linux-kernel - Re: [PATCH 1/2] watchdog: dw: RMW the control register

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Date:   Fri, 9 Mar 2018 21:04:32 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-watchdog@...r.kernel.org,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH 1/2] watchdog: dw: RMW the control register

On Fri, Mar 9, 2018 at 8:02 PM, Guenter Roeck <linux@...ck-us.net> wrote:
> On 03/09/2018 07:28 PM, Brian Norris wrote:
>> I guess I could mention it. I was assuming that was an intended behavior
>> of the existing driver: that we set resp_mode=0 (via clobber), so we
>> always get a system reset (we don't try to handle any interrupt in this
>> driver).
>>
> I don't think it was intended behavior. We don't even know for sure (or at
> least
> I don't know) if all implementations of this IP have the same configuration
> bit
> layout. All we can do is hope for the best.

Huh, OK. I did try to look for any sort of generic DesignWare register
documentation, and I couldn't find one easily (even with a proper
Synopsys account -- maybe I wasn't looking in the right place). But
besides the Rockchip TRMs, I did find some openly accessible Altera
SoCFPGA docs [1] which also use this, and they have a few things to
add:
(1) they have the same 'reset pulse length' field, except it's labeled RO
(2) they have the same 'response mode' field
(3) the docs for the entire register say:

"The value of a reserved bit must be maintained in software. When you
modify registers containing reserved bit fields, you must use a
read-modify-write operation to preserve state and prevent
indeterminate system behavior."

So, that pretty well corroborates my patch. Nice.

> Still, clobbering just 1 bit is better than clobbering 30 bit.

Yeah, that's the idea. Well, as long as it's only the 1 bit I want to clobber ;)

I guess if we really find that any of this becomes more problematic
(and varies enough from IP to IP), then we'll need chip-specific
compatible properties.

Brian

[1] e.g. https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/hb/arria-10/a10_5v4.pdf