linux-kernel - Re: [RFC PATCH v4 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

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Date:   Tue, 7 Jul 2020 19:12:08 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>
Cc:     linux-arch@...r.kernel.org, suzuki.poulose@....com,
        Zhenyu Ye <yezhenyu2@...wei.com>, linux-kernel@...r.kernel.org,
        xiexiangyou@...wei.com, steven.price@....com,
        zhangshaokun@...ilicon.com, linux-mm@...ck.org, arm@...nel.org,
        prime.zeng@...ilicon.com, guohanjun@...wei.com, olof@...om.net,
        kuhn.chenqun@...wei.com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v4 2/2] arm64: tlb: Use the TLBI RANGE feature in
 arm64

On 2020-07-07 18:46, Catalin Marinas wrote:
> On Tue, Jul 07, 2020 at 06:43:35PM +0100, Marc Zyngier wrote:
>> On 2020-07-07 18:36, Catalin Marinas wrote:
>>> On Mon, Jun 01, 2020 at 10:47:13PM +0800, Zhenyu Ye wrote:
>>>> @@ -59,6 +69,47 @@
>>>>   		__ta;						\
>>>>   	})
>>>>
>>>> +/*
>>>> + * __TG defines translation granule of the system, which is decided
>>>> by
>>>> + * PAGE_SHIFT.  Used by TTL.
>>>> + *  - 4KB	: 1
>>>> + *  - 16KB	: 2
>>>> + *  - 64KB	: 3
>>>> + */
>>>> +#define __TG	((PAGE_SHIFT - 12) / 2 + 1)
>>>
>>> Nitpick: maybe something like __TLBI_TG to avoid clashes in case someone
>>> else defines a __TG macro.
>>
>> I have commented on this in the past, and still maintain that this
>> would be better served by a switch statement similar to what is used
>> for TTL already (I don't think this magic formula exists in the
>> ARM ARM).
> 
> Good point, it would be cleaner indeed.

FWIW, we use the somewhat obtuse "(shift - 10) / 2" in the SMMUv3 
driver, but that's because we support multiple different granules at 
runtime and want to generate efficient code. Anything based on 
PAGE_SHIFT that resolves to a compile-time constant has no excuse for 
not being written in a clear and obvious manner ;)

Robin.