Re: [PATCH] i2c: designware: size the RX FIFO threshold to the queued transfer

[Navon John Lukose <navonjohnlukose@gmail.com>]

On Tue, Sep 22, 2026 at 02:29:02PM +0200, Mika Westerberg wrote:
> Yeah I don't think 0.7ms is much but we have things like touchscreens and
> the like with various report sizes so at least it would be good to check if
> those have any issues.
>
> Unfortunately I don't have any of such devices anymore but I asked our
> validation to check if they have something that could be used to test this.

I have a Goodix touchscreen on a second controller on the same machine, so
I ran it at three report sizes. GET_REPORT at 100 Hz, 2000 transfers per
cell, three interleaved passes, ~normal desktop use in the background:

    bytes   interrupts/report      mean latency
       7      4.17 ->  2.00     1776 -> 2093 us
      21     10.06 ->  2.00     2131 -> 2676 us
      64     63.97 ->  3.21     2303 -> 3512 us

64 bytes is past rx_fifo_depth on this part, so that is also the first
exercise of the half-FIFO cap.

So the latency is not a fixed 0.7 ms as I implied earlier, it grows with
the report size. Breaking down where it goes, with kprobes on i2c_dw_xfer
and i2c_dw_isr, for 64 bytes:

    segment                       stock   patched
    xfer start -> first ISR          78       146
    first ISR -> last ISR          2157      2748
    last ISR -> xfer return         101       147
    i2c-hid and ioctl overhead       12        49

84% of it is between the first and last interrupt. Stock's 65 interrupts
are 33 us apart, too close together for the CPU to idle deeply, so each
wakeup is cheap. Patched leaves 3 gaps of around 900 us, deep enough to
idle properly, and pays the exit latency on each one.

So the power saving and the added latency are the same effect seen from
either end. Lowering the cap trades one back for the other at roughly the
same rate, so if the latency needs bounding (I personally think it is
overkill) it wants a QoS request as Andy suggested.

I can send the raw data and the scripts if needed.

Assisted-by: LLM # for the kprobe measurements :)