Re: [PATCH v2] watchdog: ebc-c384_wdt: Migrate to the regmap API

[Guenter Roeck]

On 3/14/23 09:31, William Breathitt Gray wrote:
> On Tue, Mar 14, 2023 at 05:50:42PM +0200, Andy Shevchenko wrote:
> > On Tue, Mar 14, 2023 at 11:29:16AM -0400, William Breathitt Gray wrote:
> > > The regmap API supports IO port accessors so we can take advantage of
> > > regmap abstractions rather than handling access to the device registers
> > > directly in the driver.
> >
> > ...
> >
> > >   - Utilize watchdog_set_drvdata() and watchdog_get_drvdata()
> >
> > I'm wondering why you can't use dev_get_regmap() instead.
>
> We can set `wdd->parent = dev` in ebc_c384_wdt_probe(), and then use
> `dev_get_regmap(wdev->parent)` to retrieve the regmap. The only downside
> I see if perhaps the added latency a call to devres_find(), whereas
> using watchdog_get_drvdata() is just a pointer dereference.
>
> I'm indifferent to either choice, so if Guenter or Wim have a preference
> here I'll follow their decision.

I am not inclined to accept a patch which calls dev_get_regmap() more
than once. It is not just added latency, it is unnecessarily executing
a lot of code. Maybe that call is abused nowadays, and/or maybe people do not
care about wasting CPU cycles anymore, but that is not its intended use case.

> > >   - Map watchdog control registers based on offset 0x1 and adjust regmap
> > >     configurations accordingly; offset 0x0 is unused in this driver so we
> > >     should avoid unnecessary exposure of it
> >
> > I'm wondering what bad could happen if you expose it.
>
> The WINSYSTEMS EBC-C384 documentation I have does not specify what
> offset 0x0 does (nor offsets 0x3-0x4), so I don't know if there are side
> effects to reading those addresses. Really, I'm just avoiding the hassle
> of writing an explicit precious registers table for those offsets by not
> exposing them at all.

Counter questions:
- What would be the purpose of exposing register addresses if they are not needed ?
- What bad can happen by _not_ exposing those register addresses ?

Guenter