linux-kernel - Re: [PATCH] regmap: fix writes to non incrementing registers

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Message-ID: <CAF3==iuZvCnmAg9hqs8ivHw0wHaUQEf8k9U8=KTekMMjdyyEKg@mail.gmail.com>
Date:   Wed, 14 Aug 2019 14:09:11 +0100
From:   Ben Whitten <ben.whitten@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andreas Färber <afaerber@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, nandor.han@...sala.com
Subject: Re: [PATCH] regmap: fix writes to non incrementing registers

On Wed, 14 Aug 2019 at 11:01, Mark Brown <broonie@...nel.org> wrote:
>
> On Tue, Aug 13, 2019 at 10:22:51PM +0100, Ben Whitten wrote:
>
> > @@ -1489,10 +1489,11 @@ static int _regmap_raw_write_impl(struct regmap *map, unsigned int reg,
> >       WARN_ON(!map->bus);
> >
> >       /* Check for unwritable registers before we start */
> > -     for (i = 0; i < val_len / map->format.val_bytes; i++)
> > -             if (!regmap_writeable(map,
> > -                                  reg + regmap_get_offset(map, i)))
> > -                     return -EINVAL;
> > +     if (!regmap_writeable_noinc(map, reg))
> > +             for (i = 0; i < val_len / map->format.val_bytes; i++)
> > +                     if (!regmap_writeable(map,
> > +                                          reg + regmap_get_offset(map, i)))
> > +                             return -EINVAL;
>
> This feels like we're getting ourselves confused about nonincrementing
> registers and probably have other breakage somewhere else - we're
> already checking for nonincrementability in regmap_write_noinc(), and
> here we're only checking if the first register in the block has that
> property which might blow up on us if there's a register in the middle
> of the block that is nonincrementable.  If we're going to check this
> here I think we should check on every register, but this is
> _raw_write_impl() which is part of the call path for implementing
> regmap_noinc_write() so checking here will break the API purpose
> designed for nonincrementing writes.

So it appeared that the last patch in this area for validating a register
block [1] broke the regmap_noinc_write use case.
Because regmap_noinc_write calls _regmap_raw_write and in
turn hits the _regmap_raw_write_impl, the val_len is the depth of the
one register to write to and not a block of registers which is assumed
by the previous check. By inserting a check that the first (and only)
register is a noinc one allows me to start writing to my FIFO again.

I'm all for an alternative solution though if there is a cleaner approach.

Kind regards,
Ben

[1] https://lore.kernel.org/patchwork/patch/1057184/