linux-kernel - Re: [PATCH V5 1/3] ARM64 LPC: Indirect ISA port IO introduced

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Date:   Tue, 8 Nov 2016 17:05:50 +0000
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will.deacon@....com>
CC:     "zhichang.yuan" <yuanzhichang@...ilicon.com>,
        <mark.rutland@....com>, <devicetree@...r.kernel.org>,
        <lorenzo.pieralisi@....com>, <benh@...nel.crashing.org>,
        <minyard@....org>, <arnd@...db.de>, <catalin.marinas@....com>,
        <gabriele.paoloni@...wei.com>, <zhichang.yuan02@...il.com>,
        <liviu.dudau@....com>, <linux-kernel@...r.kernel.org>,
        <xuwei5@...ilicon.com>, <linuxarm@...wei.com>, <olof@...om.net>,
        <robh+dt@...nel.org>, <zourongrong@...il.com>,
        <linux-serial@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <bhelgaas@...gle.com>, <kantyzc@....com>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH V5 1/3] ARM64 LPC: Indirect ISA port IO introduced

On 08/11/2016 16:49, Will Deacon wrote:
> On Tue, Nov 08, 2016 at 04:33:44PM +0000, John Garry wrote:
>> On 08/11/2016 16:12, Will Deacon wrote:
>>> On Tue, Nov 08, 2016 at 11:47:07AM +0800, zhichang.yuan wrote:
>>>> +static inline void arm64_set_extops(struct extio_ops *ops)
>>>> +{
>>>> +	if (ops)
>>>> +		WRITE_ONCE(arm64_extio_ops, ops);
>>>
>>> Why does this need to be WRITE_ONCE? You don't have READ_ONCE on the reader
>>> side. Also, what if multiple drivers want to set different ops for distinct
>>> address ranges?
>>
>> I think that the idea here is that we only have possibly one master in the
>> system which offers indirectIO backend, so another one could not possibly
>> re-set this value.
>
> Why is that assumption valid, and why does WRITE_ONCE help there? It's not
> ONCE as in WARN_ONCE, more ONCE as in exactly-once-per-invocation.

It's only valid based on the inherent assumption that all indirectIO is 
redirected to one backend master, i.e. LPC driver.

Anyway, right, I don't think that WRITE_ONCE is correct. Zhichang was 
looking for something which would only allow the pointer to be written 
once ever.

>
>>>> diff --git a/arch/arm64/kernel/extio.c b/arch/arm64/kernel/extio.c
>>>> new file mode 100644
>>>> index 0000000..647b3fa
>>>> --- /dev/null
>>>> +++ b/arch/arm64/kernel/extio.c
>>>> @@ -0,0 +1,27 @@
>>>> +/*
>>>> + * Copyright (C) 2016 Hisilicon Limited, All Rights Reserved.
>>>> + * Author: Zhichang Yuan <yuanzhichang@...ilicon.com>
>>>> + *
>>>> + * This program is free software; you can redistribute it and/or modify
>>>> + * it under the terms of the GNU General Public License version 2 as
>>>> + * published by the Free Software Foundation.
>>>> + *
>>>> + * This program is distributed in the hope that it will be useful,
>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>>>> + * GNU General Public License for more details.
>>>> + *
>>>> + * You should have received a copy of the GNU General Public License
>>>> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
>>>> + */
>>>> +
>>>> +#include <linux/io.h>
>>>> +
>>>> +struct extio_ops *arm64_extio_ops;
>>>> +
>>>> +
>>>> +BUILD_EXTIO(b, u8)
>>>> +
>>>> +BUILD_EXTIO(w, u16)
>>>> +
>>>> +BUILD_EXTIO(l, u32)
>>>
>>> Is there no way to make this slightly more generic, so that it can be
>>> re-used elsewhere? For example, if struct extio_ops was common, then
>>> you could have the singleton (which maybe should be an interval tree?),
>>> type definition, setter function and the BUILD_EXTIO invocations
>>> somewhere generic, rather than squirelled away in the arch backend.
>>>
>> The concern would be that some architecture which uses generic higher-level
>> ISA accessor ops, but have IO space, could be affected.
>
> You're already adding a Kconfig symbol for this stuff, so you can keep
> that if you don't want it on other architectures. I'm just arguing that
> plumbing drivers directly into arch code via arm64_set_extops is not
> something I'm particularly fond of, especially when it looks like it
> could be avoided with a small amount of effort.

We'll check this.

Cheers,
John

>
> Will
>
> .
>