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

> > +++ 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@denx.de
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