linux-kernel - Re: [PATCH] watchdog: sbsa: use 32-bit read for WCV

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Message-ID: <20180307100403.GB14432@rric.localdomain>
Date:   Wed, 7 Mar 2018 11:04:04 +0100
From:   Robert Richter <robert.richter@...ium.com>
To:     Jayachandran C <jnair@...iumnetworks.com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-watchdog@...r.kernel.org, Fu Wei <fu.wei@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: sbsa: use 32-bit read for WCV

On 28.02.18 02:52:20, Jayachandran C wrote:
> According to SBSA spec v3.1 section 5.3:
>   All registers are 32 bits in size and should be accessed using
>   32-bit reads and writes. If an access size other than 32 bits
>   is used then the results are IMPLEMENTATION DEFINED.
>   [...]
>   The Generic Watchdog is little-endian
>
> The current code uses readq to read the watchdog compare register
> which does a 64-bit access. This fails on ThunderX2 which does not
> implement 64-bit access to this register.
>
> Fix this by using lo_hi_readq() that does two 32-bit reads.
>
> Signed-off-by: Jayachandran C <jnair@...iumnetworks.com>

Reviewed-by: Robert Richter <rrichter@...ium.com>

I have looked into the non-atomic use of the register access and it
looks sane as the WCV register is only incremented by hardware. There
is no concurrent (write) access from the kernel.

-Robert