netdev - Re: [PATCH net-next v2 0/8] net: extend ethtool link mode bitmaps to 48 bits

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Date:	Tue, 6 Jan 2015 09:36:36 -0800
From:	David Decotigny <ddecotig@...il.com>
To:	Amir Vadai <amirv@...lanox.com>
Cc:	Florian Fainelli <f.fainelli@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	Saeed Mahameed <saeedm@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ben Hutchings <ben@...adent.org.uk>,
	Masatake YAMATO <yamato@...hat.com>, Xi Wang <xii@...gle.com>,
	Neil Horman <nhorman@...driver.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Flavio Leitner <fbl@...hat.com>, Tom Gundersen <teg@...m.no>,
	Jiri Pirko <jiri@...nulli.us>,
	Vlad Yasevich <vyasevic@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Venkata Duvvuru <VenkatKumar.Duvvuru@...lex.com>,
	Govindarajulu Varadarajan <_govind@....com>
Subject: Re: [PATCH net-next v2 0/8] net: extend ethtool link mode bitmaps to
 48 bits

Interesting. It seems that the band-aid I was proposing is already
obsolete. We could still use the remaining reserved 16 bits to encode
5 more bits per mask (that is: 53 bits / mask total). But if I
understand you, it would allow us to survive only a few months longer,
as opposed to a few weeks.

One short-term alternative solution I can imagine is the following:
/* For example bitmap-based for variable length: */
struct ethtool_link_mode {
  __u32 cmd;
  __u8 autoneg :1;
  __u8 duplex :2;
 __u16 supported_nbits;
  __u16 advertising_nbits;
  __u16 lp_advertising_nbits;
  __u32 reserved[4];
  __u8 masks[0];
};
/* Or simpler, statically limited to 64b / mask, but easier to migrate
to for driver authors: */
struct ethtool_link_mode {
  __u32 cmd;
  __u8 autoneg :1;
  __u8 duplex :2;
   __u64 supported;
  __u64 advertising;
  __u64 lp_advertising;
  __u32 reserved[4];
};
#define ETHTOOL_GLINK_MODE 0x0000004a
#define ETHTOOL_SLINK_MODE 0x0000004b
struct ethtool_ops {
...
   int (*get_link_mode)(struct net_device *, struct ethtool_link_mode *);
   int (*set_link_mode)(struct net_device *, struct ethtool_link_mode *);
};

The same thing required for EEE.

I am not sure about moving the autoneg and duplex fields into the new
struct. Especially the "duplex" field.

Then the idea would be to update the ethtool user-space tool to try
get/set_link mode when reporting/changing the autoneg/advertising
settings.

Both will require significant effort from the driver authors.
Especially if the variable-length bitmap approach is preferred:
 - most drivers currently use simple bitwise arithmetic in their code,
and that goes far beyond get/set_settings, it is sometimes part of the
core driver logic. They will have to migrate to the bitmap API if they
want to use the larger bitmaps (note: no change needed if they are
happy with <= 32b / mask)
 - we would have to progressively deprecate the use of #define
ADVERTISED_1000baseT_Full in favor of an enum of the bit indices.

Any feedback welcome. In the meantime, I am going to propose a v3 of
current option with 53 bits / mask. I can also propose a prototype of
the scheme described above, please let me know.

On Tue, Jan 6, 2015 at 5:56 AM, Amir Vadai <amirv@...lanox.com> wrote:
> On 1/6/2015 4:54 AM, David Decotigny wrote:
>> This patch series increases the width of the supported/advertising
>> ethtool masks from 32 bits to 48. This should allow to breathe for a
>> couple more years (or... months?).
> Hi,
>
> Nice work.
> What worries me is that it is going to be for weeks more than years or
> months...
>
> Mellanox is about to release next month a driver for a new NIC, with 3
> new speeds * few link modes for each + new link modes for 10G.
> It seems that we will need to consume almost all the new bits.
>
> Amir
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