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SubjectRe: [PATCH HACK] powerpc: quick hack to get a functional eHEA with hardirq preemption
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 <sebastien.dugue@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);



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