[PATCH] i2c: aspeed: Retain delay/setup/hold values when configuring bus frequency

[Andrew Jeffery]

On Mon, 2017-08-28 at 18:07 +0200, Wolfram Sang wrote:
> On Tue, Aug 15, 2017 at 04:51:02PM +0930, Andrew Jeffery wrote:
> > In addition to the base, low and high clock configuration, the AC timing
> > register #1 on the AST2400 houses fields controlling:
> > 
> > 1. tBUF: Minimum delay between Stop and Start conditions
> > 2. tHDSTA: Hold time for the Start condition
> > 3. tACST: Setup time for Start and Stop conditions, and hold time for the
> >    Repeated Start condition
> > 
> > These values are defined in hardware on the AST2500 and therefore don't
> > need to be set.
> > 
> > aspeed_i2c_init_clk() was performing a direct write of the generated
> > clock values rather than a read/mask/modify/update sequence to retain
> > tBUF, tHDSTA and tACST, and therefore cleared the tBUF, tHDSTA and tACST
> > fields on the AST2400. This resulted in a delay/setup/hold time of 1
> > base clock, which in some configurations is not enough for some devices
> > (e.g. the MAX31785 fan controller, with an APB of 48MHz and a desired
> > bus speed of 100kHz).
> > 
> > Signed-off-by: Andrew Jeffery <andrew at aj.id.au>
> 
> Applied to for-next, thanks! 

Thanks!

> I even considered for-current but it does
> not apply there. So, I leave the backporting for the interested parties
> :)
> 

It depends on Brendan's clock divisor calculation fix, which appears to
be in for-next but not for-current:

    87b59ff8d1d9 i2c: aspeed: add proper support fo 24xx clock params

I'd argue that Brendan's patch should go in for-current as well,
because it fixes a divisor rounding error for the ast2500 (bus is
clocked faster than requested).

Cheers,

Andrew
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