Messages in this thread | | | Date | Wed, 15 Mar 2006 17:22:35 -0500 | From | Jeff Garzik <> | Subject | Re: libata/sata_nv latency on NVIDIA CK804 [was Re: AMD64 X2 lost ticks on PM timer] |
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Ingo Molnar wrote: > in this particular case there's only very simple (and non-IO) > instructions in that codepath (no loops either), except for > ata_bmdma_status() which does IO ops: so i agree with you that the most > likely candidate for the delay is the readb() or the inb() in > ata_bdma_status(). > > I'm wondering which one of the two. inb()s are known to be horrible on > some systems - but i've never seen them take 16 milliseconds. If it's > the inb(), then that could also involve SMM mode and IO
ata_bmdma_status() is just a single IO read, and even 1ms is highly improbable.
I'd look elsewhere. There are a ton of udelay() calls in the legacy PCI IDE BMDMA code paths (sata_nv uses these), so I'm not surprised there is latency in general, in a libata+sata_nv configuration. Status checks for example (ata_busy_wait in libata.h) are basically
while (ioreadX() != condition) udelay(10)
That delay is mainly a "don't pound too hard on the hardware" delay. If the hardware is really slow completing a command after signalling completion, you'll potentially wait up to 1000*10 us in some cases. And there are other delays, such as the per-command ndelay() plus ioread().
Welcome to the wonderful world of IDE.
Jeff
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