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

[Jani Nikula <jani.nikula@intel.com>]

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?

> 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