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

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Date:   Wed, 16 Aug 2017 09:15:39 +0200
From:   Cédric Le Goater <clg@...d.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Joel Stanley <joel@....id.au>, Andrew Jeffery <andrew@...id.au>
Cc:     Ryan Chen <ryan_chen@...eedtech.com>,
        linux-aspeed@...ts.ozlabs.org, Wolfram Sang <wsa@...-dreams.de>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-i2c@...r.kernel.org
Subject: Re: [PATCH] i2c: aspeed: Retain delay/setup/hold values when
 configuring bus frequency

On 08/16/2017 08:59 AM, Benjamin Herrenschmidt wrote:
> On Wed, 2017-08-16 at 08:53 +0200, Cédric Le Goater wrote:
>>>>          divisor = DIV_ROUND_UP(bus->parent_clk_frequency, bus->bus_frequency);
>>>> -       clk_reg_val = bus->get_clk_reg_val(divisor);
>>>> +       clk_reg_val = readl(bus->base + ASPEED_I2C_AC_TIMING_REG1);
>>>> +       clk_reg_val &= (ASPEED_I2CD_TIME_TBUF_MASK |
>>>> +                       ASPEED_I2CD_TIME_THDSTA_MASK |
>>>> +                       ASPEED_I2CD_TIME_TACST_MASK);
>>>
>>> Instead of keeping the u-boot values (which appear to be hard-coded),
>>> should we instead put the known working values in the register?
>>
>> Yes. I was wondering where the initial setting was from on the AST400.
> 
> Well, the AST2500 hard codes them in HW, so it makes some amount of
> sense to use whatever aspeed platform.S hard codes in u-boot for the
> 2400. The values look reasonably sane.
> 
> If we ever see a need to change them, we can add DT props etc... but
> for now I'd just not bother.
> 
> The way it is now, at least, if I have problems, I can tweak the values
> with devmem and try again without the driver overwriting them :-)

ah yes. that is useful I agree. 

C.