[Open-graphics] Video interrupt enable/disable

[Timothy Normand Miller]

On 4/16/07, Paul Brook <paul at codesourcery.com> wrote:
> > Perhaps it would be even simpler to have two interrupts.  One as you
> > describe -- a sync interrupt -- which could not be turned off,  A second
> > for a service request which would be triggered by firmware on the board
> > or the DMA controller.
> >
> > Then the issue of turning the sync interrupt off and on is moved to the
> > driver since the APIC Intel acronym == Advanced Programmable Interrupt
> > Controller) on the motherboard already has the hardware to turn off
> > (ignore) an interrupt.
>
> On a typical PCI system each device only gets a single interrupt pin (a PCI
> bus has 4, but each device is only supposed to use 1), and several devices
> share an interrupt line.
>
> Thus all interrupts should be maskable on the device, and probably combined
> into a single output after masking.

What we did on TROZ was have the following sorts of controls for
interrupts (inspired by what we'd seen with other chips):

- Specify the condition under which an interrupt would be triggered
- Enable the condition to occur
- Enable a triggered interrupt to assert the interrupt line on the bus
- Reset the interrupt condition state

For video, the they are, respectively:

- The bit in the VC instruction that specifies when the interrupt happens
- The config register that lets us pay attention to that bit, setting
a flag when it happens.
- A configuration flag that gates the interrupt signal onto the PCI bus
- A register to write to that clears the flag

Oh, and course, you need a register to read that tells you which of
potentially multiple interrupt conditions are being asserted.

This isn't overkill.  There's a difference between the triggering
event, the flag that gets set as a result of the triggering event, and
the signal to the host that causes a CPU interrupt.  There are
situations where you'd want the event to happen and be noted (by
setting the flag) without it interrupting the CPU.

-- 
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Open Graphics Project