Re: [PATCH 1/2] dma-buf/dma-fence: Add dma_fence_init_noref()

["Christian König" <christian.koenig@amd.com>]

Am 13.03.23 um 08:13 schrieb Christian König:
> Am 11.03.23 um 18:35 schrieb Rob Clark:
>> From: Rob Clark <robdclark@chromium.org>
>>
>> Add a way to initialize a fence without touching the refcount. This is
>> useful, for example, if the fence is embedded in a drm_sched_job.  In
>> this case the refcount will be initialized before the job is queued.
>> But the seqno of the hw_fence is not known until job_run().
>>
>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>
> Well that approach won't work. The fence can only be initialized in 
> the job_run() callback because only then the sequence number can be 
> determined.

Ah, wait a second! After reading the msm code I realized you are going 
to use this exactly the other way around that I think you use it.

In this case it would work, but that really needs better documentation. 
And I'm pretty sure it's not a good idea for msm, but let's discuss that 
on the other patch.

Regards,
Christian.

>
> Regards,
> Christian.
>
>> ---
>>   drivers/dma-buf/dma-fence.c | 43 ++++++++++++++++++++++++++++---------
>>   include/linux/dma-fence.h   |  2 ++
>>   2 files changed, 35 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
>> index 74e36f6d05b0..97c05a465cb4 100644
>> --- a/drivers/dma-buf/dma-fence.c
>> +++ b/drivers/dma-buf/dma-fence.c
>> @@ -989,28 +989,27 @@ void dma_fence_describe(struct dma_fence 
>> *fence, struct seq_file *seq)
>>   EXPORT_SYMBOL(dma_fence_describe);
>>     /**
>> - * dma_fence_init - Initialize a custom fence.
>> + * dma_fence_init_noref - Initialize a custom fence without 
>> initializing refcount.
>>    * @fence: the fence to initialize
>>    * @ops: the dma_fence_ops for operations on this fence
>>    * @lock: the irqsafe spinlock to use for locking this fence
>>    * @context: the execution context this fence is run on
>>    * @seqno: a linear increasing sequence number for this context
>>    *
>> - * Initializes an allocated fence, the caller doesn't have to keep its
>> - * refcount after committing with this fence, but it will need to 
>> hold a
>> - * refcount again if &dma_fence_ops.enable_signaling gets called.
>> - *
>> - * context and seqno are used for easy comparison between fences, 
>> allowing
>> - * to check which fence is later by simply using dma_fence_later().
>> + * Like &dma_fence_init but does not initialize the refcount.  Suitable
>> + * for cases where the fence is embedded in another struct which has 
>> it's
>> + * refcount initialized before the fence is initialized.  Such as 
>> embedding
>> + * in a &drm_sched_job, where the job is created before knowing the 
>> seqno
>> + * of the hw_fence.
>>    */
>>   void
>> -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops 
>> *ops,
>> -           spinlock_t *lock, u64 context, u64 seqno)
>> +dma_fence_init_noref(struct dma_fence *fence, const struct 
>> dma_fence_ops *ops,
>> +             spinlock_t *lock, u64 context, u64 seqno)
>>   {
>>       BUG_ON(!lock);
>>       BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name);
>> +    BUG_ON(!kref_read(&fence->refcount));
>>   -    kref_init(&fence->refcount);
>>       fence->ops = ops;
>>       INIT_LIST_HEAD(&fence->cb_list);
>>       fence->lock = lock;
>> @@ -1021,4 +1020,28 @@ dma_fence_init(struct dma_fence *fence, const 
>> struct dma_fence_ops *ops,
>>         trace_dma_fence_init(fence);
>>   }
>> +EXPORT_SYMBOL(dma_fence_init_noref);
>> +
>> +/**
>> + * dma_fence_init - Initialize a custom fence.
>> + * @fence: the fence to initialize
>> + * @ops: the dma_fence_ops for operations on this fence
>> + * @lock: the irqsafe spinlock to use for locking this fence
>> + * @context: the execution context this fence is run on
>> + * @seqno: a linear increasing sequence number for this context
>> + *
>> + * Initializes an allocated fence, the caller doesn't have to keep its
>> + * refcount after committing with this fence, but it will need to 
>> hold a
>> + * refcount again if &dma_fence_ops.enable_signaling gets called.
>> + *
>> + * context and seqno are used for easy comparison between fences, 
>> allowing
>> + * to check which fence is later by simply using dma_fence_later().
>> + */
>> +void
>> +dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops 
>> *ops,
>> +           spinlock_t *lock, u64 context, u64 seqno)
>> +{
>> +    kref_init(&fence->refcount);
>> +    dma_fence_init_noref(fence, ops, lock, context, seqno);
>> +}
>>   EXPORT_SYMBOL(dma_fence_init);
>> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
>> index d54b595a0fe0..f617c78a2e0a 100644
>> --- a/include/linux/dma-fence.h
>> +++ b/include/linux/dma-fence.h
>> @@ -279,6 +279,8 @@ struct dma_fence_ops {
>>       void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
>>   };
>>   +void dma_fence_init_noref(struct dma_fence *fence, const struct 
>> dma_fence_ops *ops,
>> +              spinlock_t *lock, u64 context, u64 seqno);
>>   void dma_fence_init(struct dma_fence *fence, const struct 
>> dma_fence_ops *ops,
>>               spinlock_t *lock, u64 context, u64 seqno);
>