Re: [Linaro-acpi] [PATCH v8 5/5] Watchdog: introduce ARM SBSA watchdog driver

[Timur Tabi]

On 11/12/2015 06:06 PM, Al Stone wrote:
> If it is a NAK, that's fine, but I also want to be sure I understand what the
> objections are.  Based on my understanding of the discussion so far over the
> multiple versions, I think the primary objection is that the use of pretimeout
> makes this driver too complex, and indeed complex enough that there is some
> concern that it could destabilize a running system.  Do I have that right?

I don't have a problem with the concept of pre-timeout per se.  My 
primary objection is this code:

> +static irqreturn_t sbsa_gwdt_interrupt(int irq, void *dev_id)
> +{
> +       struct sbsa_gwdt *gwdt = (struct sbsa_gwdt *)dev_id;
> +       struct watchdog_device *wdd = &gwdt->wdd;
> +
> +       /* We don't use pretimeout, trigger WS1 now */
> +       if (!wdd->pretimeout)
> +               sbsa_gwdt_set_wcv(wdd, 0);

This driver depends on an interrupt handler in order to properly program 
the hardware.  Unlike some other devices, the SBSA watchdog does not 
need assistance to reset on a timeout -- it is a "fire and forget" 
device.  What happens if there is a hard lockup, and interrupts no 
longer work?

The reason why Fu does this is because he wants to support a pre-timeout 
value that's independent of the timeout value.  The SBSA watchdog is 
normally programmed where real timeout equals twice the pre-timeout.  I 
would prefer that the driver adhere to this limitation.  That would 
eliminate the need to pre-program the hardware in the interrupt handler.

> And finally, a simpler, single stage timeout watchdog driver would be a
> reasonable thing to accept, yes?  I can see where that would make sense.

I would be okay with merging such a driver, and then enhancing it later 
to add pre-timeout support.

> The issue for me in that case is that the SBSA requires a two stage timeout,
> so a single stage driver has no real value for me.

There are plenty of existing watchdog devices that have a two-stage 
timeout but the driver treats it as a single stage.  The PowerPC 
watchdog driver is like that.  The hardware is programmed for the second 
stage to cause a hardware reset, and the interrupt handler is typically 
a no-op or just a printk().

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