Date: Mon, 01 Jul 2019 11:41:12 -0600 From: Ian Lepore <ian@freebsd.org> To: Warner Losh <imp@bsdimp.com>, Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: Andriy Gapon <avg@freebsd.org>, FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: i2c bit banging timeout for SCL Message-ID: <291f26bcc4b4b30cda4f5b23855e889aa5883ee3.camel@freebsd.org> In-Reply-To: <CANCZdfp9MKkq7s4F%2BOHBa%2Bu%2Bb_OfExe0B0cWeZ1aPS5xBG8mXA@mail.gmail.com> References: <a3538c10-2438-a287-77db-ad390078b274@FreeBSD.org> <7e5d8a06bc2d46263262369464e9348b82263847.camel@freebsd.org> <CANCZdfoFBvmxPtnEL4GOqXTvp6Zd-xrtja4rmUO1rAcy0JdeSw@mail.gmail.com> <93148.1562001288@critter.freebsd.dk> <CANCZdfp9MKkq7s4F%2BOHBa%2Bu%2Bb_OfExe0B0cWeZ1aPS5xBG8mXA@mail.gmail.com>
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On Mon, 2019-07-01 at 11:25 -0600, Warner Losh wrote: > On Mon, Jul 1, 2019 at 11:14 AM Poul-Henning Kamp <phk@phk.freebsd.dk > > > wrote: > > > -------- > > In message < > > CANCZdfoFBvmxPtnEL4GOqXTvp6Zd-xrtja4rmUO1rAcy0JdeSw@mail.gmail.com> > > , > > Warner Losh writes: > > > > > The only issue, really, is that this timeout is a busy loop and > > > there may > > > be I/O bus contention introduced on these systems. > > > > Does it have to be a busy loop for the entire duration ? > > > > Spin for the median, timeout+poll for the rest of the time ? > > > > That's a good suggestion. I'd be inclined to spin for 1 tick or so, > then do > a timeout per tick after that (eg, shift from DELAY to pause(1)). It > won't > be super accurate or high performance, but when the devices are slow, > that > would add only a little extra time. > > Ideally, that's what we'd do. In the short term, bumping the timeout > wouldn't be horrible. > > Warner Most of the DELAY() in i2c bitbang is just the idle time before toggling the clock line to achieve the 100khz bus rate. That's a 10us delay, and on modern hardware those delays should be pause() calls because that's enough time to get useful work done. When polling for ack at the end of a byte, using a DELAY(1) loop makes more sense (actually, just polling without delay may make even more sense, since DELAY() is generally just polling a clock register). Hmm, actually, it looks like iicbb hardcodes the bus frequency delay as 10us and delays after every toggle, so I guess it's really running the bus at 50khz. -- Ian
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