[Intel-gfx] [PATCH] drm/i915/hsw: Fix workaround for server AUX channel clock divisor

[Jani Nikula]

On Thu, 21 May 2015, Jim Bride <jim.bride at linux.intel.com> wrote:
> On Thu, May 21, 2015 at 03:04:40PM +0300, Jani Nikula wrote:
>> On Thu, 21 May 2015, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
>> > On Wed, May 20, 2015 at 04:07:37PM -0700, Todd Previte wrote:
>> >> Hi Jim,
>> >> 
>> >> I checked the BSpec as well and there's nothing indicating that these 
>> >> two bits are mutually exclusive. They are both sticky though, and if the 
>> >> loop times out 5 times, both _DONE and _TIMEOUT may very well be set. In 
>> >> that case the current code would just exit and never bother to change 
>> >> clock dividers. So I think your code here is valid.
>> >
>> > Shouldn't we we checking receive error as well then?
>> 
>> In other words,
>> 
>> 
>> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> index 7cf3fd43071a..179af62d803d 100644
>> --- a/drivers/gpu/drm/i915/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/intel_dp.c
>> @@ -893,10 +893,8 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>>  				continue;
>>  			}
>>  			if (status & DP_AUX_CH_CTL_DONE)
>> -				break;
>> +				goto done;
>>  		}
>> -		if (status & DP_AUX_CH_CTL_DONE)
>> -			break;
>>  	}
>>  
>>  	if ((status & DP_AUX_CH_CTL_DONE) == 0) {
>> @@ -921,7 +919,7 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>>  		ret = -ETIMEDOUT;
>>  		goto out;
>>  	}
>> -
>> +done:
>>  	/* Unload any bytes sent back from the other side */
>>  	recv_bytes = ((status & DP_AUX_CH_CTL_MESSAGE_SIZE_MASK) >>
>>  		      DP_AUX_CH_CTL_MESSAGE_SIZE_SHIFT);
>
> This looks good, except I think we might want something more like:
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 0edc305..89ab714 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -893,10 +893,8 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>                                 continue;
>                         }
>                         if (status & DP_AUX_CH_CTL_DONE)
> -                               break;
> +                               goto done;;
>                 }
> -               if (status & DP_AUX_CH_CTL_DONE)
> -                       break;
>         }
>  
>         if ((status & DP_AUX_CH_CTL_DONE) == 0) {
> @@ -905,6 +903,7 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>                 goto out;
>         }
>  
> +done:
>         /* Check for timeout or receive error.
>          * Timeouts occur when the sink is not connected
>          */
>
> This way we actually set ret appropriately in the event of a receive error
> or a timeout.
>
> Thoughts?

When we hit the "goto done" part, I think we have all the error
conditions already covered within the innermost loop, and everything is
okay. We have to drop out of the loops to hit the errors.

But I'm fine either way.

BR,
Jani


>
> Jim
>
>
>> 
>> 
>> 
>> >
>> > In fact looking at our code after the loop, it's expecting DONE to be
>> > set before it'll even report receive/timeout errors to the user as
>> > EIO/ETIMEDOUT, otherwise it'll just return EBUSY.
>> >
>> >> 
>> >> The only thing you may want to do is capture some of the IRC discussion 
>> >> though. In particular, whether or not this is really HSW-specific, how 
>> >> this affects other platforms and and the additional delays incurred by 
>> >> running through the loop again after changing AUX channel clock dividers 
>> >> would be good information to put in there. That's probably more of a 
>> >> bikeshed though.
>> >> 
>> >> Reviewed-by: Todd Previte <tprevite at gmail.com>
>> >> 
>> >> -T
>> >> 
>> >> On 5/19/2015 9:13 AM, jim.bride at linux.intel.com wrote:
>> >> > From: Jim Bride <jim.bride at linux.intel.com>
>> >> >
>> >> > According to the HSW b-spec we need to try clock divisors of 63
>> >> > and 72, each 3 or more times, when attempting DP AUX channel
>> >> > communication on a server chipset.  This actually wasn't happening
>> >> > due to a short-circuit that only checked the DP_AUX_CH_CTL_DONE bit
>> >> > in status rather than checking that the operation was done and
>> >> > that DP_AUX_CH_CTL_TIME_OUT_ERROR was not set.
>> >> >
>> >> > Signed-off-by: Jim Bride <jim.bride at linux.intel.com>
>> >> > ---
>> >> >   drivers/gpu/drm/i915/intel_dp.c | 3 ++-
>> >> >   1 file changed, 2 insertions(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> >> > index 0edc305..c01a3f9 100644
>> >> > --- a/drivers/gpu/drm/i915/intel_dp.c
>> >> > +++ b/drivers/gpu/drm/i915/intel_dp.c
>> >> > @@ -895,7 +895,8 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>> >> >   			if (status & DP_AUX_CH_CTL_DONE)
>> >> >   				break;
>> >> >   		}
>> >> > -		if (status & DP_AUX_CH_CTL_DONE)
>> >> > +		if ((status & DP_AUX_CH_CTL_DONE) &&
>> >> > +		    !(status & DP_AUX_CH_CTL_TIME_OUT_ERROR))
>> >> >   			break;
>> >> >   	}
>> >> >   
>> >> 
>> >> _______________________________________________
>> >> Intel-gfx mailing list
>> >> Intel-gfx at lists.freedesktop.org
>> >> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>> >
>> > -- 
>> > Ville Syrjälä
>> > Intel OTC
>> > _______________________________________________
>> > Intel-gfx mailing list
>> > Intel-gfx at lists.freedesktop.org
>> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>> 
>> -- 
>> Jani Nikula, Intel Open Source Technology Center
>> _______________________________________________
>> Intel-gfx mailing list
>> Intel-gfx at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Jani Nikula, Intel Open Source Technology Center