netdev - Re: [PATCH ethtool] ethtool: dump nested registers

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Date:   Mon, 5 Aug 2019 10:52:16 -0400
From:   Vivien Didelot <vivien.didelot@...il.com>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     netdev@...r.kernel.org, f.fainelli@...il.com, andrew@...n.ch,
        davem@...emloft.net, linville@...hat.com, cphealy@...il.com
Subject: Re: [PATCH ethtool] ethtool: dump nested registers

Hi Michal!

On Mon, 5 Aug 2019 10:04:48 +0200, Michal Kubecek <mkubecek@...e.cz> wrote:
> On Fri, Aug 02, 2019 at 03:34:54PM -0400, Vivien Didelot wrote:
> > Usually kernel drivers set the regs->len value to the same length as
> > info->regdump_len, which was used for the allocation. In case where
> > regs->len is smaller than the allocated info->regdump_len length,
> > we may assume that the dump contains a nested set of registers.
> > 
> > This becomes handy for kernel drivers to expose registers of an
> > underlying network conduit unfortunately not exposed to userspace,
> > as found in network switching equipment for example.
> > 
> > This patch adds support for recursing into the dump operation if there
> > is enough room for a nested ethtool_drvinfo structure containing a
> > valid driver name, followed by a ethtool_regs structure like this:
> > 
> >     0      regs->len                        info->regdump_len
> >     v              v                                        v
> >     +--------------+-----------------+--------------+-- - --+
> >     | ethtool_regs | ethtool_drvinfo | ethtool_regs |       |
> >     +--------------+-----------------+--------------+-- - --+
> > 
> > Signed-off-by: Vivien Didelot <vivien.didelot@...il.com>
> > ---
> 
> I'm not sure about this approach. If these additional objects with their
> own registers are represented by a network device, we can query their
> registers directly. If they are not (which, IIUC, is the case in your
> use case), we should use an appropriate interface. AFAIK the CPU ports
> are already represented in devlink, shouldn't devlink be also used to
> query their registers?

Yet another interface wasn't that much appropriate for DSA, making the stack
unnecessarily complex. In fact we are already glueing the statistics of the CPU
port into the master's ethtool ops (both physical ports are wired together).
Adding support for nested registers dump in ethtool makes it simple to
(pretty) dump CPU port's registers without too much userspace addition.

> 
> >  ethtool.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/ethtool.c b/ethtool.c
> > index 05fe05a08..c0e2903c5 100644
> > --- a/ethtool.c
> > +++ b/ethtool.c
> > @@ -1245,7 +1245,7 @@ static int dump_regs(int gregs_dump_raw, int gregs_dump_hex,
> >  
> >  	if (gregs_dump_raw) {
> >  		fwrite(regs->data, regs->len, 1, stdout);
> > -		return 0;
> > +		goto nested;
> >  	}

You're right regarding your comment about raw output. I can keep the return
0 here.

> >  
> >  	if (!gregs_dump_hex)
> > @@ -1253,7 +1253,7 @@ static int dump_regs(int gregs_dump_raw, int gregs_dump_hex,
> >  			if (!strncmp(driver_list[i].name, info->driver,
> >  				     ETHTOOL_BUSINFO_LEN)) {
> >  				if (driver_list[i].func(info, regs) == 0)
> > -					return 0;
> > +					goto nested;
> >  				/* This version (or some other
> >  				 * variation in the dump format) is
> >  				 * not handled; fall back to hex
> > @@ -1263,6 +1263,15 @@ static int dump_regs(int gregs_dump_raw, int gregs_dump_hex,
> >  
> >  	dump_hex(stdout, regs->data, regs->len, 0);
> >  
> > +nested:
> > +	/* Recurse dump if some drvinfo and regs structures are nested */
> > +	if (info->regdump_len > regs->len + sizeof(*info) + sizeof(*regs)) {
> > +		info = (struct ethtool_drvinfo *)(&regs->data[0] + regs->len);
> > +		regs = (struct ethtool_regs *)(&regs->data[0] + regs->len + sizeof(*info));
> > +
> > +		return dump_regs(gregs_dump_raw, gregs_dump_hex, info, regs);
> > +	}
> > +
> >  	return 0;
> >  }
> >  
> 
> For raw and hex dumps, this will dump only the payloads without any
> metadata allowing to identify what are the additional blocks for the
> other related objects, i.e. where they start, how long they are and what
> they belong to. That doesn't seem very useful.


Thanks,

	Vivien