Hello Tao, Our recommend about clk divider setting is follow the datasheet clock setting table for clock divisor. Ryan -----Original Message----- From: Linux-aspeed [mailto:linux-aspeed-bounces+ryan_chen=aspeedtech.com@xxxxxxxxxxxxxxxx] On Behalf Of Tao Ren Sent: Thursday, June 20, 2019 6:33 AM To: Brendan Higgins <brendanhiggins@xxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx>; devicetree <devicetree@xxxxxxxxxxxxxxx>; linux-aspeed@xxxxxxxxxxxxxxxx; OpenBMC Maillist <openbmc@xxxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>; Rob Herring <robh+dt@xxxxxxxxxx>; Linux ARM <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>; linux-i2c@xxxxxxxxxxxxxxx Subject: Re: [PATCH 1/2] i2c: aspeed: allow to customize base clock divisor On 6/19/19 2:25 PM, Brendan Higgins wrote: > On Wed, Jun 19, 2019 at 2:00 PM Tao Ren <taoren@xxxxxx> wrote: >> >> Some intermittent I2C transaction failures are observed on Facebook >> CMM and Minipack (ast2500) BMC platforms, because slave devices (such >> as CPLD, BIC and etc.) NACK the address byte sometimes. The issue can >> be resolved by increasing base clock divisor which affects ASPEED I2C >> Controller's base clock and other AC timing parameters. >> >> This patch allows to customize ASPEED I2C Controller's base clock >> divisor in device tree. > > First off, are you sure you actually need this? > > You should be able to achieve an effectively equivalent result by just > lowering the `bus-frequency` property specified in the DT. The > `bus-frequency` property ultimately determines all the register > values, and you should be able to set it to whatever you want by > refering to the Aspeed documentation. > > Nevertheless, the code that determines the correct dividers from the > frequency is based on the tables in the Aspeed documentation. I don't > think the equation makes sense when the base_clk_divisor is fixed; I > mean it will probably just set the other divisor to max or min > depending on the values chosen. I think if someone really wants to > program this parameter manually, they probably want to set the other > parameters manually too. Thank you for the quick response, Brendan. Aspeed I2C bus frequency is defined by 3 parameters (base_clk_divisor, clk_high_width, clk_low_width), and I choose base_clk_divisor because it controls all the Aspeed I2C timings (such as setup time and hold time). Once base_clk_divisor is decided (either by the current logic in i2c-aspeed driver or manually set in device tree), clk_high_width and clk_low_width will be calculated by i2c-aspeed driver to meet the specified I2C bus speed. For example, by setting I2C bus frequency to 100KHz on AST2500 platform, (base_clock_divisor, clk_high_width, clk_low_width) is set to (3, 15, 14) by our driver. But some slave devices (on CMM i2c-8 and Minipack i2c-0) NACK byte transactions with the default timing setting: the issue can be resolved by setting base_clk_divisor to 4, and (clk_high_width, clk_low_width) will be set to (7, 7) by our i2c-aspeed driver to achieve similar I2C bus speed. Not sure if my answer helps to address your concerns, but kindly let me know if you have further questions/suggestions. Thanks, Tao
