Inconsistent fence info between 6.1 and 5.15 kernel

[Mengchi Cheng <mengcc@amazon.com>]

Hello,                                                                  
                                                                        
We are upleveling kernel from 5.15.32 to 6.1.62 for our system. The     
kernels we use are all vanila kernel from upstream without any extra    
patches. But we see our graphics application crashed in the 6.1.        
                                                                        
And later we found that 6.1 has DMA_BUF_IOCTL_EXPORT_SYNC_FILE option   
added into dma_buf_ioctl. It initialized dma_fence_stub and returned    
with a sync_file. So then our app called sync_file_ioctl_fence_info to  
inquire fence info, it returns info of dma_fence_stub, which was not    
expected. As the result, the app aborted because of it.                 
                                                                        
Meanwhile, with the 5.15 kernel, our app somehow also called            
DMA_BUF_IOCTL_EXPORT_SYNC_FILE. Since it returned -ENOTTY, the app did  
not further ask for fence info.                                         
                                                                        
I am not a graphics engineer. I do not know if userspace is handling    
dma_fence_stub correctly. But since the addition of                     
DMA_BUF_IOCTL_EXPORT_SYNC_FILE is causing differnet behavior, I am      
wondering if there is a way to keep things consistent in the kernel.    
For example, add a macro to provide a choice disabling it? There is
probably a better solution to it. Look forward to your reply.                                                          
                                                                        
                                                                        
Best,                                                                   
Mengchi Cheng

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2.40.1