linux-kernel - Re: [PATCH v2 0/3] Add support for SBI v0.2

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Date:   Fri, 27 Sep 2019 22:57:45 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "hch@...radead.org" <hch@...radead.org>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "alankao@...estech.com" <alankao@...estech.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "gary@...yguo.net" <gary@...yguo.net>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "anup@...infault.org" <anup@...infault.org>,
        "allison@...utok.net" <allison@...utok.net>,
        "palmer@...ive.com" <palmer@...ive.com>
Subject: Re: [PATCH v2 0/3] Add support for SBI v0.2

On Fri, 2019-09-27 at 15:19 -0700, Christoph Hellwig wrote:
> On Thu, Sep 26, 2019 at 05:09:12PM -0700, Atish Patra wrote:
> > The Supervisor Binary Interface(SBI) specification[1] now defines a
> > base extension that provides extendability to add future extensions
> > while maintaining backward compatibility with previous versions.
> > The new version is defined as 0.2 and older version is marked as
> > 0.1.
> > 
> > This series adds support v0.2 and a unified calling convention
> > implementation between 0.1 and 0.2. It also adds minimal SBI
> > functions
> > from 0.2 as well to keep the series lean. 
> 
> So before we do this game can be please make sure we have a clean 0.2
> environment that never uses the legacy extensions as discussed
> before?
> Without that all this work is rather futile.
> 

As per our discussion offline, here are things need to be done to
achieve that.

1. Replace timer, sfence and ipi with better alternative APIs
	- sbi_set_timer will be same but with new calling convention
	- send_ipi and sfence_* apis can be modified in such a way that
		- we don't have to use unprivileged load anymore
		- Make it scalable

2. Drop clear_ipi, console, and shutdown in 0.2.

We will have a new kernel config (LEGACY_SBI) that can be manually
enabled if older firmware need to be used. By default, LEGACY_SBI will
be disabled and kernel with new SBI will be built. We will have to set
a flag day in a year or so when we can remove the LEGACY_SBI
completely.

Let us know if it is not an acceptable approach to anybody.
I will post a RFC patch with new alternate v0.2 APIs sometime next
week.

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-- 
Regards,
Atish

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