Re: [PATCH] mfd: intel-lpss: Clamp latency tolerance to the register value mask

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Nov 09, 2026 at 11:39:12PM +0200, Andy Shevchenko wrote:
> On Mon, Nov 09, 2026 at 05:42:42PM +0800, raoxu wrote:
> > 
> > The PM QoS latency tolerance interface accepts non-negative s32 values,
> > but the LPSS active and idle LTR registers have only a ten-bit value
> > field. intel_lpss_ltr_set() selects the larger scale for requests above
> > 1023 us and ORs val >> 5 into the register without limiting its width.
> > 
> > For a request of 32768 us, the shifted value is 1024 and the latency
> > value field becomes zero. A request of 100000 us leaves only 53 in the
> > value field. Still larger requests can set bits outside both the value
> > and scale fields. Thus a relaxed latency request can produce a much
> > stricter tolerance and unnecessarily restrict power saving.
> > 
> > Clamp the shifted value to LPSS_PRIV_LTR_VALUE_MASK before combining it
> > with LPSS_PRIV_LTR_SCALE_32US. This saturates oversized requests instead
> > of wrapping the field, matching the Intel LTR handling in sdhci-pci.
> > Negative values and PM_QOS_LATENCY_ANY retain their existing handling.
> 
> Shouldn't we rather forbid to apply wrong values?
> 
> > The supplied fix was reviewed and this commit message prepared with
> > OpenCode AI assistance.
> 
> This paragraph should go after '---' line below.

...

> > Fixes: 4b45efe85263 ("mfd: Add support for Intel Sunrisepoint LPSS devices")
> > Signed-off-by: Xu Rao <raoxu@uniontech.com>
> > Cc: stable@vger.kernel.org

Ah, and it's not a fix, it's just improving user experience. The interface even
not a sysfs one IIRC.

> > ---

...

> >  		if (val > LPSS_PRIV_LTR_VALUE_MASK)
> > -			ltr |= LPSS_PRIV_LTR_SCALE_32US | val >> 5;
> > +			ltr |= LPSS_PRIV_LTR_SCALE_32US |
> > +			       min_t(u32, val >> 5, LPSS_PRIV_LTR_VALUE_MASK);
> 
> No min_t() in a new code. You mentioned clamping, use clamp(),
> it's exactly for that.
> 
> >  		else
> >  			ltr |= LPSS_PRIV_LTR_SCALE_1US | val;

-- 
With Best Regards,
Andy Shevchenko