Re: [RFC PATCH 38/97] drm/i915/guc: Optimize CTB writes and reads

On 06.05.2021 21:13, Matthew Brost wrote:

CTB writes are now in the path of command submission and should be optimized for performance. Rather than reading CTB descriptor values (e.g. head, tail, size) which could result in accesses across the PCIe

size was removed from the descriptor in 25/97

bus, store shadow local copies and only read/write the descriptor values when absolutely necessary.

maybe worth to add some words about caching available space ?

Signed-off-by: John Harrison John.C.Harrison@Intel.com Signed-off-by: Matthew Brost matthew.brost@intel.com

---

drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 78 +++++++++++++---------- drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h | 6 ++ 2 files changed, 52 insertions(+), 32 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c index 4eab319d61be..77dfbc94dcc3 100644 --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c @@ -127,6 +127,10 @@ static void guc_ct_buffer_desc_init(struct guc_ct_buffer_desc *desc) static void guc_ct_buffer_reset(struct intel_guc_ct_buffer *ctb) { ctb->broken = false;

• ctb->tail = 0;
• ctb->head = 0;
• ctb->space = CIRC_SPACE(ctb->tail, ctb->head, ctb->size);
• guc_ct_buffer_desc_init(ctb->desc);

}

@@ -371,10 +375,8 @@ static int ct_write(struct intel_guc_ct *ct, { struct intel_guc_ct_buffer *ctb = &ct->ctbs.send; struct guc_ct_buffer_desc *desc = ctb->desc;

• u32 head = desc->head;
• u32 tail = desc->tail;

• u32 tail = ctb->tail; u32 size = ctb->size;

• u32 used; u32 header; u32 hxg; u32 *cmds = ctb->cmds;

@@ -386,25 +388,14 @@ static int ct_write(struct intel_guc_ct *ct, if (unlikely(desc->status)) goto corrupted;

• if (unlikely((tail | head) >= size)) {

+#ifdef CONFIG_DRM_I915_DEBUG_GUC

• if (unlikely((desc->tail | desc->head) >= size)) { CT_ERROR(ct, "Invalid offsets head=%u tail=%u (size=%u)\n",

• 	 head, tail, size);
  

• 	 desc->head, desc->tail, size);
  

  desc->status |= GUC_CTB_STATUS_OVERFLOW; goto corrupted;

nit: as we are now caching tail value, we can start comparing it with the value in descriptor and report GUC_CTB_STATUS_MISMATCH if needed

}

• /*

• * tail == head condition indicates empty. GuC FW does not support
  

• * using up the entire buffer to get tail == head meaning full.
  

• */
  

• if (tail < head)

• used = (size - head) + tail;
  

• else

• used = tail - head;
  

• /* make sure there is a space including extra dw for the fence */
• if (unlikely(used + len + 1 >= size))

• return -ENOSPC;
  

+#endif

/* * dw0: CT header (including fence) @@ -444,7 +435,9 @@ static int ct_write(struct intel_guc_ct *ct, write_barrier(ct);

/* now update descriptor */

• ctb->tail = tail; WRITE_ONCE(desc->tail, tail);

• ctb->space -= len + 1;

  return 0;

@@ -460,7 +453,7 @@ static int ct_write(struct intel_guc_ct *ct,

• @req: pointer to pending request
• @status: placeholder for status

• • For each sent request, Guc shall send bac CT response message.

• • For each sent request, GuC shall send back CT response message.
  • Our message handler will update status of tracked request once
  • response message with given fence is received. Wait here and
  • check for valid response status value.

@@ -508,24 +501,35 @@ static inline bool ct_deadlocked(struct intel_guc_ct *ct) return ret; }

-static inline bool ctb_has_room(struct intel_guc_ct_buffer *ctb, u32 len_dw) +static inline bool h2g_has_room(struct intel_guc_ct *ct, u32 len_dw)

this function was introduced moment ago ... can we minimize number of changes between patches ?

{

• struct guc_ct_buffer_desc *desc = ctb->desc;
• u32 head = READ_ONCE(desc->head);

• struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
• u32 head; u32 space;

• space = CIRC_SPACE(desc->tail, head, ctb->size);

• if (ctb->space >= len_dw)

• return true;
  

• head = READ_ONCE(ctb->desc->head);
• if (unlikely(head > ctb->size)) {

• CT_ERROR(ct, "Corrupted descriptor head=%u tail=%u size=%u\n",
  

• 	  ctb->desc->head, ctb->desc->tail, ctb->size);
  

• ctb->desc->status |= GUC_CTB_STATUS_OVERFLOW;
  

• ctb->broken = true;
  

• return false;
  

• }
• space = CIRC_SPACE(ctb->tail, head, ctb->size);
• ctb->space = space;

shouldn't we update our ctb->head with new head value?

return space >= len_dw; }

static int has_room_nb(struct intel_guc_ct *ct, u32 len_dw) {

• struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;

• lockdep_assert_held(&ct->ctbs.send.lock);

• if (unlikely(!ctb_has_room(ctb, len_dw))) {

• if (unlikely(!h2g_has_room(ct, len_dw))) { if (ct->stall_time == KTIME_MAX) ct->stall_time = ktime_get();

@@ -593,11 +597,11 @@ static int ct_send(struct intel_guc_ct *ct, * rare. */ retry:

• spin_lock_irqsave(&ct->ctbs.send.lock, flags);
• if (unlikely(!ctb_has_room(ctb, len + 1))) {

• spin_lock_irqsave(&ctb->lock, flags);
• if (unlikely(!h2g_has_room(ct, len + 1))) { if (ct->stall_time == KTIME_MAX) ct->stall_time = ktime_get();

• spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);
  

• spin_unlock_irqrestore(&ctb->lock, flags);
  

this ...

if (unlikely(ct_deadlocked(ct)))
	return -EDEADLK;

@@ -620,7 +624,7 @@ static int ct_send(struct intel_guc_ct *ct,

err = ct_write(ct, action, len, fence, 0);

• spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);

• spin_unlock_irqrestore(&ctb->lock, flags);

and this likely could be done in earlier patch

if (unlikely(err)) goto unlink; @@ -708,7 +712,7 @@ static int ct_read(struct intel_guc_ct *ct, struct ct_incoming_msg **msg) { struct intel_guc_ct_buffer *ctb = &ct->ctbs.recv; struct guc_ct_buffer_desc *desc = ctb->desc;

• u32 head = desc->head;

• u32 head = ctb->head; u32 tail = desc->tail; u32 size = ctb->size; u32 *cmds = ctb->cmds;

@@ -723,12 +727,21 @@ static int ct_read(struct intel_guc_ct *ct, struct ct_incoming_msg **msg) if (unlikely(desc->status)) goto corrupted;

• if (unlikely((tail | head) >= size)) {

+#ifdef CONFIG_DRM_I915_DEBUG_GUC

• if (unlikely((desc->tail | desc->head) >= size)) { CT_ERROR(ct, "Invalid offsets head=%u tail=%u (size=%u)\n", head, tail, size); desc->status |= GUC_CTB_STATUS_OVERFLOW; goto corrupted; }

+#else

• if (unlikely((tail | ctb->head) >= size)) {

we are in control of cached 'ctb->head' so it shall never be > size

• CT_ERROR(ct, "Invalid offsets head=%u tail=%u (size=%u)\n",
  

• 	 head, tail, size);
  

• desc->status |= GUC_CTB_STATUS_OVERFLOW;
  

• goto corrupted;
  

• }

+#endif

/* tail == head condition indicates empty */ available = tail - head; @@ -778,6 +791,7 @@ static int ct_read(struct intel_guc_ct *ct, struct ct_incoming_msg **msg) } CT_DEBUG(ct, "received %*ph\n", 4 * len, (*msg)->msg);

• ctb->head = head; /* now update descriptor */ WRITE_ONCE(desc->head, head);

diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h index 55ef7c52472f..9924335e2ee6 100644 --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h @@ -33,6 +33,9 @@ struct intel_guc;

• @desc: pointer to the buffer descriptor
• @cmds: pointer to the commands buffer
• @size: size of the commands buffer in dwords

• • @head: local shadow copy of head in dwords
• • @tail: local shadow copy of tail in dwords
• • @space: local shadow copy of space in dwords
  • @broken: flag to indicate if descriptor data is broken
  */

struct intel_guc_ct_buffer { @@ -40,6 +43,9 @@ struct intel_guc_ct_buffer { struct guc_ct_buffer_desc *desc; u32 *cmds; u32 size;

• u32 tail;
• u32 head;
• u32 space; bool broken;

};