Re: [PATCH v3 1/3] dsa: marvell: Provide per device information about max frame size

From: Lukasz Majewski
Date: Thu Jan 05 2023 - 05:38:21 EST


Hi Andrew, Alexander,

> Hi Andrew,
>
> > > @@ -3548,7 +3548,9 @@ static int mv88e6xxx_get_max_mtu(struct
> > > dsa_switch *ds, int port) if
> > > (chip->info->ops->port_set_jumbo_size) return 10240 -
> > > VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN; else if
> > > (chip->info->ops->set_max_frame_size)
> > > - return 1632 - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN;
> > > + return (max_t(int, chip->info->max_frame_size,
> > > 1632)
> > > + - VLAN_ETH_HLEN - EDSA_HLEN -
> > > ETH_FCS_LEN); +
> > > return 1522 - VLAN_ETH_HLEN - EDSA_HLEN - ETH_FCS_LEN;
> > >
> >
> > I would also prefer if all this if/else logic is removed, and the
> > code simply returned chip->info->max_frame_size - VLAN_ETH_HLEN -
> > EDSA_HLEN - ETH_FCS_LEN;
> >
>
> So then the mv88e6xxx_get_max_mtu shall look like:
>
> WARN_ON_ONCE(!chip->info->max_frame_size)
>
> if (chip->info->ops->port_set_jumbo_size)
> ...
> else
> return chip->info->max_frame_size - VLAN_ETH_HLEN -
> EDSA_HLEN - ETH_FCS_LEN;
>
>
> Or shall I put WARN_ON_ONCE to the mv88e6xxx_probe() function?
>
>
> The above approach is contrary to one proposed by Alexander, who
> wanted to improve the defensive approach in this driver (to avoid
> situation where the max_frame_size callback is not defined and
> max_frame_size member of *_info struct is not added by developer).
>
> Which approach is the recommended one for this driver?

Is there any decision regarding the preferred approach to rewrite this
code?

>
> > > +++ b/drivers/net/dsa/mv88e6xxx/chip.h
> > > @@ -132,6 +132,7 @@ struct mv88e6xxx_info {
> > > unsigned int num_gpio;
> > > unsigned int max_vid;
> > > unsigned int max_sid;
> > > + unsigned int max_frame_size;
> >
> > It might be worth adding a comment here what this value actually
> > represents.
>
> Ok. I will add proper comment.
>
> > We don't want any mixups where the value already has the
> > frame checksum removed for example.
>
> Could you be more specific here about this use case?
>
> The max_frame_size is the maximal size of the ethernet frame for which
> the IC designer provided specified amount of RAM (it is a different
> value for different SoCs in the Link Street family).
>
> >
> > Andrew
>
>
> Best regards,
>
> Lukasz Majewski
>
> --
>
> DENX Software Engineering GmbH, Managing Director: Erika Unter
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email:
> lukma@xxxxxxx




Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH, Managing Director: Erika Unter
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@xxxxxxx

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