Subject: Re: [PATCH HACK] powerpc: quick hack to get a functional eHEA with hardirq preemption

From: Thomas Klein <[email-masked]>
Date: Mon, 15 Sep 2008 14:35:16 +0200

Hi,

we are a bit worried about putting this into the mainstream part of non real
time linux. There interrupts work perfectly fine, and it was a bit of a
challenge to get there for all cases / configurations / machines.

Could you try to enable these changes only for RT-Linux via a real-time
kconfig switch? This way we make sure we don't break the scheme for
eHEA / eHCA.

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 <[email-masked]>
> ---
>  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);