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 08:50:41AM +0200, Mika Westerberg wrote:
> Yes power savings are always good but in this case it also affects the
> latency visible to the user so we don't want devices like touchpads become
> "sluggish" either. Do you see any such issues with the devices when you
> have this enabled and not using /dev/cpu_dma_latency or any other interface
> to limit the CPU low power states?

No. I have been running it as my only kernel for a week with nothing set on
cpu_dma_latency, and I cannot tell the difference on the touchpad.

This patch increases latency by 0.7 ms on the mean, which I don't think
is easily noticeable by humans. Against that, the i2c traffic alone cost
2.24 W on the machine I measured, more than its entire 1.87 W idle draw.

However, if anyone tests and can feel the difference, please let me know.

Also, i2c_dw_configure_mode() sets TX_TL to half the FIFO and RX_TL to 0
on adjacent lines, so a transmit interrupt moves up to half a FIFO of
data and a receive interrupt moves one byte, which is what this patch
changes.