linux-kernel - Re: [PATCH v2 20/28] ARCv2: barriers

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Message-ID: <20150623084905.GE31504@arm.com>
Date:	Tue, 23 Jun 2015 09:49:05 +0100
From:	Will Deacon <will.deacon@....com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>
Subject: Re: [PATCH v2 20/28] ARCv2: barriers

Hi Vineet,

On Tue, Jun 23, 2015 at 08:58:03AM +0100, Vineet Gupta wrote:
> ARCv2 based HS38 cores are weakly ordered and thus explicit barriers for
> kernel proper.
> 
> SMP barrier is provided by DMB instruction which also guarantees local
> barrier hence used as backend of smp_*mb() as well as *mb() APIs
> 
> Also hookup barriers into MMIO accessors to avoid ordering issues in IO
> 
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Will Deacon <will.deacon@....com>
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> ---
> Changes since v1
>  * Better changelog and comments
>  * local/mandatory barriers to NOT use DSYNC, but DMB
>  * define DMB based mandatory barriers even for !SMP
> ---

Functionally, all looks good to me. However, my comment is completely
misleading ;)

> +/*
> + * MMIO can also get buffered/optimized in micro-arch, so barriers needed
> + * Based on ARM model for the typical use case
> + *
> + *	<writel_relaxed DMA buffer>
> + *	<writel MMIO "go" reg>
> + *   or:
> + *   	<readl MMIO "status" reg>
> + *   	<readl_relaxed DMA buffer>

The writel_relaxed/readl_relaxed parts here would actually just be
bog-standard loads and stores to an in-memory buffer. I was trying too hard
to show the barrier semantics and accidentally turned the DMA buffers into
__iomem regions.

If you fix the comment:

  Reviewed-by: Will Deacon <will.deacon@....com>

Sorry for messing you about.

Will
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