBJ_PORT_VLAN:
	    vlan = obj;
	    err = foo_port_vlan_add(foo, vlan);
	    break;
	default:
	    err = -EOPNOTSUPP;
	    break;
	}

	return err;
    }

Where foo_port_{fdb,vlan}_add can still return -EOPNOTSUPP depending on
their current state.

Thanks,
-v
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