linux-kernel - Re: [PATCH] rtc: sun6i: Add NVMEM provider

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Message-ID: <a3b03a06-c8fc-7dbe-7c0b-ffd1f194ecbc@sholland.org>
Date:   Sun, 9 May 2021 22:39:30 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        linux-rtc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: sun6i: Add NVMEM provider

On 4/30/21 4:02 AM, Maxime Ripard wrote:
> Hi,
> 
> On Sun, Apr 18, 2021 at 08:45:49PM -0500, Samuel Holland wrote:
>> The sun6i RTC provides 32 bytes of general-purpose data registers.
>> They can be used to save data in the always-on RTC power domain.
>> The registers are writable via 32-bit MMIO accesses only.
>>
>> Expose the region as a NVMEM provider so it can be used by userspace and
>> other drivers.
>>
>> Signed-off-by: Samuel Holland <samuel@...lland.org>
> 
> As far as I understood, you want to use those registers to implement
> super-standby? If so, while it makes sense for the kernel to be able to
> be able to write to those registers, I guess it would be a bit unwise to
> allow the userspace to access it?

I want the user to be able to pass information to the bootloader (to
select a boot device, e.g. reboot to FEL). I also want the user to be
able to read data stored to these registers by system firmware (e.g.
crust writes exception information there). It's not really related to
standby.

I would want to stack a nvmem-reboot-mode on top to give friendlier
names to some of the numbers, but I don't see a problem with root having
direct access to the registers. It's no different from /dev/nvram
providing access to the PC CMOS RAM.

Regards,
Samuel