netdev - Re: [PATCH 2/3] can: add support for Janz VMOD-ICAN3 Intelligent CAN module

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Message-ID: <20100402004311.GB25412@ovro.caltech.edu>
Date:	Thu, 1 Apr 2010 17:43:11 -0700
From:	"Ira W. Snyder" <iws@...o.caltech.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, socketcan-core@...ts.berlios.de,
	netdev@...r.kernel.org, sameo@...ux.intel.com
Subject: Re: [PATCH 2/3] can: add support for Janz VMOD-ICAN3 Intelligent
 CAN module

On Thu, Apr 01, 2010 at 01:03:59PM -0700, Andrew Morton wrote:
> On Mon, 29 Mar 2010 09:58:51 -0700
> "Ira W. Snyder" <iws@...o.caltech.edu> wrote:
> 
> > The Janz VMOD-ICAN3 is a MODULbus daughterboard which fits onto any
> > MODULbus carrier board. It is an intelligent CAN controller with a
> > microcontroller and associated firmware.
> > 
> 
> A neat-looking driver.
> 
> > ...
> >
> > +	spin_lock_irqsave(&mod->lock, flags);
> >
> > ...
> 
> It does this rather a lot.  it seems to be doing quite a lot of work
> under that lock, too - quite a lot of memcpy_toio(), other stuff.
> 

Like most similar cards, the host computer communicates to the
microcontroller through a dual ported memory (DPM) interface. In this
card, it is split into 256x 256 byte pages/windows.

The lock ensures that once code sets a window, it doesn't change while
the memcpy/iowrite happens.

> Is there potential here to disable interrupt for too long?  Not
> possible to use spin_lock_bh() here?
> 

The largest possible memcpy_(to|from)io() in the driver is 256 bytes.
Not too huge, but I understand the concern.

Looking at this again, I don't take the lock in the interrupt handler
(nor do I need to). What contexts do the network driver's xmit() and
napi() routines run in? hardirq and softirq respectively, right? In that
case, I think spin_lock_bh() is probably enough.

Ira
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