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Date: Mon, 30 Nov 2015 13:12:45 +1100 (AEDT) From: Finn Thain <fthain@...egraphics.com.au> To: Geert Uytterhoeven <geert@...ux-m68k.org> cc: Ondrej Zary <linux@...nbow-software.org>, "James E.J. Bottomley" <JBottomley@...n.com>, Michael Schmitz <schmitzmic@...il.com>, linux-m68k <linux-m68k@...r.kernel.org>, linux-scsi <linux-scsi@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 22/71] ncr5380: Eliminate selecting state On Sun, 29 Nov 2015, Geert Uytterhoeven wrote: > If an ISA access takes 8 us, while the CPU runs at 1 GHz, i.e. 500M > loops/s, the difference will be huge. Ondrej showed that an ISA access can take about 1.6 us. I don't know what to make of the "8 uS" comment in the mainline driver. Maybe it was an even slower ISA card. Anyway, I made a measurement on my hardware and confirmed that lpj is a very bad proxy for device register access throughput. The "loops per access" gap is several orders of magnitude: lpj HZ access time (us) lpa ------------------------------------------------------------------------ Ondrej's 5380 ISA card (PC): 4797252 250 1.6 1.9k My DMX3191D PCI card (PowerMac): 167079 100 0.42 7.0 > > Perhaps you can calibrate an NCR5380_read() loop at driver init time, > and use the calibration value later? I had the same idea but I didn't think that the complexity was justified by the low precision requirement. But now that I have some timings I have to agree. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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