Messages in this thread | | | Date | Tue, 7 Apr 2015 13:05:43 -0700 | Subject | Re: [PATCH v4 2/4] mmc: core: Add mmc_regulator_set_vqmmc() | From | Doug Anderson <> |
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Hi,
On Fri, Mar 20, 2015 at 4:28 AM, Mark Brown <broonie@kernel.org> wrote: > As previously discussed the problem is that there can be a *lot* of > voltages on a modern regulator with fine grained voltage steps and > tolerances are also used for things like cpufreq where we care about > performance. We need something that doesn't require a linear scan of > possible values.
Finally getting back to this (sorry for the delay!). I tried modifying my patches to keep using the simple implementation of regulator_set_voltage_tol() but to take two tolerances (lower and upper). ...but when I thought about it I decided it wasn't enough. I think that doing a proper implementation of regulator_set_voltage_tol() is going to be a requirement for getting the MMC core changes posted.
Specifically, I think the right implementation for the MMC core's 3.3V signaling is something like this:
/* * Bus operating conditions say that card should accept input * between (0.625 * VDD) and (VDD + 0.3), so we'll use those * as tolerances. */ return mmc_regulator_set_voltage_if_supported(mmc->supply.vqmmc, vmmc_voltage, vmmc_voltage * 375 / 1000, 300000);
Ulf says he has a board where vmmc is 3.4V and the max vqmmc is 2.9V. If we think about that board, we'll end up calling regulator_set_voltage_tol() with a lower/upper tolerance of 1.275V and .3V.
Extending the current simple implementation of regulator_set_voltage_tol() to take an upper and lower, that will translate to first trying to set the voltage to 3.4V - 3.7V, which will fail. We'll then try to set the voltage to 2.125V - 3.7V. Presumably that will end up picking 2.125V, which is really non-ideal compared to 2.9V and it seems likely to cause some cards to start failing.
Mark: I know you said you were considering writing a better regulator_set_voltage_tol() yourself, but I don't know if you've already started work on it.
I'm expecting to maybe have time to take a crack at it in a few weeks if you haven't already done it by then.
Thanks!
-Doug
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