Messages in this thread Patch in this message | | | Date | Wed, 7 Dec 2022 13:39:58 +0200 | From | Vladimir Oltean <> | Subject | Re: [PATCH net-next v3 1/2] dsa: lan9303: Add port_max_mtu API |
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On Tue, Dec 06, 2022 at 11:44:58PM +0000, Jerry.Ray@microchip.com wrote: > > > +/* For non-cpu ports, the max frame size is 1518. > > > + * The CPU port supports a max frame size of 1522. > > > + * There is a JUMBO flag to make the max size 2048, but this driver > > > + * presently does not support using it. > > > + */ > > > +static int lan9303_port_max_mtu(struct dsa_switch *ds, int port) > > > +{ > > > + struct net_device *p = dsa_port_to_master(dsa_to_port(ds, port)); > > > > You can put debugging prints in the code, but please, in the code that > > you submit, do remove gratuitous poking in the master net_device. > > > > > + struct lan9303 *chip = ds->priv; > > > + > > > + dev_dbg(chip->dev, "%s(%d) entered. NET max_mtu is %d", > > > + __func__, port, p->max_mtu); > > > + > > > + if (dsa_port_is_cpu(dsa_to_port(ds, port))) > > > > The ds->ops->port_max_mtu() function is never called for the CPU port. > > You must know this, you put a debugging print right above. If this would > > have been called for anything other than user ports, dsa_port_to_master() > > would have triggered a NULL pointer dereference (dp->cpu_dp is set to > > NULL for CPU ports). > > > > So please remove dead code. > > > > I've written the function to handle being called with any port. While I > couldn't directly exercise calling the port_max_mtu with the cpu port, I did > simulate it to verify it would work. > > I'm using the dsa_to_port() rather than the dsa_port_to_master() function.
No, you're using the dsa_to_port() *and* the dsa_port_to_master() functions. See? It's in the code you posted:
static int lan9303_port_max_mtu(struct dsa_switch *ds, int port) { struct net_device *p = dsa_port_to_master(dsa_to_port(ds, port)); ~~~~~~~~~~~~~~~~~~
> I'd rather include support for calling the api with the cpu port. I didn't > want to assume otherwise. That's why I don't consider this dead code. > > > > + return 1522 - ETH_HLEN - ETH_FCS_LEN; > > > + else > > > + return 1518 - ETH_HLEN - ETH_FCS_LEN; > > > > Please replace "1518 - ETH_HLEN - ETH_FCS_LEN" with "ETH_DATA_LEN". > > > > Which brings me to a more serious question. If you say that the max_mtu > > is equal to the default interface MTU (1500), and you provide no means > > for the user to change the MTU to a different value, then why write the > > patch? What behaves differently with and without it? > > > > I began adding the port_max_mtu api to attempt to get rid of the following > error message: > "macb f802c000.ethernet eth0: error -22 setting MTU to 1504 to include DSA overhead"
And how well did that go? That error message is saying that the macb driver (drivers/net/ethernet/cadence/macb_main.c) does not accept the MTU of 1504. Maybe because it doesn't have MACB_CAPS_JUMBO, I don't know. But this patch is clearly unrelated to the problem you've observed.
> If someone were to check the max_mtu supported on the CPU port of the LAN9303, > they would see that 1504 is okay.
No, they would not see that 1504 is okay. They would get a NULL pointer dereference in your function, if port_max_mtu() was ever called for a CPU port.
Don't believe me? You don't even have to. Please look at this patch, study it, run it, and see what happens with your port_max_mtu() implementation.
diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c index e5f156940c67..636e4b4df79a 100644 --- a/net/dsa/dsa.c +++ b/net/dsa/dsa.c @@ -473,6 +473,12 @@ static int dsa_port_setup(struct dsa_port *dp) break; dsa_port_enabled = true; + if (ds->ops->port_max_mtu) { + dev_info(ds->dev, "max MTU of CPU port %d is %d\n", + dp->index, + ds->ops->port_max_mtu(ds, dp->index)); + } + break; case DSA_PORT_TYPE_DSA: if (dp->dn) { The max_mtu of the CPU port is simply a question that the DSA core does not ask, so there's no reason to report it. How things are supposed to work is that the max_mtu of the user ports is propagated to their net_devices, and when the MTU of any user port is changed, the port_change_mtu() of that user port is called, and the maximum MTU of all user ports is recalculated and all CPU and DSA ports also get a port_change_mtu() call with that maximum value. If those ports need to program their hardware with something that also includes their tagging protocol overhead, they do so privately.
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