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

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On 06.07.2021 21:00, John Harrison wrote:
> On 7/1/2021 10:15, 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:
>>   (Michel)
>>    - Add additional sanity checks for head / tail pointers
>>    - Use GUC_CTB_HDR_LEN rather than magic 1
>>
>> Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
>> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
>> ---
>>   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 a9cb7b608520..5b8b4ff609e2 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 *cmds = ctb->cmds;
>> @@ -395,25 +397,22 @@ 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)) {
>>           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 fence */
>> -    if (unlikely(used + len + GUC_CTB_HDR_LEN >= size))
>> -        return -ENOSPC;
>> +#endif
>>         /*
>>        * dw0: CT header (including fence)
>> @@ -454,7 +453,9 @@ 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;
>>   @@ -470,7 +471,7 @@ 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.
>> @@ -526,24 +527,35 @@ 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();
>>   @@ -613,7 +625,7 @@ 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);
>> @@ -733,7 +745,7 @@ 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;
>> @@ -748,12 +760,29 @@ 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);
> Is the BUG_ON necessary given that both options below do the same check
> but as a corrupted buffer test (with subsequent recovery by GT reset?)
> rather than killing the driver.

"head" and "size" are now fully owned by the driver.
BUGON here is to make sure driver is coded correctly.

> 
>> +
>> +#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)) {
>> +        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)) {
> Could just be 'head' rather than 'ctb->head'.

or drop "ctb->head" completely since this is driver owned field and
above you already have BUGON to test it

Michal

> 
> John.
> 
>>           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;
>> @@ -803,6 +832,7 @@ 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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