[Open-graphics] Video interrupt enable/disable

[James Richard Tyrer]

Jean-Baptiste Note wrote:

> What are we talking about here anyway ? You need an acknowledgement of
> the IRQ anyway, so there will be a write on the PCI. We're talking
> about saving a couple microseconds for reading an IRQ status register
> on PCI, this against alienating customers, having to delve into
> low-level linux PCI code and generally having a headache about
> debugging the stuff.

The OS code isn't something that customers need to worry about.

What are we talking about?  The question is whether or not service of 
the 'service' interrupt will introduce too much latency in the service 
of the 'sync' interrupt.

What your 'analysis' fails to consider is that, if shared, that the 
'sync' interrupt cannot be serviced until service of the 'service' 
interrupt has completed to the point that the interrupt has been cleared 
  (in hardware) and interrupt is enabled.  Also not considered is that 
these two interrupts should have different priorities.

IAC, this doesn't make any difference unless the system the board is 
plugged into has enough interrupt controller hardware to make a separate 
hardware interrupt available for the 'sync' interrupt -- the 'service' 
interrupt can be shared with other interrupts since it doesn't need the 
high priority.

I also note (for example) that AGP cards have two interrupts, but many 
systems share these.  So, it only matters if the system has the hardware 
interrupts available.  But, if the two interrupts are not separate, then 
the system must share them even if it has the hardware available to 
provide separate interrupts.  That is what I am talking about.

If multiple hardware interrupts weren't any advantage, they why do we 
have multiple hardware interrupts -- why don't all I/O devices just 
share one interrupt?

-- 
JRT