[PATCH net-next 2/4] io: add function to flush the write combine buffer to device immediately

[Xiongfeng Wang]

Hi, Will

On 2021/7/30 17:00, Will Deacon wrote:
> Hi,
> 
> On Fri, Jul 30, 2021 at 11:14:22AM +0800, Guangbin Huang wrote:
>> From: Xiongfeng Wang <wangxiongfeng2 at huawei.com>
>>
>> Device registers can be mapped as write-combine type. In this case, data
>> are not written into the device immediately. They are temporarily stored
>> in the write combine buffer and written into the device when the buffer
>> is full. But in some situation, we need to flush the write combine
>> buffer to device immediately for better performance. So we add a general
>> function called 'flush_wc_write()'. We use DGH instruction to implement
>> this function for ARM64.
>>
>> Signed-off-by: Xiongfeng Wang <wangxiongfeng2 at huawei.com>
>> Signed-off-by: Guangbin Huang <huangguangbin2 at huawei.com>
>> ---
>>  arch/arm64/include/asm/io.h | 2 ++
>>  include/linux/io.h          | 6 ++++++
>>  2 files changed, 8 insertions(+)
> 
> -ENODOCUMENTATION
> 
>> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
>> index 7fd836bea7eb..5315d023b2dd 100644
>> --- a/arch/arm64/include/asm/io.h
>> +++ b/arch/arm64/include/asm/io.h
>> @@ -112,6 +112,8 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
>>  #define __iowmb()		dma_wmb()
>>  #define __iomb()		dma_mb()
>>  
>> +#define flush_wc_write()	dgh()
> 
> I think it would be worthwhile to look at what architectures other than
> arm64 offer here. For example, is there anything similar to this on riscv,
> x86 or power? Doing a quick survery of what's out there might help us define
> a macro that can be used across multiple architectures.

I searched in 'barrier.h' of different architectures and didn't find similar
merge preventing instructions. Could you give me some advice on naming this
common interface ?

Thanks,
Xiongfeng

> 
> Thanks,
> 
> Will
> 
>>  /*
>>   * Relaxed I/O memory access primitives. These follow the Device memory
>>   * ordering rules but do not guarantee any ordering relative to Normal memory
>> diff --git a/include/linux/io.h b/include/linux/io.h
>> index 9595151d800d..469d53444218 100644
>> --- a/include/linux/io.h
>> +++ b/include/linux/io.h
>> @@ -166,4 +166,10 @@ static inline void arch_io_free_memtype_wc(resource_size_t base,
>>  }
>>  #endif
>>  
>> +/* IO barriers */
>> +
>> +#ifndef flush_wc_write
>> +#define flush_wc_write()		do { } while (0)
>> +#endif
>> +
>>  #endif /* _LINUX_IO_H */
>> -- 
>> 2.8.1
>>
> .
>