[RFC PATCH 38/97] drm/i915/guc: Optimize CTB writes and reads
Michal Wajdeczko
michal.wajdeczko at intel.com
Mon May 24 13:31:25 UTC 2021
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 at Intel.com>
> Signed-off-by: Matthew Brost <matthew.brost at 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;
> };
>
>
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