[U-Boot] [PATCH 01/14] dm: Initial import of design documents

[Marek Vasut <marex@denx.de>]

Dear Stephen Warren,

> On 08/08/2012 05:42 AM, Marek Vasut wrote:
> > From: Marek Vasut <marek.vasut@gmail.com>
> > 
> > This patch contains UDM-design.txt, which is document containing
> > general description of the driver model. The remaining files contains
> > descriptions of conversion process of particular subsystems.
> > 
> > diff --git a/doc/driver-model/UDM-design.txt
> > b/doc/driver-model/UDM-design.txt
> > 
> > +II) Driver core initialization stages
> > +-------------------------------------
> > +
> > +The drivers have to be initialized in two stages, since the U-Boot
> > bootloader +runs in two stages itself. The first stage is the one which
> > is executed before +the bootloader itself is relocated. The second stage
> > then happens after +relocation.
> > +
> > +  1) First stage
> > +  --------------
> > +
> > +  The first stage runs after the bootloader did very basic hardware
> > init. This +  means the stack pointer was configured, caches disabled
> > and that's about it. +  The problem with this part is the memory
> > management isn't running at all. To +  make things even worse, at this
> > point, the RAM is still likely uninitialized +  and therefore
> > unavailable.
> > +
> > +  2) Second stage
> > +  ---------------
> > +
> > +  At this stage, the bootloader has initialized RAM and is running from
> > it's +  final location. Dynamic memory allocations are working at this
> > point. Most of +  the driver initialization is executed here.
> 
> Given the above descriptions of the two stages, ...
> 
> > +III) The drivers
> > +----------------
> > +
> > +  1) The structure of a driver
> > +  ----------------------------
> > +
> > +  The driver will contain a structure located in a separate section,
> > which +  will allow linker to create a list of compiled-in drivers at
> > compile time. +  Let's call this list "driver_list".
> > +
> > +  struct driver __attribute__((section(driver_list))) {
> > +    /* The name of the driver */
> > +    char		name[STATIC_CONFIG_DRIVER_NAME_LENGTH];
> > +
> > +    /*
> > +     * This function should connect this driver with cores it depends on
> > and +     * with other drivers, likely bus drivers
> > +     */
> > +    int			(*bind)(struct instance *i);
> 
> ... the comments here should probably say which stage each function will
> be run at.

The drivers must be agnostic to where they're executed

> > +
> > +    /* This function actually initializes the hardware. */
> > +    int			(*probe)(struct instance *i);
> > +
> > +    /*
> > +     * The function of the driver called when U-Boot finished
> > relocation. +     * This is particularly important to eg. move pointers
> > to DMA buffers +     * and such from the location before relocation to
> > their final location. +     */
> > +    int			(*reloc)(struct instance *i);
> 
> The need for this function implies that some other functions (both bind
> and probe?) are to be called before relocation.

They can be called after reloc too.

> Isn't the pre-relocation
> environment rather strict

Yes

> and hence this will require a bunch of
> changes to driver code to initialize differently?

No

> Why not just
> initialize everything after relocation; IIRC, that's how most things are
> initialized now, isn't it?

You want eg. i2c bus running before relocation to init your DRAM controller. Or 
serial console for debug output.

> Related, if you're intending to allow drivers to be loaded from e.g. the
> filesystem later

Not now, but we keep that in mind. Btw. (!) we're writing a bootloader here, not 
an operating system.

> that will happen after relocation. There will then be
> a discrepancy between the environment where bind/probe get run for a
> built-in driver vs. a dynamically loaded driver.

Will there?

Best regards,
Marek Vasut