RISC-V: Define pgprot_dmacoherent() for non-coherent devices

The pgprot_dmacoherent() is used when allocating memory for
non-coherent devices and by default pgprot_dmacoherent() is
same as pgprot_noncached() unless architecture overrides it.

Currently, there is no pgprot_dmacoherent() definition for
RISC-V hence non-coherent device memory is being mapped as
IO thereby making CPU access to such memory slow.

Define pgprot_dmacoherent() to be same as pgprot_writecombine()
for RISC-V so that CPU access non-coherent device memory as
NOCACHE which is better than accessing it as IO.

Fixes: ff689fd ("riscv: add RISC-V Svpbmt extension support")
Signed-off-by: Anup Patel <[email protected]>
Tested-by: Han Gao <[email protected]>
Tested-by: Guo Ren (Alibaba DAMO Academy) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paul Walmsley <[email protected]>

Author: avpatel

arch/riscv/include/asm/pgtable.h

@@ -654,6 +654,8 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
 	return __pgprot(prot);
 }
 
+#define pgprot_dmacoherent pgprot_writecombine
+
 /*
  * Both Svade and Svadu control the hardware behavior when the PTE A/D bits need to be set. By
  * default the M-mode firmware enables the hardware updating scheme when only Svadu is present in