Re: [PATCH] drm/i915: Fix correct FIFO size for Baytrail

[Vijay Purushothaman <vijay.a.purushothaman@intel.com>]

On 2/7/2014 9:28 PM, Ville Syrjälä wrote:
> On Fri, Feb 07, 2014 at 08:43:12PM +0530, Vijay Purushothaman wrote:
>> B-spec says the FIFO total size is 512. So fix this to 512.
>>
>> Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
>> ---
>>   drivers/gpu/drm/i915/i915_reg.h |    2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
>> index cc3ea04..fb73031 100644
>> --- a/drivers/gpu/drm/i915/i915_reg.h
>> +++ b/drivers/gpu/drm/i915/i915_reg.h
>> @@ -3395,7 +3395,7 @@
>>   #define I915_FIFO_LINE_SIZE	64
>>   #define I830_FIFO_LINE_SIZE	32
>>
>> -#define VALLEYVIEW_FIFO_SIZE	255
>> +#define VALLEYVIEW_FIFO_SIZE	511
>>   #define G4X_FIFO_SIZE		127
>>   #define I965_FIFO_SIZE		512
>>   #define I945_FIFO_SIZE		127
>
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Thanks for the review.

>
> Not that we actually use the value anywhere at the moment.

This value is used when the display controller is configured in Max FIFO 
mode. This is working fine in the android tree. At this moment i am 
rewriting some logic related to this Max FIFO, memory arbiter credits 
and drain latency handling for the sprite planes. I should have the 
patches ready over the week end, will test it on monday once i get to 
office.

>
> As a side note the FIFO sizing for gmch platforms seems to be a place
> where the documentation is rather poor. It kind of looks like there
> are off by one errors in the text, and yet when I was playing around
> with this stuff on gen2/gen4 machines it kind of looks like the
> hardware has the same off by one issues too. IIRC my conlusion was
> that the last cacheline in the FIFO can't actually be used. So
> specifying 511 matches with my conclusion.

I agree.. I was thrown off by this oddity as well and it took some time 
for me to understand this completely. The display block in Baytrail is a 
mix and match of features from gen4 & gen5 (Cantiga, Crestline and 
Ironlake). No wonder this chip has the same h/w issues..

>
> One other thing I did notice now that I look at our g4x/vlv watermark
> code. We seem to assume the watermarks for g4x/vlv work the same way
> as pch platforms. Ie. you specify the minimum level of data left in the
> FIFO before it needs to start fetching more. But the documentation
> suggests that it's the other way around, where you specify the max
> amount of free space allowed in the FIFO before more data needs to be
> fetched. We use the latter logic for gen2-gen4. I wonder if the spec
> is wrong, or if your code is wrong. I guess I just need to verify it
> on real hardware at some point...
>
I think this is implemented correctly in the android kernel.. On a high 
level the split is something like 32 KB FIFO per pipe - 16 KB for 
primary plane and 8 KB for each sprite. When we are in single display 
mode we can configure the entire 64KB FIFO for the same pipe. There is 
another trick to enable trickle feed.. With all these tricks i am seeing 
good memory self refresh numbers - almost on par with the theoretical 
target. I should be able to post the patch series on monday once i do 
some sanity testing..

Thanks,
Vijay