linux-kernel - Re: [PATCH v3 1/9] ASoC: sun4i-i2s: Adjust regmap settings

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Message-ID: <20190109184604.GJ10405@sirena.org.uk>
Date:   Wed, 9 Jan 2019 18:46:04 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Code Kipper <codekipper@...il.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-ALSA <alsa-devel@...a-project.org>,
        "Andrea Venturi (pers)" <be17068@...rbole.bo.it>
Subject: Re: [PATCH v3 1/9] ASoC: sun4i-i2s: Adjust regmap settings

On Sat, Dec 22, 2018 at 12:44:07AM +0800, Chen-Yu Tsai wrote:
> On Fri, Dec 21, 2018 at 11:21 PM <codekipper@...il.com> wrote:

> > +       regcache_cache_bypass(i2s->regmap, true);
> >         regmap_update_bits(i2s->regmap, SUN4I_I2S_FIFO_CTRL_REG,
> >                            SUN4I_I2S_FIFO_CTRL_FLUSH_RX,
> >                            SUN4I_I2S_FIFO_CTRL_FLUSH_RX);
> > +       regcache_cache_bypass(i2s->regmap, false);

> IIRC the flush cache bit is self-clearing. So you likely want to mark
> this register as volatile. If it is marked as volatile, then all access
> to that register bypasses the cache, so the regcache_cache_bypass calls
> are unneeded.

Yes, that should be the case.

> However, looking at the code, the write would seem to be ignored if the
> regmap is in the cache_only state. We only set this when the bus clock
> is disabled. Under such a condition, bypassing the cache and forcing a
> write would be unwise, as the system either drops the write, or stalls
> altogether.

Right, access to a cache only register while the device is in cache only
mode is not a great idea - the usual reason we're in cache only mode is
that the device is in a state where I/O isn't going to work.  One thing
that can work for this if you need the register to be cached (but is a
bit gross) is to do a write setting the self clearing bit then another
immediately after resetting it back to the cleared state.  That works OK
for cases where the bit is a strobe and never retains state, though if
the device isn't operational then needing to write to the register might
indicate a bigger picture logic error (or it could be that the register
map mixes random things into one register).

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