Hi Geert,
On Mon, Jan 15, 2024 at 02:18:12PM +0100, Geert Uytterhoeven wrote:
> Switch the R-Mobile A1, R-Mobile APE6, and SH-Mobile AG5 SoCs to the new
> frequency calculation formula, to (a) avoid running the I2C bus too fast,
> and (b) bring the low/high ratio closer to the recommended ratio 5/4.
>
> Measurement results on R-Mobile APE6 and SH-Mobile AG5 (fck=104 MHz,
> clks_per_count=2):
> 100 kHz: 106 kHz LH=1.12 before, 99.6 kHz L/H=1.22 after
> 400 kHz: 384 kHz LH=1.67 before, 392 kHz L/H=1.27 after
>
> Measurement results on R-Mobile A1 (fck=49.5 MHz, clks_per_count=1):
> 100 kHz: 106 kHz L/H=1.09 before, 99.6 kHz L/H=1.20 after
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Looks good to me, and it seems to comply with the datasheets? (I haven't
checked)
> Notes:
> - fast_clock_dt_config is now unused, so I guess we should drop it,
> and rename v2_freq_calc_dt_config to fast_clock_dt_config?
Yes, we can (and should do that).
> - The formulas in the documentation for R-Mobile A1 do not include the
> "-1" and "-5", presumably this is for rounding?
Sounds reasonable. The formulas evolved over time :)
> - The formulas in the documentation for SH-Mobile AG5 include
> the "-1" and "-5" in the example for 400 kHz, but not in the example
> for 100 kHz.
*shrug*
> - The SH-Mobile AG5 documentation suggests to use an L/H ratio of 5/3
> instead of 5/4 for 400 kHz, which means the old formula is better
> for 400 kHz?
Both ratios are within the I2C specifications for minimum SCL lo/hi
time. 5/3 is a little further away from the minimum requirements, so
maybe a tad more robust. But both should work(tm).
> - Remaining users of the old formula are sh7343, sh7366, and sh772[234].
> At least for sh772[234], the "Transfer Rate" formula in the
> documentation is the same as for R-Mobile A1. Hence probably we
> should switch the default_dt_config and the default fallback to the
> "v2" formula, too?
I think we could. But touching old systems without testing and a need is
more risky than leaving them alone.
> - I am still a bit puzzled, as the "v2" formula introduced in commit
> 4ecfb9d3b229fff5 ("i2c: sh_mobile: add new frequency calculation for
> later SoC") is basically what the driver did before commit
> 23a612916a51cc37 ("i2c: i2c-sh_mobile: optimize ICCH/ICCL values
> according to I2C bus speed")?
Interesting. I can't recall that I analyzed the back then existing
formula when I implemented v2. I recall that I used R-Car Gen2
datasheets as the reference. I probably thought those are the "new"
algorithms. I likely didn't have the old datasheets back then. Still,
my memory is not clear about this.
Obvious to say, this I2C core is not used anymore in newer R-Car SoCs.
So, I won't dive too deep into it...
Happy hacking,
Wolfram
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