Re: [PATCH] drm/i915/perf: Update handling of MMIO triggered reports

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Mon, 18 Dec 2023 16:05:43 -0800, Umesh Nerlige Ramappa wrote:
>

Hi Umesh,

> On XEHP platforms user is not able to find MMIO triggered reports in the
> OA buffer since i915 squashes the context ID fields. These context ID
> fields hold the MMIO trigger markers.
>
> Update logic to not squash the context ID fields of MMIO triggered
> reports.
>
> Fixes: cba94bbcff08 ("drm/i915/perf: Determine context valid in OA reports")
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_perf.c | 39 ++++++++++++++++++++++++++++----
>  1 file changed, 34 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index 7b1c8de2f9cb..2d695818f006 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -772,10 +772,6 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
>		 * The reason field includes flags identifying what
>		 * triggered this specific report (mostly timer
>		 * triggered or e.g. due to a context switch).
> -		 *
> -		 * In MMIO triggered reports, some platforms do not set the
> -		 * reason bit in this field and it is valid to have a reason
> -		 * field of zero.
>		 */
>		reason = oa_report_reason(stream, report);
>		ctx_id = oa_context_id(stream, report32);
> @@ -787,8 +783,41 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
>		 *
>		 * Note: that we don't clear the valid_ctx_bit so userspace can
>		 * understand that the ID has been squashed by the kernel.
> +		 *
> +		 * Update:
> +		 *
> +		 * On XEHP platforms the behavior of context id valid bit has
> +		 * changed compared to prior platforms. To describe this, we
> +		 * define a few terms:
> +		 *
> +		 * context-switch-report: This is a report with the reason type
> +		 * being context-switch. It is generated when a context switches
> +		 * out.
> +		 *
> +		 * context-valid-bit: A bit that is set in the report ID field
> +		 * to indicate that a valid context has been loaded.
> +		 *
> +		 * gpu-idle: A condition characterized by a
> +		 * context-switch-report with context-valid-bit set to 0.
> +		 *
> +		 * On prior platforms, context-id-valid bit is set to 0 only
> +		 * when GPU goes idle. In all other reports, it is set to 1.
> +		 *
> +		 * On XEHP platforms, context-valid-bit is set to 1 in a context
> +		 * switch report if a new context switched in. For all other
> +		 * reports it is set to 0.
> +		 *
> +		 * This change in behavior causes an issue with MMIO triggered
> +		 * reports. MMIO triggered reports have the markers in the
> +		 * context ID field and the context-valid-bit is 0. The logic
> +		 * below to squash the context ID would render the report
> +		 * useless since the user will not be able to find it in the OA
> +		 * buffer. Since MMIO triggered reports exist only on XEHP,
> +		 * we should avoid squashing these for XEHP platforms.

Hmm I am wondering if this is over-information and this comment should be
made brief. For the record, here's the explanation of what is happening
from Robert Krzemien's email (which at least makes it simpler for me to
understand what is happening):

	For Gen12HP+ (ATS/DG2/PVC/MTL+) platforms, context id valid bit is
	set only for context switch reports and when a context is being
	loaded. When exiting a context, a context switch report is
	generated, ctx id is not zero, but the bit is not set. It allows us
	to distinguish whether context switch reports are generated due to
	entering or exiting GPU contexts. Ctx id field is non-zero for
	context switches and mmio triggers. Other types always have ctx id
	set to 0.

	For previous platforms (like Gen12LP, Gen9/11), the bit is set for
	all types of reports if a context is loaded. But those older
	platforms don’t have mmio triggers. Ctx id field is non-zero for
	all types of reports if a context is loaded.

	I don’t understand why i915 needs to set ctx id to 0xffffffff if
	the flag is not set. It has been removed from XE KMD as I remember
	correctly.

>		 */
> -		if (oa_report_ctx_invalid(stream, report)) {
> +
> +		if (oa_report_ctx_invalid(stream, report) &&
> +		    GRAPHICS_VER_FULL(stream->engine->i915) < IP_VER(12, 50)) {
>			ctx_id = INVALID_CTX_ID;
>			oa_context_id_squash(stream, report32);
>		}

So I am assuming there's some unknown reason (maybe not hearing from Mesa?)
why we can't get rid of the squashing even for legacy platforms. But that's
ok I guess. So this is:

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>