M. Warner Losh wrote:
In message: <4947d474.9040...@samsco.org>
Scott Long <sco...@samsco.org> writes:
: Alexander Motin wrote:
: > Author: mav
: > Date: Tue Dec 16 16:04:40 2008
: > New Revision: 186182
: > URL: http://svn.freebsd.org/changeset/base/186182
: >
: > Log:
: > Call ata_legacy() only once on attach and save it's result. Scanning PCI
: > configuration registers (which are not going to change) on every interrupt
: > looks expensive, especially when interrupt is shared. Profiling shows me
3%
: > of time spent by atapci0 on pure network load due to IRQ sharing with em0.
: >
:
: Nice change. PCI Config registers are exceptionally slow to access on
: most systems.
And we've been recommending to people for years that they avoid config
space access in interrupt handlers. Maybe it is time for something
that checks and prints a warning?
With the move to memory-mapped pci config registers, there was an
intention to allow low-end devices to put their registers into config
space. I think I recall some legacy ultra-low end devices that also
put a few required registers into config space. So while it's not ideal
to access it from an interrupt handler, I can't think of why it should
be expressly forbidden.
Scott
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