Re: [PATCH v3 2/2] MIPS: io: add a barrier after register read in readX()

[Sinan Kaya <okaya@codeaurora.org>]

On 4/12/2018 5:51 PM, James Hogan wrote:
> But why don't we always use wmb() in the writeX() case? Might not the
> cached write to DMA buffer be reordered with the uncached write to MMIO
> register from the coherent DMA point of view? I'm waiting on feedback
> from MIPS hardware folk on this topic.

Are you asking about this?

 #if defined(CONFIG_CPU_CAVIUM_OCTEON) || defined(CONFIG_LOONGSON3_ENHANCEMENT)
 #define war_io_reorder_wmb()		wmb()
 #else
-#define war_io_reorder_wmb()		do { } while (0)
+#define war_io_reorder_wmb()		barrier()
 #endif

There is a write barrier in writeX() but seem to be different from platform
to platform. 

I'm not familiar with the MIPS architecture. We can always use a wmb() but it
could hurt performance where it is not needed. 

This is the kind of input we need from the MIPS folks if compiler barrier is
enough or we need a wmb() for all cases.

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.