linux-kernel - Re: [PATCH net-next 1/3] arm64: barrier: add DGH macros to control memory accesses merging

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Date:   Thu, 24 Jun 2021 22:18:03 +0800
From:   "huangguangbin (A)" <huangguangbin2@...wei.com>
To:     Mark Rutland <mark.rutland@....com>, Will Deacon <will@...nel.org>
CC:     <davem@...emloft.net>, <kuba@...nel.org>,
        <catalin.marinas@....com>, <maz@...nel.org>, <dbrazdil@...gle.com>,
        <qperret@...gle.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <lipeng321@...wei.com>,
        <peterz@...radead.org>
Subject: Re: [PATCH net-next 1/3] arm64: barrier: add DGH macros to control
 memory accesses merging



On 2021/6/22 20:32, Mark Rutland wrote:
> On Tue, Jun 22, 2021 at 01:16:31PM +0100, Will Deacon wrote:
>> On Tue, Jun 22, 2021 at 07:11:09PM +0800, Guangbin Huang wrote:
>>> From: Xiongfeng Wang <wangxiongfeng2@...wei.com>
>>>
>>> DGH prohibits merging memory accesses with Normal-NC or Device-GRE
>>> attributes before the hint instruction with any memory accesses
>>> appearing after the hint instruction. Provide macros to expose it to the
>>> arch code.
>>
>> Hmm.
>>
>> The architecture states:
>>
>>    | DGH is a hint instruction. A DGH instruction is not expected to be
>>    | performance optimal to merge memory accesses with Normal Non-cacheable
>>    | or Device-GRE attributes appearing in program order before the hint
>>    | instruction with any memory accesses appearing after the hint instruction
>>    | into a single memory transaction on an interconnect.
>>
>> which doesn't make a whole lot of sense to me, in all honesty.
> 
> I think there are some missing words, and this was supposed to say
> something like:
> 
> | DGH is a hint instruction. A DGH instruction *indicates that it* is
> | not expected to be performance optimal to merge memory accesses with
> | Normal Non-cacheable or Device-GRE attributes appearing in program
> | order before the hint instruction with any memory accesses appearing
> | after the hint instruction into a single memory transaction on an
> | interconnect.
> 
> ... i.e. it's a hint to the CPU to avoid merging accesses which are
> either side of the DGH, so that the prior accesses don't get
> indefinitely delayed waiting to be merged.
> 
> I'll try to get the documentation fixed, since as-is the wording does
> not make sense.
> 
> Thanks,
> Mark.
> .
> 
Thanks very much, we will fix the documentation.

Thanks,
Guangbin,
.