Re: [PATCH 10/13] i2c: nomadik: support Mobileye EyeQ5 I2C controller

Théo Lebrun
2024-02-19T14:52:40Z

Hello,

On Mon Feb 19, 2024 at 3:35 PM CET, Linus Walleij wrote:
> On Thu, Feb 15, 2024 at 5:52 PM Théo Lebrun <theo.lebrun@bootlin.com> wrote:
>
> > Add compatible for the integration of the same DB8500 IP block into the
> > Mobileye EyeQ5 platform. Two quirks are present:
> >
> > - The memory bus only supports 32-bit accesses. One writeb() is done to
> > fill the Tx FIFO which we replace with a writel().
> >
> > - A register must be configured for the I2C speed mode; it is located
> > in a shared register region called OLB. We access that memory region
> > using a syscon & regmap that gets passed as a phandle (mobileye,olb).
> >
> > A two-bit enum per controller is written into the register; that
> > requires us to know the global index of the I2C
> > controller (mobileye,id).
> >
> > We add #include <linux/mfd/syscon.h> and <linux/regmap.h> and sort
> > headers.
> >
> > Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
>
> (...)
>
> > - writeb(*priv->cli.buffer, priv->virtbase + I2C_TFR);
> > + if (priv->has_32b_bus)
> > + writel(*priv->cli.buffer, priv->virtbase + I2C_TFR);
> > + else
> > + writeb(*priv->cli.buffer, priv->virtbase + I2C_TFR);
>
> Are the other byte accessors working flawlessly? I get the shivers.
> If it's needed in one place I bet the others prefer 32bit access too.

I see where your shivers come from; I'll investigate as I don't remember
my conclusion from the time when I worked on this driver (a few months
ago).

> Further the MIPS is big-endian is it not? It feels that this just happens
> to work because of byte order access? writel() is little-endian by
> definition.

Actually, no. Our platform is little-endian.

The full story, summarised: the endianness of our cores in kernel and
hypervisor mode is defined by a pin read at reset. User mode can toggle
the endianness at runtime I believe, but that is not of our concern.
Our endianness in kernel mode is little-endian because the pin in
question is hardwired to the value meaning little-endian.

> What happens if you replace all writeb():s with something like
>
> static void nmk_write_reg(struct nmk_i2c_dev *priv, u32 reg, u8 val)
> {
> if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
> writeb(val, priv->virtbase + reg + 3);
> // if this doesn't work then use writeb((u32)val,
> priv->virtbase + reg) I guess
> else
> writeb(val, priv->virtbase + reg);
> }
>
> and conversely for readb()?

As mentionned above, big endian isn't the worry for us. I'll be checking
the readb() calls found in i2c_irq_handler() though.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering