What is the significance of coherent_dma_mask?

[Venu Byravarasu]

Hi,

Some of the USB drivers are setting coherent_dma_mask & many others are 
not doing it.
From the description present in the comment beside this field in 
device.h, I could not understand its real significance.

Can some one plz let me know its real need and use case where it is 
mandatory to use it?

Thanks,
Venu


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