Re: [PATCH 2.6.13-rc1 03/10] IOCHK interface for I/O error handling/detecting

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> Are you assuming that a device driver will use an iochk_read() for
> every DMA operation? for every MMIO to the card?  
> 
> For high performance devices, it seems to me that this will cause
> a rather large performance burden, especially if its envisioned that
> all architectures will do something similar.
> 
> My concern is that (at least on ppc64) the call pci_read_config_word()
> requires a call into "firmware" aka "BIOS", which takes thousands upon
> thousands of cpu cycles.  There are hundreds of cycles of gratuitous
> crud just to get into the firmware, and then lord-knows-what the
> firmware does while its in there; probably doing all sorts of crazy
> math to compute bus addresses and other arcane things.  I would imagine
> that most architectures, includig ia64, are similar.
> 
> Thus, one wouldn't want to perform an iochk_read() in this way unless
> one was already pretty sure that an error had already occured ... 
> 
> Am I misunderstanding something?

I would expect pSeries not to use the "default" error checking (that
tests the status register) but rather use EEH.

Ben.