Porting the clock driver (July 8 2020 Week 6)

The current device drivers in the Beagle BSP do the clock initialization locally this is not bad but a seperate driver that handles clock initialization part is better. And specially when you have to deal with multiple BSP variants a separate clock driver is more neater approach.

Similar to the pinmux driver imported previously from FreeBSD we will be importing the clock driver from FreeBSD too. And use the implemented FreeBSD structures to make the porting easier.

FreeBSD provides an API allow drivers to configure their devices clock. This API is provided by the ti_prcm.h header.

The API provides the following functions that let the driver configure its devices clocks.

int ti_prcm_clk_valid(clk_ident_t clk);
int ti_prcm_clk_enable(clk_ident_t clk);
int ti_prcm_clk_disable(clk_ident_t clk);
int ti_prcm_clk_accessible(clk_ident_t clk);
int ti_prcm_clk_disable_autoidle(clk_ident_t clk);
int ti_prcm_clk_set_source(clk_ident_t clk, clk_src_t clksrc);
int ti_prcm_clk_get_source_freq(clk_ident_t clk, unsigned int *freq);

The way the API works is, the drivers tell the API the clock to configure using the clock indentifier clk_ident_t. This is then used by the SOC specific implementation to activate the right set of clocks.

The SOC specific driver for AM335X variant is present in (am335x_prcm.c)[http://web.mit.edu/freebsd/head/sys/arm/ti/am335x/am335x_prcm.c].

So as far as now, we have to import the following files:

1. ti_prcm.h
2. ti_prcm.c
3. am335x_prcm.h
4. am335x_prcm.c

But there is one part missing, how does the driver know about the clock identifier? This is done using the ti,hwmods prop in the device tree. Each node(device) that has a clock contains this prop. This prop used to find the clock indenifier for the device. The parsing of this property is done separately in ti_hwmods.c. Thus these files also have to be imported. And there is also other dependency, ti_scm.c this driver is a dependency of am335x_prcm and is responsible for reading and writing the register values.

Thus the final list of files that have to be ported are:

1. ti_prcm.h
2. ti_prcm.c
3. am335x_prcm.h
4. am335x_prcm.c
5. ti_hwmods.h
6. ti_hwmods.c
7. ti_scm.h
8. ti_scm.c

One thing to note is that we will not be removing the ti_hwmods driver from libBSD after importing it to RTEMS this is because the driver is not possible to port to RTEMS as it is since it has dependency on the device structure in libBSD instead we will be modifying it slightly to cater the local BSP drivers and let the driver present in libBSD to cater the libBSD drivers.

Once we import these files to RTEMS, we use the FreeBSD structure to do the required modifications to make them work with RTEMS.

The commits that imports and port these files can be found in this branch.