linux-kernel - Re: [RFC 4/7] mfd: ocelot-spi: Change the regmap stride to reflect the real one

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Date:   Fri, 24 Mar 2023 08:48:18 -0700
From:   Colin Foster <colin.foster@...advantage.com>
To:     Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc:     Andrew Lunn <andrew@...n.ch>, Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rafael@...nel.org, Vladimir Oltean <vladimir.oltean@....com>,
        Lee Jones <lee@...nel.org>, davem@...emloft.net,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        thomas.petazzoni@...tlin.com
Subject: Re: [RFC 4/7] mfd: ocelot-spi: Change the regmap stride to reflect
 the real one

Hi Maxime,

On Fri, Mar 24, 2023 at 01:48:17PM +0100, Maxime Chevallier wrote:
> Hello Andrew,
> 
> On Fri, 24 Mar 2023 13:11:07 +0100
> Andrew Lunn <andrew@...n.ch> wrote:
> 
> > On Fri, Mar 24, 2023 at 10:36:41AM +0100, Maxime Chevallier wrote:
> > > When used over SPI, the register addresses needs to be translated,
> > > compared to when used over MMIO. The translation consists in
> > > applying an offset with reg_base, then downshifting the registers
> > > by 2. This actually changes the register stride from 4 to 1.
> > > 
> > > Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
> > > ---
> > >  drivers/mfd/ocelot-spi.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/mfd/ocelot-spi.c b/drivers/mfd/ocelot-spi.c
> > > index 2d1349a10ca9..107cda0544aa 100644
> > > --- a/drivers/mfd/ocelot-spi.c
> > > +++ b/drivers/mfd/ocelot-spi.c
> > > @@ -124,7 +124,7 @@ static int ocelot_spi_initialize(struct device
> > > *dev) 
> > >  static const struct regmap_config ocelot_spi_regmap_config = {
> > >  	.reg_bits = 24,
> > > -	.reg_stride = 4,
> > > +	.reg_stride = 1,
> > >  	.reg_shift = REGMAP_DOWNSHIFT(2),
> > >  	.val_bits = 32,  
> > 
> > This does not look like a bisectable change? Or did it never work
> > before?
> 
> Actually this works in all cases because of "regmap: check for alignment
> on translated register addresses" in this series. Before this series,
> I think using a stride of 1 would have worked too, as any 4-byte-aligned
> accesses are also 1-byte aligned.
> 
> But that's also why I need review on this, my understanding is that
> reg_stride is used just as a check for alignment, and I couldn't test
> this ocelot-related patch on the real HW, so please take it with a
> grain of salt :(

You're exactly right. reg_stride wasn't used anywhere in the
ocelot-spi path before this patch series. When I build against patch 3
("regmap: allow upshifting register addresses before performing
operations") ocelot-spi breaks.

[    3.207711] ocelot-soc spi0.0: error -EINVAL: Error initializing SPI bus

When I build against the whole series, or even just up to patch 4 ("mfd:
ocelot-spi: Change the regmap stride to reflect the real one")
functionality returns.

If you keep patch 4 and apply it before patch 2, everything should
work.

Sorry for the bug. Thanks for the fix. And I'm glad I'm not the only one
taking advantage of the "reg_shift" regmap operation! I thought I'd be
the only one.


Let me know if you want me to take any action on this fix.


Colin