[Intel-gfx] [RFC PATCH 36/97] drm/i915/guc: Add non blocking CTB send function
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Wed May 26 08:57:10 UTC 2021
On 25/05/2021 18:21, Matthew Brost wrote:
> On Tue, May 25, 2021 at 10:21:00AM +0100, Tvrtko Ursulin wrote:
>>
>> On 06/05/2021 20:13, Matthew Brost wrote:
>>> Add non blocking CTB send function, intel_guc_send_nb. In order to
>>> support a non blocking CTB send function a spin lock is needed to
>>> protect the CTB descriptors fields. Also the non blocking call must not
>>> update the fence value as this value is owned by the blocking call
>>> (intel_guc_send).
>>
>> Could the commit message say why the non-blocking send function is needed?
>>
>
> Sure. Something like:
>
> 'CTBs will be used in the critical patch of GuC submission and there is
> no need to wait for each CTB complete before moving on the i915'
A bit more, like also mentioning the critical path is with interrupts disabled or so. And not just that there is no need to wait but waiting is not possible because this or that. So only choice is to do this busy loop send. It's a bit horrible so justification needs to be documented.
>>> The blocking CTB now must have a flow control mechanism to ensure the
>>> buffer isn't overrun. A lazy spin wait is used as we believe the flow
>>> control condition should be rare with properly sized buffer.
>>>
>>> The function, intel_guc_send_nb, is exported in this patch but unused.
>>> Several patches later in the series make use of this function.
>>>
>>> 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.h | 12 ++-
>>> drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 96 +++++++++++++++++++++--
>>> drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h | 7 +-
>>> 3 files changed, 105 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc.h b/drivers/gpu/drm/i915/gt/uc/intel_guc.h
>>> index c20f3839de12..4c0a367e41d8 100644
>>> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc.h
>>> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc.h
>>> @@ -75,7 +75,15 @@ static inline struct intel_guc *log_to_guc(struct intel_guc_log *log)
>>> static
>>> inline int intel_guc_send(struct intel_guc *guc, const u32 *action, u32 len)
>>> {
>>> - return intel_guc_ct_send(&guc->ct, action, len, NULL, 0);
>>> + return intel_guc_ct_send(&guc->ct, action, len, NULL, 0, 0);
>>> +}
>>> +
>>> +#define INTEL_GUC_SEND_NB BIT(31)
>>> +static
>>> +inline int intel_guc_send_nb(struct intel_guc *guc, const u32 *action, u32 len)
>>> +{
>>> + return intel_guc_ct_send(&guc->ct, action, len, NULL, 0,
>>> + INTEL_GUC_SEND_NB);
>>> }
>>> static inline int
>>> @@ -83,7 +91,7 @@ intel_guc_send_and_receive(struct intel_guc *guc, const u32 *action, u32 len,
>>> u32 *response_buf, u32 response_buf_size)
>>> {
>>> return intel_guc_ct_send(&guc->ct, action, len,
>>> - response_buf, response_buf_size);
>>> + response_buf, response_buf_size, 0);
>>> }
>>> static inline void intel_guc_to_host_event_handler(struct intel_guc *guc)
>>> 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 a76603537fa8..af7314d45a78 100644
>>> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
>>> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
>>> @@ -3,6 +3,11 @@
>>> * Copyright © 2016-2019 Intel Corporation
>>> */
>>> +#include <linux/circ_buf.h>
>>> +#include <linux/ktime.h>
>>> +#include <linux/time64.h>
>>> +#include <linux/timekeeping.h>
>>> +
>>> #include "i915_drv.h"
>>> #include "intel_guc_ct.h"
>>> #include "gt/intel_gt.h"
>>> @@ -308,6 +313,7 @@ int intel_guc_ct_enable(struct intel_guc_ct *ct)
>>> if (unlikely(err))
>>> goto err_deregister;
>>> + ct->requests.last_fence = 1;
>>> ct->enabled = true;
>>> return 0;
>>> @@ -343,10 +349,22 @@ static u32 ct_get_next_fence(struct intel_guc_ct *ct)
>>> return ++ct->requests.last_fence;
>>> }
>>> +static void write_barrier(struct intel_guc_ct *ct) {
>>> + struct intel_guc *guc = ct_to_guc(ct);
>>> + struct intel_gt *gt = guc_to_gt(guc);
>>> +
>>> + if (i915_gem_object_is_lmem(guc->ct.vma->obj)) {
>>> + GEM_BUG_ON(guc->send_regs.fw_domains);
>>> + intel_uncore_write_fw(gt->uncore, GEN11_SOFT_SCRATCH(0), 0);
>>
>> It's safe to write to this reg? Does it need a comment to explain it?
>>
>
> Yes, it is same. IMO 'SCRATCH' in the name is enough documentation.
Why would it be enough? It requires digging to figure it out since it appears these are some sort of GuC special registers and not generic scratch:
commit 2d4ed3a988e6b1ff9729d0edd74bf4890571253e
Author: Michal Wajdeczko <michal.wajdeczko at intel.com>
Date: Mon May 27 18:36:05 2019 +0000
drm/i915/guc: New GuC scratch registers for Gen11
If it was a normal scratch then async trashing of those from a random driver thread isn't per se safe if used from a GPU context running in parallel.
But then according to bspec they are called VF_SW_FLAG_<n> and not GEN11_SOFT_SCRATCH so yeah.
>
>>> + } else {
>>> + wmb();
>>> + }
>>> +}
>>> +
>>> static int ct_write(struct intel_guc_ct *ct,
>>> const u32 *action,
>>> u32 len /* in dwords */,
>>> - u32 fence)
>>> + u32 fence, u32 flags)
>>> {
>>> struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
>>> struct guc_ct_buffer_desc *desc = ctb->desc;
>>> @@ -393,9 +411,13 @@ static int ct_write(struct intel_guc_ct *ct,
>>> FIELD_PREP(GUC_CTB_MSG_0_NUM_DWORDS, len) |
>>> FIELD_PREP(GUC_CTB_MSG_0_FENCE, fence);
>>> - hxg = FIELD_PREP(GUC_HXG_MSG_0_TYPE, GUC_HXG_TYPE_REQUEST) |
>>> - FIELD_PREP(GUC_HXG_REQUEST_MSG_0_ACTION |
>>> - GUC_HXG_REQUEST_MSG_0_DATA0, action[0]);
>>> + hxg = (flags & INTEL_GUC_SEND_NB) ?
>>> + (FIELD_PREP(GUC_HXG_MSG_0_TYPE, GUC_HXG_TYPE_EVENT) |
>>> + FIELD_PREP(GUC_HXG_EVENT_MSG_0_ACTION |
>>> + GUC_HXG_EVENT_MSG_0_DATA0, action[0])) :
>>> + (FIELD_PREP(GUC_HXG_MSG_0_TYPE, GUC_HXG_TYPE_REQUEST) |
>>> + FIELD_PREP(GUC_HXG_REQUEST_MSG_0_ACTION |
>>> + GUC_HXG_REQUEST_MSG_0_DATA0, action[0]));
>>> CT_DEBUG(ct, "writing (tail %u) %*ph %*ph %*ph\n",
>>> tail, 4, &header, 4, &hxg, 4 * (len - 1), &action[1]);
>>> @@ -412,6 +434,12 @@ static int ct_write(struct intel_guc_ct *ct,
>>> }
>>> GEM_BUG_ON(tail > size);
>>> + /*
>>> + * make sure H2G buffer update and LRC tail update (if this triggering a
>>> + * submission) are visable before updating the descriptor tail
>>> + */
>>> + write_barrier(ct);
>>> +
>>> /* now update descriptor */
>>> WRITE_ONCE(desc->tail, tail);
>>> @@ -466,6 +494,46 @@ static int wait_for_ct_request_update(struct ct_request *req, u32 *status)
>>> return err;
>>> }
>>> +static inline bool ctb_has_room(struct intel_guc_ct_buffer *ctb, u32 len_dw)
>>> +{
>>> + struct guc_ct_buffer_desc *desc = ctb->desc;
>>> + u32 head = READ_ONCE(desc->head);
>>> + u32 space;
>>> +
>>> + space = CIRC_SPACE(desc->tail, head, ctb->size);
>>> +
>>> + return space >= len_dw;
>>> +}
>>> +
>>> +static int ct_send_nb(struct intel_guc_ct *ct,
>>> + const u32 *action,
>>> + u32 len,
>>> + u32 flags)
>>> +{
>>> + struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
>>> + unsigned long spin_flags;
>>> + u32 fence;
>>> + int ret;
>>> +
>>> + spin_lock_irqsave(&ctb->lock, spin_flags);
>>> +
>>> + ret = ctb_has_room(ctb, len + 1);
>>> + if (unlikely(ret))
>>> + goto out;
>>> +
>>> + fence = ct_get_next_fence(ct);
>>> + ret = ct_write(ct, action, len, fence, flags);
>>> + if (unlikely(ret))
>>> + goto out;
>>> +
>>> + intel_guc_notify(ct_to_guc(ct));
>>> +
>>> +out:
>>> + spin_unlock_irqrestore(&ctb->lock, spin_flags);
>>> +
>>> + return ret;
>>> +}
>>> +
>>> static int ct_send(struct intel_guc_ct *ct,
>>> const u32 *action,
>>> u32 len,
>>> @@ -473,6 +541,7 @@ static int ct_send(struct intel_guc_ct *ct,
>>> u32 response_buf_size,
>>> u32 *status)
>>> {
>>> + struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
>>> struct ct_request request;
>>> unsigned long flags;
>>> u32 fence;
>>> @@ -482,8 +551,20 @@ static int ct_send(struct intel_guc_ct *ct,
>>> GEM_BUG_ON(!len);
>>> GEM_BUG_ON(len & ~GUC_CT_MSG_LEN_MASK);
>>> GEM_BUG_ON(!response_buf && response_buf_size);
>>> + might_sleep();
>>
>> Sleep is just cond_resched below or there is more?
>>
>
> Yes, the cond_resched.
>
>>> + /*
>>> + * We use a lazy spin wait loop here as we believe that if the CT
>>> + * buffers are sized correctly the flow control condition should be
>>> + * rare.
>>> + */
>>> +retry:
>>> spin_lock_irqsave(&ct->ctbs.send.lock, flags);
>>> + if (unlikely(!ctb_has_room(ctb, len + 1))) {
>>> + spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);
>>> + cond_resched();
>>> + goto retry;
>>> + }
>>
>> If this patch is about adding a non-blocking send function, and below we can
>> see that it creates a fork:
>>
>> intel_guc_ct_send:
>> ...
>> if (flags & INTEL_GUC_SEND_NB)
>> return ct_send_nb(ct, action, len, flags);
>>
>> ret = ct_send(ct, action, len, response_buf, response_buf_size, &status);
>>
>> Then why is there a change in ct_send here, which is not the new
>> non-blocking path?
>>
>
> There is not a change to ct_send(), just to intel_guc_ct_send.
I was doing by the diff which says:
static int ct_send(struct intel_guc_ct *ct,
const u32 *action,
u32 len,
@@ -473,6 +541,7 @@ static int ct_send(struct intel_guc_ct *ct,
u32 response_buf_size,
u32 *status)
{
+ struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
struct ct_request request;
unsigned long flags;
u32 fence;
@@ -482,8 +551,20 @@ static int ct_send(struct intel_guc_ct *ct,
GEM_BUG_ON(!len);
GEM_BUG_ON(len & ~GUC_CT_MSG_LEN_MASK);
GEM_BUG_ON(!response_buf && response_buf_size);
+ might_sleep();
+ /*
+ * We use a lazy spin wait loop here as we believe that if the CT
+ * buffers are sized correctly the flow control condition should be
+ * rare.
+ */
+retry:
spin_lock_irqsave(&ct->ctbs.send.lock, flags);
+ if (unlikely(!ctb_has_room(ctb, len + 1))) {
+ spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);
+ cond_resched();
+ goto retry;
+ }
So it looks like a change to ct_send to me. Is that wrong?
Regards,
Tvrtko
> As for why intel_guc_ct_send is updated and we don't just a new public
> function, this was another reviewers suggestion. Again can't make
> everyone happy.
>
>>> fence = ct_get_next_fence(ct);
>>> request.fence = fence;
>>> @@ -495,7 +576,7 @@ static int ct_send(struct intel_guc_ct *ct,
>>> list_add_tail(&request.link, &ct->requests.pending);
>>> spin_unlock(&ct->requests.lock);
>>> - err = ct_write(ct, action, len, fence);
>>> + err = ct_write(ct, action, len, fence, 0);
>>> spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);
>>> @@ -537,7 +618,7 @@ static int ct_send(struct intel_guc_ct *ct,
>>> * Command Transport (CT) buffer based GuC send function.
>>> */
>>> int intel_guc_ct_send(struct intel_guc_ct *ct, const u32 *action, u32 len,
>>> - u32 *response_buf, u32 response_buf_size)
>>> + u32 *response_buf, u32 response_buf_size, u32 flags)
>>> {
>>> u32 status = ~0; /* undefined */
>>> int ret;
>>> @@ -547,6 +628,9 @@ int intel_guc_ct_send(struct intel_guc_ct *ct, const u32 *action, u32 len,
>>> return -ENODEV;
>>> }
>>> + if (flags & INTEL_GUC_SEND_NB)
>>> + return ct_send_nb(ct, action, len, flags);
>>> +
>>> ret = ct_send(ct, action, len, response_buf, response_buf_size, &status);
>>> if (unlikely(ret < 0)) {
>>> CT_ERROR(ct, "Sending action %#x failed (err=%d status=%#X)\n",
>>> 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 1ae2dde6db93..55ef7c52472f 100644
>>> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h
>>> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.h
>>> @@ -9,6 +9,7 @@
>>> #include <linux/interrupt.h>
>>> #include <linux/spinlock.h>
>>> #include <linux/workqueue.h>
>>> +#include <linux/ktime.h>
>>> #include "intel_guc_fwif.h"
>>> @@ -42,7 +43,6 @@ struct intel_guc_ct_buffer {
>>> bool broken;
>>> };
>>> -
>>> /** Top-level structure for Command Transport related data
>>> *
>>> * Includes a pair of CT buffers for bi-directional communication and tracking
>>> @@ -69,6 +69,9 @@ struct intel_guc_ct {
>>> struct list_head incoming; /* incoming requests */
>>> struct work_struct worker; /* handler for incoming requests */
>>> } requests;
>>> +
>>> + /** @stall_time: time of first time a CTB submission is stalled */
>>> + ktime_t stall_time;
>>
>> Unused in this patch.
>>
>
> Yea, wrong patch. Will fix.
>
> Matt
>
>>> };
>>> void intel_guc_ct_init_early(struct intel_guc_ct *ct);
>>> @@ -88,7 +91,7 @@ static inline bool intel_guc_ct_enabled(struct intel_guc_ct *ct)
>>> }
>>> int intel_guc_ct_send(struct intel_guc_ct *ct, const u32 *action, u32 len,
>>> - u32 *response_buf, u32 response_buf_size);
>>> + u32 *response_buf, u32 response_buf_size, u32 flags);
>>> void intel_guc_ct_event_handler(struct intel_guc_ct *ct);
>>> #endif /* _INTEL_GUC_CT_H_ */
>>>
>>
>> Regards,
>>
>> Tvrtko
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