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

[Mika Westerberg]

Hi,

On Tue, Sep 22, 2026 at 05:42:38PM +0530, Navon John Lukose wrote:
> 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.

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.

> 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.

Right and there is no explanation why 0 but it has been working fine so far
;-)