linux-kernel - Re: [PATCH 2/7] ARM: Add Broadcom Brahma-B15 readahead cache support

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Date:   Wed, 18 Jan 2017 16:18:33 -0800
From:   Florian Fainelli <florian.fainelli@...adcom.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Alamy Liu <alamyliu@...adcom.com>,
        "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Jonathan Austin <jonathan.austin@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Nicolas Pitre <nico@...aro.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>, will.deacon@....com
Subject: Re: [PATCH 2/7] ARM: Add Broadcom Brahma-B15 readahead cache support

On 01/18/2017 02:56 PM, Russell King - ARM Linux wrote:
> On Wed, Jan 18, 2017 at 12:29:21PM -0800, Florian Fainelli wrote:
>> The readahead cache only intercepts reads, not writes, as such, some
>> data can remain stale in any of its buffers, such that we need to flush
>> it, which is an operation that needs to happen in a particular order:
>>
>> - disable the readahead cache
>> - flush it
>> - call the appropriate cache-v7.S function
>> - re-enable
> 
> I really do hope that the above explanation is wrong, because if that's
> really how it's implemented, it's going to cause coherency problems.
> 
> It's got to at least monitor writes, otherwise how do you guarantee
> that the CPU doesn't see stale data?  IOW:

Yes, it does monitor writes, the explanation given here was wrong. Thanks!

> 
> Consider this at the L2 memory-side interface (iow, downstream of the
> point-of-coherency):
> 
> 	CPU1		CPU2		Read-ahead buffer
> 			read cache line C
> 					reads cache line C and C+1
> 	writes cache line C+1
> 			read cache line C+1
> 
> What ensures that CPU2 sees the written out cache line from CPU1?
-- 
Florian