Re: [Intel-gfx] [PATCH 6/7] drm/i915/guc: Optimize CTB writes and reads

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Jul 06, 2021 at 03:51:00PM -0700, John Harrison wrote:
> On 7/6/2021 15:20, 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) which could result in accesses across the PCIe bus,
> > store shadow local copies and only read/write the descriptor values when
> > absolutely necessary. Also store the current space in the each channel
> > locally.
> > 
> > v2:
> >   (Michal)
> >    - Add additional sanity checks for head / tail pointers
> >    - Use GUC_CTB_HDR_LEN rather than magic 1
> > v3:
> >   (Michal / John H)
> >    - Drop redundant check of head value
> > 
> > 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 | 88 +++++++++++++++--------
> >   drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h |  6 ++
> >   2 files changed, 65 insertions(+), 29 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 db3e85b89573..4a73a1f03a9b 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> > @@ -130,6 +130,10 @@ 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);
> >   }
> > @@ -383,10 +387,8 @@ 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 type;
> > @@ -396,25 +398,22 @@ static int ct_write(struct intel_guc_ct *ct,
> >   	if (unlikely(desc->status))
> >   		goto corrupted;
> > -	if (unlikely((tail | head) >= size)) {
> > +	GEM_BUG_ON(tail > size);
> > +
> > +#ifdef CONFIG_DRM_I915_DEBUG_GUC
> > +	if (unlikely(tail != READ_ONCE(desc->tail))) {
> > +		CT_ERROR(ct, "Tail was modified %u != %u\n",
> > +			 desc->tail, ctb->tail);
> > +		desc->status |= GUC_CTB_STATUS_MISMATCH;
> > +		goto corrupted;
> > +	}
> > +	if (unlikely((desc->tail | desc->head) >= size)) {
> Same arguments below about head apply to tail here. Also, there is no #else

Yes, desc->tail can be removed from this check. Same for head below. Can
you fix this when merging?

> check on ctb->head?

ctb->head variable isn't used in this path nor is ctb->tail in the
other. In the other path desc->tail is checked as it is read while
desc->head isn't needed to be read here. The other path can also likely
be reworked to pull the tail check outside of the if / else define
block.

> 
> >   		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;
> >   	}
> > -
> > -	/*
> > -	 * 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 header */
> > -	if (unlikely(used + len + GUC_CTB_HDR_LEN >= size))
> > -		return -ENOSPC;
> > +#endif
> >   	/*
> >   	 * dw0: CT header (including fence)
> > @@ -453,7 +452,9 @@ 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 + GUC_CTB_HDR_LEN;
> >   	return 0;
> > @@ -469,7 +470,7 @@ 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.
> > @@ -525,24 +526,35 @@ static inline bool ct_deadlocked(struct intel_guc_ct *ct)
> >   	return ret;
> >   }
> > -static inline bool h2g_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)
> >   {
> > -	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;
> >   	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(!h2g_has_room(ctb, len_dw))) {
> > +	if (unlikely(!h2g_has_room(ct, len_dw))) {
> >   		if (ct->stall_time == KTIME_MAX)
> >   			ct->stall_time = ktime_get();
> > @@ -612,7 +624,7 @@ static int ct_send(struct intel_guc_ct *ct,
> >   	 */
> >   retry:
> >   	spin_lock_irqsave(&ctb->lock, flags);
> > -	if (unlikely(!h2g_has_room(ctb, len + GUC_CTB_HDR_LEN))) {
> > +	if (unlikely(!h2g_has_room(ct, len + GUC_CTB_HDR_LEN))) {
> >   		if (ct->stall_time == KTIME_MAX)
> >   			ct->stall_time = ktime_get();
> >   		spin_unlock_irqrestore(&ctb->lock, flags);
> > @@ -732,7 +744,7 @@ 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;
> > @@ -747,12 +759,29 @@ 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)) {
> > +	GEM_BUG_ON(head > size);
> > +
> > +#ifdef CONFIG_DRM_I915_DEBUG_GUC
> > +	if (unlikely(head != READ_ONCE(desc->head))) {
> > +		CT_ERROR(ct, "Head was modified %u != %u\n",
> > +			 desc->head, ctb->head);
> > +		desc->status |= GUC_CTB_STATUS_MISMATCH;
> > +		goto corrupted;
> > +	}
> > +	if (unlikely((desc->tail | desc->head) >= size)) {
> As per comment in other thread, the check on head here is redundant because
> you have already hit a BUG_ON(ctb->head > size) followed by
> CT_ERROR(ctb->head != desc->head). Therefore, you can't get here if
> 'desc->head > size'.
>

Yep. See above we can likely just delete this.
 
> >   		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 >= size)) {
> > +		CT_ERROR(ct, "Invalid offsets tail=%u (size=%u)\n",
> > +			 tail, size);
> > +		desc->status |= GUC_CTB_STATUS_OVERFLOW;
> > +		goto corrupted;
> > +	}

Now we can move this outside if/else define block as it is same check as
above. Again can you do this when you merge this?

Matt

> > +#endif
> >   	/* tail == head condition indicates empty */
> >   	available = tail - head;
> > @@ -802,6 +831,7 @@ 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 bee03794c1eb..edd1bba0445d 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;
> >   };
> 
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