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

[Jan-Bernd Themann]

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 <sebastien.dugue at bull.net> wrote on 15.09.2008 10:04:06:

> 
> 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 interruptswhen 
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 at bull.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);
> -- 
> 1.6.0.1.308.gede4c
> 
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