Re: [RFC] Move BDW workarounds to ring init fn

["Siluvery, Arun" <arun.siluvery@linux.intel.com>]

On 28/07/2014 18:26, Ville Syrjälä wrote:
> On Mon, Jul 28, 2014 at 05:31:45PM +0100, arun.siluvery@linux.intel.com wrote:
>> From: Arun Siluvery <arun.siluvery@linux.intel.com>
>>
>> This patch moves BDW workarounds from init_clock_gating() to render ring
>> init fn otherwise they are lost when gpu is reset.
>> In case of execlists, some of the workarounds modify registers that are
>> part of register state context which doesn't get initialized until
>> init_clock_gating(); this results in default context with incorrect values
>> as it is restored and saved before updated by workarounds.
>
> I don't think it has to do with execlists. Many of the registers are
> part of the context image even in ring buffer mode AFAIK.
>
>>
>> Open issue:
>> For Wa4x4STCOptimizationDisable, we set CACHE_MODE_1[6:6] = 1
>> At the time when HW contexts are enabled after rings are initialized with
>> default context this workaround is valid but followed by a context switch
>> this is getting reset, please see below log snippet.
>
> This is a bit weird. The default context should have restore inhibit==1
> so it shouldn't clobber the CACHE_MODE_1 register. There was a specific magic
> dance you're supposed to do when accessing such registers with mmio, but here
> we do the write even before the first context switch.
>
> Apparently there was some kind of problem with CACHE_MODE_0 on snb too:
>   commit 3a69ddd6f872180b6f61fda87152b37202118fbc
>   Author: Kenneth Graunke <kenneth@whitecape.org>
>   Date:   Fri Apr 27 12:44:41 2012 -0700
>
>      drm/i915: Set the Stencil Cache eviction policy to non-LRA mode.
>
> but IIRC I wasn't able to reproduce it when I tried.

Similar to this register I am also applying this in render ring init fn.

>
> Maybe we need to delay these register writes until we've switched to the default
> context?
>
In its current state (WAs applied in init_clock_gating()) we are writing 
these registers after switching to default context.

When a new hw context is created does all the registers part of context 
start with default values or they sample the current state? and at what 
point this sampling takes place?

As a test I have updated CACHE_MODE_1 after mi_set_context() then the 
workaround was valid with every context switch but I think it may not be 
the right way otherwise we will have to update other WA registers also 
at this point with every context switch.

regards
Arun

>>
>> ...
>> [    5.978209] [drm:i915_pages_create_for_stolen] offset=0x0, size=8294400
>> [    5.978213] [drm:intel_alloc_plane_obj] plane fb obj ffff8801472e0000
>> [    5.978215] [drm:i915_gem_setup_global_gtt] reserving preallocated space: 0 + 7e9000
>> [    5.978216] [drm:i915_gem_setup_global_gtt] clearing unused GTT space: [7e9000, fffff000]
>> [    5.979613] [drm:i915_gem_init] CACHE_MODE_1: 0x00000180
>> [    5.981372] [drm:gen8_ppgtt_init] Allocated 4 pages for page directories (0 wasted)
>> [    5.981373] [drm:gen8_ppgtt_init] Allocated 2048 pages for page tables (0 wasted)
>> [    5.981376] [drm:i915_gem_context_init] HW context support initialized
>> [    5.981462] [drm:i915_gem_init_hw] CACHE_MODE_1: 0x00000180
>> [    5.981467] [drm:i915_gem_init_rings] CACHE_MODE_1: 0x00000180
>> [    5.981704] [drm:bdw_init_workarounds] CACHE_MODE_1: 0x000001C0
>> [    5.981716] [drm:init_status_page] bsd ring hws offset: 0x0081e000
>> [    5.981792] [drm:init_status_page] blitter ring hws offset: 0x0083f000
>> [    5.981910] [drm:init_status_page] video enhancement ring hws offset: 0x00860000
>> [    5.982001] [drm:i915_gem_init_hw] CACHE_MODE_1: 0x000001C0
>> [    5.982104] [drm:i915_gem_context_enable] Switch render ring to default_context
>> [    5.982106] [drm:i915_gem_render_state_init] render ring: Render state init
>> [    5.982120] [drm:do_switch] render ring, CACHE_MODE_1: 0x000001C0, uninitialized: 1
>> [    5.982121] [drm:i915_gem_context_enable] Switch bsd ring to default_context
>> [    5.982122] [drm:do_switch] bsd ring, CACHE_MODE_1: 0x000001C0, uninitialized: 0
>> [    5.982123] [drm:i915_gem_context_enable] Switch blitter ring to default_context
>> [    5.982126] [drm:do_switch] blitter ring, CACHE_MODE_1: 0x000001C0, uninitialized: 0
>> [    5.982126] [drm:i915_gem_context_enable] Switch video enhancement ring to default_context
>> [    5.982128] [drm:do_switch] video enhancement ring, CACHE_MODE_1: 0x000001C0, uninitialized: 0
>> [    5.982133] [drm:i915_gem_init] CACHE_MODE_1: 0x000001C0
>> [    5.982258] [drm:intel_init_clock_gating]
>> ...
>> [   10.037019] [drm:do_switch] blitter ring, CACHE_MODE_1: 0x00000180, uninitialized: 0
>> ...
>> [   10.488145] [drm:do_switch] render ring, CACHE_MODE_1: 0x00000180, uninitialized: 0
>> ...
>>
>> I am currently testing this with an igt which triggers a gpu reset and compares
>> WA register contents before and after reset but the test fails because of
>> this register hence not sending it now.
>> Please let me know how to keep this WA valid after a context switch.
>>
>>
>> Arun Siluvery (1):
>>    drm/i915/bdw: Initialize BDW workarounds in render ring init fn
>>
>>   drivers/gpu/drm/i915/i915_debugfs.c     | 46 ++++++++++++++++++++++
>>   drivers/gpu/drm/i915/intel_pm.c         | 59 ----------------------------
>>   drivers/gpu/drm/i915/intel_ringbuffer.c | 68 +++++++++++++++++++++++++++++++++
>>   3 files changed, 114 insertions(+), 59 deletions(-)
>>
>> --
>> 1.9.2
>>
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