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While small, this is a > > > non-trivial amount of time in a hot path. Remove the descriptor H2G read > > > (potentially a VRAM access) from non-debug builds, as this > > > error-checking code is not needed outside of debug configurations. > > > > About the change itself: I understand the performance benefit and this would > > make the code consistent with the 2 other reads. > > > > Yes. > > > But the existing block under "if (IS_ENABLED(CONFIG_DRM_XE_DEBUG))" seems wrong > > because it is not just about optionally printing some debug statements, in case > > of error the function actually returns a different value if CONFIG_DRM_XE_DEBUG > > is enabled or not (goto), and this return value is used in __guc_ct_send_locked. > > > > I can't realy argue with this logic but I also see the benefits of > verbose checking of GuC state in CI configs. fwiw, this logic was added > here: > > git format-patch -1 d2c5a5a926f43 > > The cost of reads are actually huge here - this patch by itself is > incomplete as a tracepoint below does a desc_read in the argument list > and this is uncoditionally executed even if ftrace is disabled. On BMG H2G > before this patch / fixing ftrace are 3-4us, after ~300ns. So I'd > suggest we move forward with with a couple of fixes patches first, then > in a follow up either: > > The cost of reads is actually huge here — this patch by itself is > incomplete, as a tracepoint below performs a desc_read in the argument > list, and that is unconditionally executed even if ftrace is disabled. > On BMG, H2G reads before this patch / fixing ftrace are 3–4-µs; after, > they’re ~300ns. So I’d suggest we move forward with a couple of fixes > patches first, and then in a follow-up: > > - Move all really expensive things in GuC CT layer under > CONFIG_DRM_XE_DEBUG_GUC, enable this Kconfig some CI run > - Perhaps just assert if GuC state is corrupted in the path mentioned > above Sounds good. > > Matt > > > Francois > > > > > > > > Signed-off-by: Matthew Brost > > > --- > > > drivers/gpu/drm/xe/xe_guc_ct.c | 14 +++++++------- > > > 1 file changed, 7 insertions(+), 7 deletions(-) > > > > > > diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c > > > index 6a96bea40720..f200d3ee9d22 100644 > > > --- a/drivers/gpu/drm/xe/xe_guc_ct.c > > > +++ b/drivers/gpu/drm/xe/xe_guc_ct.c > > > @@ -939,22 +939,22 @@ static int h2g_write(struct xe_guc_ct *ct, const u32 *action, u32 len, > > > u32 full_len; > > > struct iosys_map map = IOSYS_MAP_INIT_OFFSET(&h2g->cmds, > > > tail * sizeof(u32)); > > > - u32 desc_status; > > > > > > full_len = len + GUC_CTB_HDR_LEN; > > > > > > lockdep_assert_held(&ct->lock); > > > xe_gt_assert(gt, full_len <= GUC_CTB_MSG_MAX_LEN); > > > > > > - desc_status = desc_read(xe, h2g, status); > > > - if (desc_status) { > > > - xe_gt_err(gt, "CT write: non-zero status: %u\n", desc_status); > > > - goto corrupted; > > > - } > > > - > > > if (IS_ENABLED(CONFIG_DRM_XE_DEBUG)) { > > > u32 desc_tail = desc_read(xe, h2g, tail); > > > u32 desc_head = desc_read(xe, h2g, head); > > > + u32 desc_status; > > > + > > > + desc_status = desc_read(xe, h2g, status); > > > + if (desc_status) { > > > + xe_gt_err(gt, "CT write: non-zero status: %u\n", desc_status); > > > + goto corrupted; > > > + } > > > > > > if (tail != desc_tail) { > > > desc_write(xe, h2g, status, desc_status | GUC_CTB_STATUS_MISMATCH); > > > -- > > > 2.34.1 > > >