linux-kernel - Re: [PATCH -mm 0/2] RapidIO: Changes to handling of RIO switches

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Date:	Fri, 22 Oct 2010 20:28:46 +0200
From:	Micha Nelissen <micha@...i.hopto.org>
To:	"Bounine, Alexandre" <Alexandre.Bounine@....com>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Matt Porter <mporter@...nel.crashing.org>,
	Li Yang <leoli@...escale.com>,
	Kumar Gala <galak@...nel.crashing.org>,
	Thomas Moll <thomas.moll@...go.com>
Subject: Re: [PATCH -mm 0/2] RapidIO: Changes to handling of RIO switches

Bounine, Alexandre wrote:
> Micha Nelissen <micha@...i.hopto.org> wrote:
>> Alexandre Bounine wrote:
>> How can you say this? The two variables have different meanings, this
>> logically implies you can't merge them. So how do you say 'this does
> not
>> prevent us from ...' without providing a reason?
> 
> Looks like I formulated it bad - better would be: they have different
> interpretation by hardware but logically in RapidIO they have single
> role - destid/hopcount are a device coordinates in the RIO network used
> to access that device.

They are logically different as well (for a non-host).

rswitch->destid with hopcount is the way to reach that switch.

rswitch->rdev->destid should be the id associated with a given switch, 
so that every (processor) device can agree what id some switch has. For 
a non-host, the path to reach a switch may use a different id than the 
switch itself has; it's just the id by which it was discovered.

However, it's possible to fix that by fixing the id+hopcount once the 
switch is found using the path with its own id: then you know the right 
hopcount.

>> can be defined to point to the switch that a given rio_dev is
> connected
>> to. This is useful for quick lookups. How else can to know to which
>> switch a given device is connected?
> 
> rdev->rswitch is not a pointer to the entire switch device object - it
> is a pointer to the switch specific extension associated with given
> rio_dev (if applicable). There is no other role for rdev->rswitch.

I know this, it doesn't answer my question.

> Why would you keep a pointer to device data extension instead of the
> pointer to attached device object itself?

There is no particular reason, but this is a useful way to define the 
fields that are there.

My point is, now that you remove the pointer field, that information (to 
which switch is a particular device connected) cannot be stored in this 
way, so do you have an alternative proposal for that? Maybe add a new field.

> BTW, I have back and forward links added in previous patches and only
> one link that may be added later is a forward link from mport to the
> attached rio_dev (ptr to rio_switch will not work here because it can be
> switchless connection). But this reference has to be added into
> rio_mport.

Possible, but I suggest to put it in the rio_net: fields rdev_host, and 
rdev_self. You can see it in the patch I sent you.

Micha
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