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

[Roland Scheidegger]

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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