Re: [net-next RFC PATCH 1/4] net: dsa: qca8k: drop qca8k_read/write/rmw for regmap variant

Christian Marangi
2022-07-18T19:48:10Z

On Mon, Jul 18, 2022 at 10:35:21PM +0300, Vladimir Oltean wrote:
> On Mon, Jul 18, 2022 at 08:40:14PM +0200, Christian Marangi wrote:
> > > I don't really have a preference, I just want to understand why you want
> > > to call regmap_read(priv->regmap) directly every time as opposed to
> > > qca8k_read(priv) which is shorter to type and allows more stuff to fit
> > > on one line.
> >
> > The main reason is that it's one less function. qca8k_read calls
> > directly the regmap ops so it seems a good time to drop it.
>
> This is before applying your patch 1/4, with an armv7 compiler:
> make drivers/net/dsa/qca/qca8k.lst
>
> I'm looking at the qca8k_read() call from qca8k_pcs_get_state():
>
> 000009d8 <qca8k_pcs_get_state>:
> {
> 9d8: e92d4030 push {r4, r5, lr}
> 9dc: e3005000 movw r5, #0
> 9dc: R_ARM_MOVW_ABS_NC __stack_chk_guard
> ret = qca8k_read(priv, QCA8K_REG_PORT_STATUS(port), &reg);
> 9e0: e590300c ldr r3, [r0, #12]
> {
> 9e4: e3405000 movt r5, #0
> 9e4: R_ARM_MOVT_ABS __stack_chk_guard
> 9e8: e24dd00c sub sp, sp, #12
> 9ec: e1a04001 mov r4, r1
> return regmap_read(priv->regmap, reg, val);
> 9f0: e5900008 ldr r0, [r0, #8]
> 9f4: e1a0200d mov r2, sp
> {
> 9f8: e595c000 ldr ip, [r5]
> ret = qca8k_read(priv, QCA8K_REG_PORT_STATUS(port), &reg);
> 9fc: e283101f add r1, r3, #31
> return regmap_read(priv->regmap, reg, val);
> a00: e1a01101 lsl r1, r1, #2
> a04: e5900010 ldr r0, [r0, #16]
> {
> a08: e58dc004 str ip, [sp, #4]
> return regmap_read(priv->regmap, reg, val);
> a0c: ebfffffe bl 0 <regmap_read>
> a0c: R_ARM_CALL regmap_read
> (portions irrelevant to regmap cut out)
>
> And this is how it looks like after applying your patch 1/4:
>
> 000009d8 <qca8k_pcs_get_state>:
> {
> 9d8: e92d4030 push {r4, r5, lr}
> 9dc: e3005000 movw r5, #0
> 9dc: R_ARM_MOVW_ABS_NC __stack_chk_guard
> ret = regmap_read(priv->regmap, QCA8K_REG_PORT_STATUS(port), &reg);
> 9e0: e590300c ldr r3, [r0, #12]
> {
> 9e4: e3405000 movt r5, #0
> 9e4: R_ARM_MOVT_ABS __stack_chk_guard
> 9e8: e24dd00c sub sp, sp, #12
> 9ec: e1a04001 mov r4, r1
> ret = regmap_read(priv->regmap, QCA8K_REG_PORT_STATUS(port), &reg);
> 9f0: e5900008 ldr r0, [r0, #8]
> 9f4: e1a0200d mov r2, sp
> {
> 9f8: e595c000 ldr ip, [r5]
> ret = regmap_read(priv->regmap, QCA8K_REG_PORT_STATUS(port), &reg);
> 9fc: e283101f add r1, r3, #31
> a00: e1a01101 lsl r1, r1, #2
> a04: e5900010 ldr r0, [r0, #16]
> {
> a08: e58dc004 str ip, [sp, #4]
> ret = regmap_read(priv->regmap, QCA8K_REG_PORT_STATUS(port), &reg);
> a0c: ebfffffe bl 0 <regmap_read>
> a0c: R_ARM_CALL regmap_read
>
> You don't even need to recognize the instructions or calling conventions
> to figure out that the generated assembly code is identical.
>
> > >
> > > I think if you run "make drivers/net/dsa/qca/qca8k.lst" and you look at
> > > the generated code listing before and after, you'll find it is identical
> > > (note, I haven't actually done that).
> > >
> > > > An alternative is to keep them for qca8k specific code and migrate the
> > > > common function to regmap api.
> > >
> > > No, that's silly and I can't even find a reason to do that.
> > > It's not like you're trying to create a policy to not call qca8k-common.c
> > > functions from qca8k-8xxx.c, right? That should work just fine (in this
> > > case, qca8k_read etc).
> >
> > The idea of qca8k-common is to keep them as generilized as possible.
> > Considering ipq4019 will have a different way to write/read regs we can't
> > lock common function to specific implementation.
>
> Wait a minute, what's the difference between having this in common.c:
>
> qca8k_read(priv)
>
> vs this:
>
> regmap_read(priv->regmap)
>
> when qca8k_read is implemented *exactly* as a call to regmap_read(priv->regmap)?
> There's nothing *specific* to a switch in the implementation of qca8k_read().
> But rather, all differences lie in the regmap_config structure and in
> the way the regmap was created. But the common code operates with a
> pointer to a generic regmap structure, regardless of how that was created.
>
> So no, sorry, there is no technical argument for which you cannot have
> calls to qca8k_read() in common.c. I can work with "that's the way I prefer",
> but let's not try to invent technical arguments when there aren't any.
>
> > > In fact, while typing this I realized that in your code structure,
> > > you'll have one struct dsa_switch_ops in qca8k-8xxx.c and another one in
> > > qca8k-ipq4019.c. But the vast majority of dsa_switch_ops are common,
> > > with the exception of .setup() which is switch-specific, correct?
> >
> > Phylink ops will also be different as ipq4019 will have qsgmii and will
> > require some calibration logic.
>
> Ok, phylink too, the point is that they aren't radically different switches
> for the majority of operations.
>
> > qca8k_setup will require major investigation and I think it would be
> > better to do do a qca8k_setup generalization when ipq4019 will be
> > proposed.
>
> Ok, "major investigation" sounds about right, that's what I was looking
> to hear. The alternative would have been to plop a separate ipq4019_setup(),
> leave qca8k_setup() alone, and call it a day. FWIW, that's essentially
> where the microchip ksz set of drivers were, before Arun Ramadoss
> started doing some major cleanup through them. After some point, this
> strategy simply stops scaling.
>
> > On the other hand I like the idea of putting the qca8k ops in common.c
> > and make the driver adds the relevant specific options.
> > Think I will also move that to common.c. That would permit to keep
> > function static aka even less delta and less bloat in the header file.
> >
> > (is it a problem if it won't be const?)
>
> yeah, it's a problem if it won't be const, why wouldn't it?
>

Tell me if I got this wrong.

The suggestion was to move the struct dsa_switch_ops to qca8k.h and add
in the specific code probe the needed ops to add to the generic
struct...

But now that I think about it I was just confused and it can totally be
const so sorry for the stupid question

Anyway to make it clear, these are the option that will be the same for
qca8k and ipq4019

.get_strings = qca8k_get_strings,
.get_ethtool_stats = qca8k_get_ethtool_stats,
.get_sset_count = qca8k_get_sset_count,
.set_ageing_time = qca8k_set_ageing_time,
.get_mac_eee = qca8k_get_mac_eee,
.set_mac_eee = qca8k_set_mac_eee,
.port_enable = qca8k_port_enable,
.port_disable = qca8k_port_disable,
.port_change_mtu = qca8k_port_change_mtu,
.port_max_mtu = qca8k_port_max_mtu,
.port_stp_state_set = qca8k_port_stp_state_set,
.port_bridge_join = qca8k_port_bridge_join,
.port_bridge_leave = qca8k_port_bridge_leave,
.port_fast_age = qca8k_port_fast_age,
.port_fdb_add = qca8k_port_fdb_add,
.port_fdb_del = qca8k_port_fdb_del,
.port_fdb_dump = qca8k_port_fdb_dump,
.port_mdb_add = qca8k_port_mdb_add,
.port_mdb_del = qca8k_port_mdb_del,
.port_mirror_add = qca8k_port_mirror_add,
.port_mirror_del = qca8k_port_mirror_del,
.port_vlan_filtering = qca8k_port_vlan_filtering,
.port_vlan_add = qca8k_port_vlan_add,
.port_vlan_del = qca8k_port_vlan_del,
.port_lag_join = qca8k_port_lag_join,
.port_lag_leave = qca8k_port_lag_leave,

Everything else has to be set by the driver. So yes the amount of shared
function is enough to have a reason to declare a general dsa_switch_ops.

> > > If I were to summarize your reason, it would be "because I prefer it
> > > that way and because now is a good time", right? That's fine with me,
> > > but I honestly didn't understand that while reading the commit message.
> >
> > I have to be honest... Yes you are right... This is really my opinion
> > and I don't have a particular strong reason on why dropping them.
> >
> > It's really that I don't like keeping function that are just leftover of
> > an old implementation. But my target here is not argue and find a
> > solution so it's OK for me if I should keep these compat function and
> > migrate them to common.c.
>
> I know that the revolutionary spirit can be strong, but it's good to keep
> in mind that "older/newer" is not always synonymous with "worse/better" ;)
>
> Again, I don't have a strong objection against the change and I'm not
> going to argue about it either. My comment was simply because I didn't
> physically UNDERSTAND you. My expectations were also a bit confused,
> because I initially thought it's a necessary change (that's why I
> replied to it last), and I just didn't understand what's so necessary
> about it.

Ok at the end I probably chose the wrong words since this is really a
cleanup and nothing necessary for the function of the code split.

Will send a v2 with RFC removed and a more friendly series hoping it
won't grow too much with the code split.

Again thanks for the all the good review :D

--
Ansuel