On Fri, Dec 23, 2022 at 09:02:35AM +0000, Tvrtko Ursulin wrote:
On 22/12/2022 15:55, Lucas De Marchi wrote:
On Thu, Dec 22, 2022 at 10:27:00AM +0000, Tvrtko Ursulin wrote:
On 22/12/2022 08:25, Lucas De Marchi wrote:
The comments are redundant to the checks being done to apply the
workarounds and very often get outdated as workarounds need to be
extended to new platforms or steppings. Remove them altogether with
the following matches (platforms extracted from intel_workarounds.c):
find drivers/gpu/drm/i915/gt/ -name '*.c' | xargs sed -i -E \
's/(Wa.*):(bdw|chv|bxt|glk|skl|kbl|cfl|cfl|whl|cml|aml|chv|cl|bw|ctg|elk|ilk|snb|dg|pvc|g4x|ilk|gen|glk|kbl|cml|glk|kbl|cml|hsw|icl|ehl|ivb|hsw|ivb|vlv|kbl|pvc|rkl|dg|adl|skl|skl|bxt|blk|cfl|cnl|glk|snb|tgl|vlv|xehpsdv).*/\1/'
find drivers/gpu/drm/i915/gt/ -name '*.c' | xargs sed -i -E \
's/(Wa.*):(bdw|chv|bxt|glk|skl|kbl|cfl|cfl|whl|cml|aml|chv|cl|bw|ctg|elk|ilk|snb|dg|pvc|g4x|ilk|gen|glk|kbl|cml|glk|kbl|cml|hsw|icl|ehl|ivb|hsw|ivb|vlv|kbl|pvc|rkl|dg|adl|skl|skl|bxt|blk|cfl|cnl|glk|snb|tgl|vlv|xehpsdv).*\*\//\1
Same things was executed in the gem directory, omitted here for brevity.
There were a few false positives that included the workaround
description. Those were manually patched.
sed -E 's/(Wa[a-zA-Z0-9_]+)[:,]([a-zA-Z0-9,-_\+\[]{2,})/\1/'
then there are false negatives. We have Was in the form
"Wa_xxx:tgl,dg2, mtl". False positives we can fixup, false negatives
we simply don't see. After running that in gt/:
$ git grep ": mtl" -- drivers/gpu/drm/i915/
drivers/gpu/drm/i915/gt/intel_gt_pm.c: /* Wa_14017073508: mtl */
drivers/gpu/drm/i915/gt/intel_gt_pm.c: /* Wa_14017073508: mtl */
drivers/gpu/drm/i915/gt/intel_gt_pm.c: /* Wa_14017073508: mtl */
drivers/gpu/drm/i915/gt/intel_gt_pm.c: /* Wa_14017073508: mtl */
drivers/gpu/drm/i915/gt/uc/intel_guc_rc.c: * Wa_14017073508: mtl
drivers/gpu/drm/i915/i915_reg.h:/* Wa_14017210380: mtl */
I was going with the platform names to avoid the false
negatives and because I was entertaining the idea of only doing this for
latest platforms where we do have the "Wa_[[:number:]]" form
Maybe..
Matt recently said he has this worked planned, but more
importantly - I gather then that the WA lookup tool definitely
does not output these strings?
Whatever it does it's true only at the time it's called. It simply
tells what
are the platforms and steppings the Wa applies to. We can change the
output to whatever we want, but that is not the point.
Those comments get stale and bring no real value as they match 1:1
what the code is supposed to be doing. Several times a patch has to
update just that comment to "extend a workaround" to a next platform.
This is not always done, so we get a comment that doesn't match what is
supposed to be there.
Tl;dr; version - lets park this until January and discuss once
everyone is back.
I'll leave my comment here since I will be out until mid January.
Longer version. I've been trying to get us talking about this a couple
times before and I'd really like to close with an explicit consensus,
discussion points addressed instead of skipped and just moving ahead
with patches.
References:
3fcf71b9-337f-6186-7b00-27cbfd116743@xxxxxxxxxxxxxxx
Y5j0b/bykbitCa4Q@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
So point I wanted to discuss is whether these comments are truly
useless or maybe they can help during review. If the tool can actually
output them then I am leaning towards that they can be.
I consider "can the tool output xyz?" asking the wrong question.
"The tool", which is our own small python script querying a database can
output anything like that if we want to. The database has information of
what are the platforms/steppings for each the WA is known to be applied
*today*. And that can change and do change often, particularly for early
steppings and recent platforms.
Thought is, when a patch comes for review adding a new platform,
stepping, whatever, to an existing if condition, if it contains the
comments reviewer can more easily spot a hyphotetical logic inversion
error or similar. It is also trivial to check that both condition and
comment have been updated. (So lets not be rash and remove something
maybe useful just because it can go stale *only if* reviewers are not
giving sufficient attention that changes are made in tandem.)
I can argue to the other side too. We don't have comments in the kernel
like
/* Add 1 to i */
i += 1;
This is exactly what these comments are doing. And they are misleading
and may introduce bugs rather than helping reviewing:
Wa_12345:tgl[a0,c0)
if (IS_TGL_GRAPHICS_STEP(STEP_A0, STEP_B0)
One might read the comment, skipping over the condition and thinking
"ok, we already extended this WA to B* steppings, which doesn't match
the code.
From a slightly different angle - do we expect anyone reviewing
workaround patches will cross-check against the tool? Would it be
simpler and more efficient that they could just cross-check against
the comment output from the tool and put into the patch by the author?
see above. Someone cross-checking the comment is cross-checking the
wrong thing. As I said, it happens more on early enabling of a platform.
And point here to stress out is that accidental logic errors (missed
workarounds) can be super expensive to debug in the field. Sometimes
it can literally take _months_ for sporadic and hard to reproduce
issues to get debugged, handed over between the teams, etc. So any way
in which we can influence the likelyhood of that happening is
something to weigh carefully.
yes, that's why I want to remove the comments: from my experience they
are more a source of bugs rather than helping.
Secondary but also important - if i915 is end of line then an extra
why we want to rip out this for ancient platforms. Is the cost/benefit
positive there?
yep, here I agree and was my argument about using the platform names
rather than a more "catch all" regex. I think doing this only for tgl+
platforms or even dg2+ would be ok.
As a side note, and going back to the question of what the tool can
output. Long time ago I had an idea where we could improve all this by
making it completely data-driven. Have the WA database inspecting tool
output a table which could be directly pasted into code and
interpreted by i915.
For reference look at intel_workarounds_table.h in
https://patchwork.freedesktop.org/patch/399377/?series=83580&rev=3 and
see what you thing. That was just a sketch of the idea, not complete,
and yes, i915 end of life point makes it moot.
now that xe is announced I can talk about this part... this was more
or less what I implemented in xe: it's a table with
"register + condition + action". There are the most common condition
checks builtin + a function hook for the more advanced ones. During
binding the driver walks the table and coalesces the entries creating
a per-register value that can be used at the proper times, depending if
they are gt, engine, context workarounds.
Lucas De Marchi
Regards,
Tvrtko
