Re: [PATCH] drm/i915: Update WaFlushCoherentL3CacheLinesAtContextSwitch

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jul 06, 2015 at 02:16:54PM +0100, Dave Gordon wrote:
> On 06/07/15 13:38, Daniel Vetter wrote:
> >On Mon, Jul 06, 2015 at 12:52:51PM +0100, Dave Gordon wrote:
> >>On 03/07/15 16:42, Chris Wilson wrote:
> >>>On Fri, Jul 03, 2015 at 02:27:31PM +0100, Arun Siluvery wrote:
> >>>>In this WA we need to set GEN8_L3SQCREG4[21:21] and reset it after PIPE_CONTROL
> >>>>instruction but there is a slight complication as this is applied in WA batch
> >>>>where the values are only initialized once.
> >>>>Dave identified an issue with the current implementation where the register value
> >>>>is read once at the beginning and it is reused; this patch corrects this by saving
> >>>>the register value to memory, update register with the bit of our interest and
> >>>>restore it back with original value.
> >>>>
> >>>>This implementation uses MI_LOAD_REGISTER_MEM which is currently only used
> >>>>by command parser and was using a default length of 0. This is now updated
> >>>>with correct length and moved to appropriate place.
> >>>>
> >>>>Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >>>>Cc: Dave Gordon <david.s.gordon@intel.com>
> >>>>Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
> >>>>---
> >>>>  drivers/gpu/drm/i915/i915_cmd_parser.c |  6 +--
> >>>>  drivers/gpu/drm/i915/i915_reg.h        |  3 +-
> >>>>  drivers/gpu/drm/i915/intel_lrc.c       | 72 +++++++++++++++++++++++++---------
> >>>>  3 files changed, 58 insertions(+), 23 deletions(-)
> >>>>
> >>>>diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
> >>>>index 306d9e4..430571b 100644
> >>>>--- a/drivers/gpu/drm/i915/i915_cmd_parser.c
> >>>>+++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
> 
> >>>>@@ -1021,7 +1021,7 @@ static bool check_cmd(const struct intel_engine_cs *ring,
> >>>>  			 * only MI_LOAD_REGISTER_IMM commands.
> >>>>  			 */
> >>>>  			if (reg_addr == OACONTROL) {
> >>>>-				if (desc->cmd.value == MI_LOAD_REGISTER_MEM) {
> >>>>+				if (desc->cmd.value == MI_LOAD_REGISTER_MEM(1)) {
> >>>
> >>>I had a double take here, but it all comes out in the wash. For one
> >>>moment, I thought the cmd matching had changed, but that has the length
> >>>masked out.
> >>>
> >>>Reviewed-by: Chris Wilson <chris@cris-wilson.co.uk>
> >
> >Queued for -next, thanks for the patch.
> >
> >>>Who will start to complain about all the extra frequent register writes,
> >>>probably into common power wells....
> >>>-Chris
> >>
> >>Hmm ... that is quite confusing, especially as the actual opcode in the
> >>instruction stream will be MI_LOAD_REGISTER_MEM(2) on GEN8+. It might almost
> >>be better to use MI_LOAD_REGISTER_MEM(0) to emphasise that the length field
> >>is a wildcard and not something that will be matched exactly.
> >
> >There's a separate _GEN8 #define to accomodate the differences, so I don't
> >fully understand your concern. We also don't do any decoding in the kernel
> >...
> >-Daniel
> 
> In the snippet:
> 
> >> -	CMD(  MI_LOAD_REGISTER_MEM,         SMI,   !F,  0xFF,   W | B,
> >> +	CMD(  MI_LOAD_REGISTER_MEM(1),      SMI,   !F,  0xFF,   W | B,
> 
> the (1) goes in the table but is ignored when matching instructions in the
> stream being parsed. It could just as well be (2) or (0) or (255).
> 
> Then, in the test:
> 
> >> -		if (desc->cmd.value == MI_LOAD_REGISTER_MEM) {
> >> +		if (desc->cmd.value == MI_LOAD_REGISTER_MEM(1)) {
> 
> the thing on the left of the == is not the instruction being examined, but
> the entry in the table that matched that instruction. So here also we're not
> really using the length field, EXCEPT that it MUST be the same as the
> (arbitrary) value in the table.
> 
> So my concern here was not about correctness but comprehensibility and hence
> maintainability -- after all, if Chris had to look twice it obviously isn't
> as clear as one would like!
> 
> My suggestion was that maybe the "ignored" length field should be 0 to make
> it less likely that a reader would think this matches exactly (and only) an
> opcode of 0xa400001. Or maybe (255) would be even more obviously
> not-a-literal-match?

Hm, given that the cmd parser is gen7 only I'm not too concerned about
this. It is indeed a bit surprising though, and I guess (0) would be less
surprising. Otoh other commands with a lenght field also use (1) in a
similar fashion, so at least this is consistent.

tbh no opinion here at all from my side, but happy to merge a fixup on top
to clarify this, if you can agree on a clear improvement.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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