[Mesa-dev] [PATCH] i965: Advertise 8 bits subpixel precision for viewport bounds on gen6+

[Danylo Piliaiev]

Hi,

Since the exact value is not important for users,  Roland offered 
compelling explanation
and the value is also 8 on Windows - could the patch be reviewed/pushed?

- Danil

On 06/19/2018 03:44 PM, Roland Scheidegger wrote:
> My guess would be 8 because that's what the rasterization subpixel
> precision is, so precision beyond that doesn't really do much, even if
> this actually is a float.
> Plus, with maximum sized fb (16kx16k dimension) you don't actually
> really get a lot more than 8 fixed points bits anyway (near those 16k).
> So imho 8 makes most sense.
>
> Roland
>
>
> Am 19.06.2018 um 10:35 schrieb danylo:
>> Hi Lionel,
>>
>> Indeed the value 8 here is questionable. I picked it because other
>> drivers advertise the same value e.g. in Mesa radeon returns 8 for gl
>> and vulkan or on Windows Intel's driver returns 8. But why 8? It's some
>> kind of mystery.
>>
>>> "If the implementation truely has floating point viewport bounds, it
>>> may report a sufficiently high value to indicate this. "
>> 8 seems to be a sufficiently high value (it seems if someone even checks
>> the value it's going like 'precision > 0' - it is used as a flag). But
>> still it's probably not good enough argument...
>>
>> Floating point (IEEE 754) has 24 bits of significand precision, in other
>> way - 6 to 9 significant decimal digits. And drivers return 8, the only
>> 8 in float-point is 8 exponent bits.
>>
>> Unless someone knows why 8, there two paths:
>>
>>    * Left it to be 8 - be the same as other drivers
>>    * Make 24 - to reflect 24 bits of significand precision of float
>>
>>
>> - Danil
>>
>>
>> On 18.06.18 17:27, Lionel Landwerlin wrote:
>>> Hey Danylo,
>>>
>>> Thanks for this patch.
>>> I'm not really an expert here but my understanding is that it should
>>> reflect the number of bits in fixed point precision.
>>> We use 32bits floats in the packets sent to the hardware.
>>> Quoting the spec :
>>>
>>> "If the implementation truely has floating point viewport bounds, it
>>> may report a sufficiently high value to indicate this. "
>>>
>>> Maybe we should use something a bit bigger than 8?
>>>
>>> Cheers,
>>>
>>> -
>>> Lionel
>>>
>>> On 18/06/18 13:50, Danylo Piliaiev wrote:
>>>> We use floating-points for viewport bounds so VIEWPORT_SUBPIXEL_BITS
>>>> should reflect this.
>>>>
>>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105975
>>>>
>>>> Signed-off-by: Danylo Piliaiev <danylo.piliaiev at globallogic.com>
>>>> ---
>>>>   src/mesa/drivers/dri/i965/brw_context.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
>>>> index 9ced230..eacf326 100644
>>>> --- a/src/mesa/drivers/dri/i965/brw_context.c
>>>> +++ b/src/mesa/drivers/dri/i965/brw_context.c
>>>> @@ -688,7 +688,7 @@ brw_initialize_context_constants(struct brw_context *brw)
>>>>      /* ARB_viewport_array, OES_viewport_array */
>>>>      if (devinfo->gen >= 6) {
>>>>         ctx->Const.MaxViewports = GEN6_NUM_VIEWPORTS;
>>>> -      ctx->Const.ViewportSubpixelBits = 0;
>>>> +      ctx->Const.ViewportSubpixelBits = 8;
>>>>   
>>>>         /* Cast to float before negating because MaxViewportWidth is unsigned.
>>>>          */
>>>
>>
>>
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