[U-Boot] How does Driver Model UART work?

[Simon Glass]

Hi Masahiro,

On 3 October 2014 07:04, Masahiro Yamada <yamada.m at jp.panasonic.com> wrote:
> Simon,
>
>
>
> I am totally being confused.
>
>
>
> As far as I looked at the dm code,
> the private data is calloc'ed in device_probe() function
>
>         if (drv->priv_auto_alloc_size) {
>                 dev->priv = calloc(1, drv->priv_auto_alloc_size);
>                 if (!dev->priv) {
>                         ret = -ENOMEM;
>                         goto fail;
>                 }
>         }
>
>
>
> So,  dev->priv is storing the address of the allocated memory.
>
> Am I understanding correctly?

Yes, it always does in driver model. This driver is no different.

BTW the ns16550 driver is probably the oddest in U-Boot - it looks
like UniPhier has its own UART driver so it would be better to convert
that I think. See below for ideas on other UART drivers to look at
which are much more normal.

>
>
>
> If so, I can't understand the following code:
>
>
> static int ns16550_serial_getc(struct udevice *dev)
> {
>         struct NS16550 *const com_port = dev_get_priv(dev);
>
>         if (!serial_in(&com_port->lsr) & UART_LSR_DR)
>                 return -EAGAIN;
>
>
>
> "com_port" is dev->priv, so it is pointing to the allocated area on RAM,
> I guess.
>
>
> It looks like
> serial_in(&com_port->lsr)  is trying to read from the hardware register ?
>
> Or reading from malloc area RAM ??

If you go one level deeper you will see that serial_in() is defined at
the top in about 5 ways, one of which is used for driver model:

#define serial_in(addr) \
     ns16550_readb(com_port, addr - (unsigned char *)com_port)

So it used com_port which is not an parameter. The parameter addr is
only used to specify the register. This is actually the same as the
non-DM code except in that case the parameter specifies the register
and hardware address at the same time.

This is done since the internal NS16550_t type is unfortunately
exported all over U-Boot. This is annoying because it is actually an
internal register format for the UART. Even worse is the fact that the
structure changes depending on CONFIG_SYS_NS16550_REG_SIZE. We can't
have this sort of thing in driver model - we need to be able to cope
with the device tree specifying all the information that the UART
needs. So for driver model:

/*
 * For driver model we always use one byte per register, and sort out the
 * differences in the driver
 */
#define CONFIG_SYS_NS16550_REG_SIZE (-1)

(see ns16550.h header for where this is used)

I would like to avoid having that type exported and use a different
method to pass the UART information around. But it is used in about 30
places . So this approach allows us to move forward bit by bit,
without duplicating the UART driver and creating a maintenance
headache.

In terns of implementing a new UART driver, are you using device tree?

If so then you should just be able to follow along with Tegra - it's
really easy - just copy that serial_tegra.c and adjust the device tree
compat string and input clock. Other examples are serial_omap and
serial_dw. (See u-boot-dm/working)

If you are using platform data, then the examples are serial_mxc and
serial_pl01x.

Regards,
Simon