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

[Danylo Piliaiev]

On 8/20/18 4:28 PM, Lionel Landwerlin wrote:
> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Thanks!
>
> Do you need me to push it?
Yes
>
> Thanks,
>
> -
> Lionel
>
> On 08/08/18 16:03, Danylo Piliaiev wrote:
>> 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.
>>>>>>          */
>>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> mesa-dev mailing list
>>>> mesa-dev at lists.freedesktop.org
>>>> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Fmailman%2Flistinfo%2Fmesa-dev&data=02%7C01%7Csroland%40vmware.com%7Ce1ac7ef5378f4227aff508d5d5bfa182%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636649941389721543&sdata=of%2FIoIzosaje7A3euJESjJMySE3eFjNifMg5SXzeEJU%3D&reserved=0 
>>>>
>>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
>