[Mesa-dev] [PATCH 01/13] i965: Factor out logic to build a send message instruction with indirect descriptor.

[Matt Turner]

On Wed, Mar 11, 2015 at 2:29 PM, Francisco Jerez <currojerez at riseup.net> wrote:
> Matt Turner <mattst88 at gmail.com> writes:
>> commit 4c4934636cb286e7d7836afc26e9d392e2f0f155
>> Author: Paul Berry <stereotype441 at gmail.com>
>> Date:   Tue Sep 24 15:18:52 2013 -0700
>>
>>     i965/blorp: retype destination register for texture SEND instruction to UW.
>>
>> The resource streamer only exists on HSW+, so the UW dest is certainly
>> needed for things after Gen5.
>
> Odd, I haven't seen a mention of that restriction in the hardware specs
> (at least not on reasonably recent ones -- the Gen4 and 5 specs do
> mention it and they actually hang if you send a message with compression
> enabled and anything bigger than a W as destination type).  Is this a
> purely empirical finding?  If so, doesn't it deserve a big fat warning
> comment?  Is this only a problem for some interaction with the resource
> streamer or has it ever been observed to fix something else?

This page [0] says:

"""
The subregister number, horizontal stride, destination mask and type
fields of <dest> are always valid and are used in part to generate the
WrEn. This is true even if <dest> is a null register (this is an
exception for null as for most cases these fields are ignored by
hardware). These parameters of <dest> follow the same restriction as
that of normal destination operand – destination region cannot cross
the 256-bit register boundary.
"""

Searching for the exact phrase quoted in Paul's commit finds another
page that says it applies to "DevSNB+,Pre-DevBDW".

I think this is one of those cases where they technically give you all
the information, they just don't tell you anything about what you're
supposed to do with it. Totally bullshit, but par for the course.

I guess the good news in all of this is that we now know we don't need
to bother with this for BDW+.

[0] 3D-Media-GPGPU Engine > EU Overview > ISA Introduction >
Instruction Set Reference > EUISA Instructions > Send Message [SNB+]