Date:	Mon, 15 Sep 2008 15:13:32 +0200
From:	Sebastien Dugue <sebastien.dugue@...l.net>
To:	Thomas Klein <osstklei@...ibm.com>
Cc:	linux-ppc <linuxppc-dev@...abs.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-rt <linux-rt-users@...r.kernel.org>,
	netdev@...r.kernel.org, themann@...ibm.com, tklein@...ibm.com,
	raisch@...ibm.com, jean-pierre.dion@...l.net,
	gilles.carry@....bull.net, tinytim@...ibm.com
Subject: Re: [PATCH HACK] powerpc: quick hack to get a functional eHEA with
 hardirq preemption


  Hi Thomas, Jan-Bernd, Christoph,

On Mon, 15 Sep 2008 14:35:16 +0200 Thomas Klein <osstklei@...ibm.com> wrote:

> Hi,
> 
> we are a bit worried about putting this into the mainstream part of non real
> time linux.

  Heck, I sure do not want this to be applied mainstream nor into any tree.
The sole purpose of this patch was to trigger some reaction from the people who
know the hardware and try to understand where the problem lies.

> There interrupts work perfectly fine, and it was a bit of a
> challenge to get there for all cases / configurations / machines.

  Agreed, but the fact that it fails with hardirq preemption leads me to
believe (without any more knowledge about the harware) that there might be
something amiss with this driver (or the code concerning the XICS)
nevertheless.

> 
> Could you try to enable these changes only for RT-Linux via a real-time
> kconfig switch?

  Nope, this is just a quick hack that allows me to have a functional eHEA under
the rt kernel. I want to understand what the problem is:

  - Is the eHEA really delivering level interrupts to the XICS?

  - Is the XICS loosing interrupts when they are masked?

  - ...?

> This way we make sure we don't break the scheme for
> eHEA / eHCA.

  Sure, I do not want to break anything, quite the opposite in fact ;-)


  Thanks,

  Sebastien.

> 
> Regards,
> Jan-Bernd, Christoph
> 
> 
> Sebastien Dugue wrote:
> > WARNING: HACK - HACK - HACK
> > 
> >   Under the RT kernel (with hardirq preemption) the eHEA driver hangs right
> > after booting. Fiddling with the hardirqs and softirqs priorities allows to
> > run a bit longer but as soon as the network gets under load, the hang
> > returns. After investigating, it appears that the driver is loosing interrupts.
> > 
> >   To make a long story short, looking at the code, it appears that the XICS
> > maps all its interrupts to level sensitive interrupts (I don't know if it's the
> > reality or if it's due to an incomplete implementation - no datasheets
> > available to check) and use the fasteoi processing flow.
> > 
> >   When entering the low level handler, level sensitive interrupts are masked,
> > then eio'd in interrupt context and then unmasked at the end of hardirq
> > processing.
> > That's fine as any interrupt comming in-between will still be processed since
> > the kernel replays those pending interrupts.
> > 
> >   However, it appears that the eHEA interrupts are behaving as edge sensitive
> > interrupts and are routed through the XICS which process those as level
> > sensitive using the fasteoi handler __OR__ the XICS loses interrupts when they
> > are masked.
> > 
> >   Therefore the masking done in the handler causes any interrupt happening while
> > in the handler to be lost.
> > 
> >   So this patch maps the interrupts being requested through
> > ibmebus_request_irq() as edge sensitive interrupts (this concerns both the eHEA
> > and the eHCA - only users of ibmebus_request_irq()) and changes the way edge
> > interrupts are processed by the fasteoi handler.
> > 
> >   It works for the eHEA, dunno for the eHCA.
> > 
> >   So, unless all the designers of the XICS & eHEA have been shot to keep it
> > a secret, could someone knowledgeable shed some light on this issue.
> > 
> >   Thanks,
> > 
> >   Sebastien.
> > 
> > Not-Signed-off-by: Sebastien Dugue <sebastien.dugue@...l.net>
> > ---
> >  arch/powerpc/kernel/ibmebus.c |   11 ++++++++++-
> >  kernel/irq/chip.c             |    5 +++--
> >  kernel/irq/manage.c           |    9 ++++++---
> >  3 files changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/ibmebus.c b/arch/powerpc/kernel/ibmebus.c
> > index 9971159..5200323 100644
> > --- a/arch/powerpc/kernel/ibmebus.c
> > +++ b/arch/powerpc/kernel/ibmebus.c
> > @@ -41,6 +41,7 @@
> >  #include <linux/kobject.h>
> >  #include <linux/dma-mapping.h>
> >  #include <linux/interrupt.h>
> > +#include <linux/irq.h>
> >  #include <linux/of.h>
> >  #include <linux/of_platform.h>
> >  #include <asm/ibmebus.h>
> > @@ -213,11 +214,19 @@ int ibmebus_request_irq(u32 ist, irq_handler_t handler,
> >  			void *dev_id)
> >  {
> >  	unsigned int irq = irq_create_mapping(NULL, ist);
> > +	struct irq_desc *desc;
> > +	int ret;
> >  
> >  	if (irq == NO_IRQ)
> >  		return -EINVAL;
> >  
> > -	return request_irq(irq, handler, irq_flags, devname, dev_id);
> > +	ret = request_irq(irq, handler, irq_flags, devname, dev_id);
> > +
> > +	desc = irq_desc + irq;
> > +	desc->status &= ~(IRQ_TYPE_SENSE_MASK | IRQ_LEVEL);
> > +	desc->status |= IRQ_TYPE_EDGE_RISING;
> > +
> > +	return ret;
> >  }
> >  EXPORT_SYMBOL(ibmebus_request_irq);
> >  
> > diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
> > index b7b397a..6d366ca 100644
> > --- a/kernel/irq/chip.c
> > +++ b/kernel/irq/chip.c
> > @@ -430,7 +430,7 @@ handle_fasteoi_irq(unsigned int irq, struct irq_desc *desc)
> >  	action = desc->action;
> >  	if (unlikely(!action || (desc->status & (IRQ_INPROGRESS |
> >  						 IRQ_DISABLED)))) {
> > -		desc->status |= IRQ_PENDING;
> > +		desc->status |= IRQ_PENDING | IRQ_MASKED;
> >  		if (desc->chip->mask)
> >  			desc->chip->mask(irq);
> >  		goto out;
> > @@ -439,9 +439,10 @@ handle_fasteoi_irq(unsigned int irq, struct irq_desc *desc)
> >  	desc->status |= IRQ_INPROGRESS;
> >  	/*
> >  	 * In the threaded case we fall back to a mask+eoi sequence:
> > +	 * excepted for edge interrupts which are not masked.
> >  	 */
> >  	if (redirect_hardirq(desc)) {
> > -		if (desc->chip->mask)
> > +		if (desc->chip->mask && !(desc->status & IRQ_TYPE_EDGE_BOTH))
> >  			desc->chip->mask(irq);
> >  		goto out;
> >  	}
> > diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> > index 3bffa20..3e39c71 100644
> > --- a/kernel/irq/manage.c
> > +++ b/kernel/irq/manage.c
> > @@ -788,9 +788,12 @@ static void do_hardirq(struct irq_desc *desc)
> >  		thread_simple_irq(desc);
> >  	else if (desc->handle_irq == handle_level_irq)
> >  		thread_level_irq(desc);
> > -	else if (desc->handle_irq == handle_fasteoi_irq)
> > -		thread_fasteoi_irq(desc);
> > -	else if (desc->handle_irq == handle_edge_irq)
> > +	else if (desc->handle_irq == handle_fasteoi_irq) {
> > +		if (desc->status & IRQ_TYPE_EDGE_BOTH)
> > +			thread_edge_irq(desc);
> > +		else
> > +			thread_fasteoi_irq(desc);
> > +	} else if (desc->handle_irq == handle_edge_irq)
> >  		thread_edge_irq(desc);
> >  	else
> >  		thread_do_irq(desc);
> 
> 
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