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dm: Add twserial device document 

Signed-off-by: Tomas Hlavacek<tmshlvck@gmail.com>
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Tomáš Hlaváček 2012-08-08 13:42:30 +02:00 committed by Wolfgang Denk
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==================================
TWserial device subsystem analysis
==================================
Tomas Hlavacek<tmshlvck@gmail.com>
2012-03-21
I) Overview
-----------
U-Boot currently implements one common API for TWSerial devices. The interface
is defined in include/tws.h and comprises of functions:
int tws_read(uchar *buffer, int len);
int tws_write(uchar *buffer, int len);
The functions are implemented by a proper device driver in drivers/twserial
directory and the driver to be compiled in is selected in a Makefile. There is
only one driver present now.
The driver depends on ad-hoc code in board specific data, namely functions:
void tws_ce(unsigned bit);
void tws_wr(unsigned bit);
void tws_clk(unsigned bit);
void tws_data(unsigned bit);
unsigned tws_data_read(void);
void tws_data_config_output(unsigned output);
implemented in include/configs/inka4x0.h .
II) Approach
------------
U-Boot TWserial drivers exports two simple functions and therefore the conversion
of the driver and creating a core for it is not needed. It should be consolidated
with include/configs/inka4x0.h and taken to the misc/ dir.
III) Analysis of in-tree drivers
--------------------------------
1) drivers/twserial/soft_tws.c
------------------------------
The driver is the only TWserial driver. The ad-hoc part in
include/configs/inka4x0.h and the core soft_tws driver should be consolidated
to one compact driver and moved to misc/ .