Re: [PATCH] drm/i915/gmbus: fix spurious timeout on 512-byte burst reads

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Feb 10, 2026 at 11:23:02AM +0200, Jani Nikula wrote:
> On Fri, 06 Feb 2026, Samasth Norway Ananda <samasth.norway.ananda@oracle.com> wrote:
> > When reading exactly 512 bytes with burst read enabled, the
> > extra_byte_added path breaks out of the inner do-while without
> > decrementing len. The outer while(len) then re-enters and gmbus_wait()
> > times out since all data has been delivered. Decrement len before the
> > break so the outer loop terminates correctly.
> 
> Nice find, and the fix looks correct. How did you figure this out? Did
> you hit the issue?
> 
> I wonder if the whole extra byte thing is a workaround for some old
> hardware that shouldn't be needed on modern hardware... Ville, thoughts?

I think it's still needed due to the weird way this is implemented
in the hardware. The byte counter rolls over at 256->1, so for that
to happen the programmed byte count must be >256 or else we'd reach
the target byte count before the rollover happens, in which case
the entire transfer would terminate already during the first 256
byte chunk.

> 
> > Also fix a typo in a nearby comment ("generata" -> "generate").
> 
> "Also" is a good hint that it should be a separate patch. ;)
> 
> BR,
> Jani
> 
> > Fixes: d5dc0f43f268 ("drm/i915/gmbus: Enable burst read")
> > Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_gmbus.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_gmbus.c b/drivers/gpu/drm/i915/display/intel_gmbus.c
> > index 2caff677600c..5fb3fee34af4 100644
> > --- a/drivers/gpu/drm/i915/display/intel_gmbus.c
> > +++ b/drivers/gpu/drm/i915/display/intel_gmbus.c
> > @@ -496,8 +496,10 @@ gmbus_xfer_read_chunk(struct intel_display *display,
> >  
> >  		val = intel_de_read_fw(display, GMBUS3(display));
> >  		do {
> > -			if (extra_byte_added && len == 1)
> > +			if (extra_byte_added && len == 1) {
> > +				len--;
> >  				break;
> > +			}
> >  
> >  			*buf++ = val & 0xff;
> >  			val >>= 8;
> > @@ -693,7 +695,7 @@ do_gmbus_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs, int num,
> >  			goto clear_err;
> >  	}
> >  
> > -	/* Generate a STOP condition on the bus. Note that gmbus can't generata
> > +	/* Generate a STOP condition on the bus. Note that gmbus can't generate
> >  	 * a STOP on the very first cycle. To simplify the code we
> >  	 * unconditionally generate the STOP condition with an additional gmbus
> >  	 * cycle. */
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel