Re: [PATCH] i2c: gpio: Enable working over slow can_sleep GPIOs

[Wolfram Sang <wsa@the-dreams.de>]

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On Fri, Dec 22, 2017 at 10:47:16PM +0100, Jan Kundrát wrote:
> "Slow" GPIOs (usually those connected over an SPI or an I2C bus) are,
> well, slow in their operation. It is generally a good idea to avoid
> using them for time-critical operation, but sometimes the hardware just
> sucks, and the software has to cope. In addition to that, the I2C bus
> itself does not actually define any strict timing limits; the bus is
> free to go all the way down to DC. The timeouts (and therefore the
> slowest acceptable frequency) are present only in SMBus.
> 
> The `can_sleep` is IMHO a wrong concept to use here. My SPI-to-quad-UART
> chip (MAX14830) is connected via a 26MHz SPI bus, and it happily drives
> SCL at 200kHz (5µs pulses) during my benchmarks. That's faster than the
> maximal allowed speed of the traditional I2C.
> 
> The previous version of this code did not really block operation over
> slow GPIO pins, anyway. Instead, it just resorted to printing a warning
> with a backtrace each time a GPIO pin was accessed, thereby slowing
> things down even more.
> 
> Finally, it's not just me. A similar patch was originally submitted in
> 2015 [1].
> 
> [1] https://patchwork.ozlabs.org/patch/450956/
> 
> Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>

Applied to for-next, thanks!


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