linux-kernel - Re: [PATCH v4 6/7] gpio: exar: switch to using regmap

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Message-ID: <CAMRc=McyHfti_9ner8vyF=J=srzWJXsa7GRmwKF1TJ=ksGQM5g@mail.gmail.com>
Date:   Tue, 10 Nov 2020 17:52:26 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        David Laight <David.Laight@...lab.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 6/7] gpio: exar: switch to using regmap

On Tue, Nov 10, 2020 at 5:43 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Tue, Nov 10, 2020 at 6:37 PM Bartosz Golaszewski
> <bgolaszewski@...libre.com> wrote:
> > On Tue, Nov 10, 2020 at 5:17 PM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
>
> ...
>
> > > > > > > +static const struct regmap_config exar_regmap_config = {
> > > > > > > +   .name           = "exar-gpio",
> > > > > > > +   .reg_bits       = 16,
> > > > > >
> > > > > > As per previous version comment.
> > > > > >
> > > > > > Hold on, the registers are 16-bit wide, but their halves are sparsed!
> > > > > > So, I guess 8 and 8 with helpers to get hi and lo parts are essential.
> > > > > >
> > > > > >
> > > > > > TABLE 5: DEVICE CONFIGURATION REGISTERS SHOWN IN BYTE ALIGNMENT
> > > > > >
> > > > > > > +   .val_bits       = 8,
> > > > > > > +};
> > > > > >
> > > > > > This is basically represents two banks out of 6 8-bit registers each.
> > > > >
> > > > > ...which makes me wonder if gpio-regmap can be utilized here...
> > > > >
> > > >
> > > > But the address width won't affect the actuall accessing of 8 bits
> > > > registers in an mmio regmap. Internally the mmio regmap does pretty
> > > > much the same thing the previous driver did: call readb()/writeb() on
> > > > 8-bit "chunks" of the banks.
> > >
> > > It will affect reg dump in debugfs. I would really narrow down the register
> > > address space in the config, otherwise that debugfs facility will screw up a
> > > lot of things.
> > >
> > > So, and to be on pedantic side...
> > >
> > > "The Device Configuration Registers and the two individual UART Configuration
> > > Registers of the XR17V352 occupy 2K of PCI bus memory address space."
> > >
> > > 11 seems the correct value for the address width.
> >
> > I take it as a typo and assume you meant 16. So the patch should be
> > correct and your review tag is good to go?
>
> It's not a typo. But thinking again. This is basically done in regmap
> to support serial buses. Here we have MMIO pretty much with 32-bit or
> 64-bit address accesses. I didn't dig into regmap implementation to
> understand the consequences of changing this to the different values
> (it seems like rather offset, and in this case 11 is a correct one,
> not a typo, and regmap is okay with that).
> But I would rather ask Jan to actually mount debugfs and dump
> registers and see if it screws up the UART (because it may go all over
> important registers), that's why I think this configuration is still
> missing some strict rules about what addresses (offsets) driver may or
> may not access.

Ok now I get it. Yes 11 seems to be right in this case for the max
address. We can implement the readable/writable callbacks to be very
strict about the register accesses but isn't it overkill? This driver
is very small and only accesses a couple registers. I don't see such
strict checking very often except for very complicated modules (like
pca953x you mentioned).

Bartosz