irq sharing between devices

On Sep 6, 2:12 am, Tim Roberts <t....DeleteThis@probo.com> wrote:
> kavi <mekavi....DeleteThis@gmail.com> wrote:
>
> >if I have two/three devices sharing the same interrupt num having
> >three interrupt handlers ,say pci devices , which interrupt handler
> >will get called when there is a interrupt.
>
> All of them.  Presumably, all but one of the handlers will realize that the
> interrupt was not for them, and do nothing (quickly).
>
> >Against the interrupt number, these two/three interrupt handlers will
> >be registered in interrupt descriptor table.Am I correct?
>
> Well, they will be registered in the kernel's interrupt tables.  The
> hardware IDT can only have one handler, which points to a kernel routine
> that calls all of the registered handlers.
>
> >Should there be register which says which actual interrupt is
> >generated? So, in which device this register will be present?
>
> Yes.  Every PCI hardware device must have a register that says "yes, I
> really did fire an interrupt."
> --
> Tim Roberts, t....DeleteThis@probo.com
> Providenza & Boekelheide, Inc.





Thanks Tim !!

On Sep 6, 2:12 am, Tim Roberts <t....DeleteThis@probo.com> wrote:
> kavi <mekavi....DeleteThis@gmail.com> wrote:
>
> >if I have two/three devices sharing the same interrupt num having
> >three interrupt handlers ,say pci devices , which interrupt handler
> >will get called when there is a interrupt.
>
> All of them.  Presumably, all but one of the handlers will realize that the
> interrupt was not for them, and do nothing (quickly).
>
> >Against the interrupt number, these two/three interrupt handlers will
> >be registered in interrupt descriptor table.Am I correct?
>
> Well, they will be registered in the kernel's interrupt tables.  The
> hardware IDT can only have one handler, which points to a kernel routine
> that calls all of the registered handlers.
>
> >Should there be register which says which actual interrupt is
> >generated? So, in which device this register will be present?
>
> Yes.  Every PCI hardware device must have a register that says "yes, I
> really did fire an interrupt."
> --
> Tim Roberts, t....DeleteThis@probo.com
> Providenza & Boekelheide, Inc.

Thanks Tim !!

On Sep 4, 10:15 pm, kavi <mekavi....DeleteThis@gmail.com> wrote:
> Hi,
> if I have two/three devices sharing the same interrupt num having
> three interrupt handlers ,say pci devices , which interrupt handler
> will get called when there is a interrupt.
> Against the interrupt number, these two/three interrupt handlers will
> be registered in interrupt descriptor table.Am I correct?
> Should there be register which says which actual interrupt is
> generated? So, in which device this register will be present?
>
> Basically I m trying to understand how irq sharing is happening in
> Linux.

Devices which support IRQ sharing have an interrupt handler that first
checks the device to see if it issued the interrupt. If it did not,
they return immediately. The kernel will call all the interrupt
handlers in succession.

Pretty much every modern device that supports IRQ sharing has a
register that indicates what IRQs it has issued and why. So if it's,
say, a network card, there will be a register that has several bit
flags, one for send interrupt, one for receive interrupt, one for
media change interrupt, and so on. The interrupt driver will read that
register and if none of the flags are set, it will return.

You need this even if there is no interrupt sharing, as it provides a
quick way to tell what interrupt handling code the driver needs to
run.

DS

On Sep 9, 2:16 am, David Schwartz <dav... DeleteThis @webmaster.com> wrote:
> On Sep 4, 10:15 pm, kavi <mekavi... DeleteThis @gmail.com> wrote:
>
> > Hi,
> > if I have two/three devices sharing the same interrupt num having
> > three interrupt handlers ,say pci devices , which interrupt handler
> > will get called when there is a interrupt.
> > Against the interrupt number, these two/three interrupt handlers will
> > be registered in interrupt descriptor table.Am I correct?
> > Should there be register which says which actual interrupt is
> > generated? So, in which device this register will be present?
>
> > Basically I m trying to understand how irq sharing is happening in
> > Linux.
>
> Devices which support IRQ sharing have an interrupt handler that first
> checks the device to see if it issued the interrupt. If it did not,
> they return immediately. The kernel will call all the interrupt
> handlers in succession.
>
> Pretty much every modern device that supports IRQ sharing has a
> register that indicates what IRQs it has issued and why. So if it's,
> say, a network card, there will be a register that has several bit
> flags, one for send interrupt, one for receive interrupt, one for
> media change interrupt, and so on. The interrupt driver will read that
> register and if none of the flags are set, it will return.
>
> You need this even if there is no interrupt sharing, as it provides a
> quick way to tell what interrupt handling code the driver needs to
> run.
>
> DS

Thanks David !!
You really got gr8 insight into these subjects

On Sep 10, 11:01 am, kavi <mekavi....RemoveThis@gmail.com> wrote:

> Thanks David !!
> You really got gr8 insight into these subjects

You're welcome. That's why they pay me the big bucks. (The green ones
that you can actually *spend*!)

DS