Landfall

Annotated docs/wiki/System_Architecture

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2019-04-13 Paul Boddie file changeset files shortlog Introduced wiki-format documents as the principal documentation.
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= System Architecture =
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A significant motivation for providing server programs is to isolate
 
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device-specific operations within separate components, thus preventing each
 
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component from having too broad access to low-level system functionality. The
 
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combination of components is then encouraged to achieve configuration
 
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objectives.
 
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For example, two computing systems that employ the same LCD controller might
 
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each employ different screen types and have different ways of controlling
 
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display behaviour. By defining explicit mechanisms for combining access to
 
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these different aspects of the hardware, common elements (such as the LCD
 
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controller) may be encapsulated by a device server that can be reused, with
 
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differing elements (such as the screen type) being described by separate
 
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resources or components that can be introduced as required.
 
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Here is how the Ben NanoNote's framebuffer is supported by components:
 
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######## A graph showing the relationship between components.
 
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{{{#!graphviz
 
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#format svg
 
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#transform notugly
 
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digraph nanonote_framebuffer {
 
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  node [shape=rectangle,fontsize="13.0",fontname="Helvetica"];
 
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  fb_drv [label="fb-drv"];
 
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  dev_cpm_jz4740 [label="dev_cpm_jz4740\nClock configuration"];
 
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  dev_display_qi_lb60 [label="dev_display_qi_lb60\nDisplay control"];
 
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  dev_backlight_spi_ili8960 [label="dev_backlight_spi_ili8960\nBacklight control"];
 
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  dev_spi_jz4740 [label="dev_spi_jz4740\nBacklight communication"];
 
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  fb_drv -> dev_cpm_jz4740;
 
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  fb_drv -> dev_display_qi_lb60 -> dev_backlight_spi_ili8960 -> dev_spi_jz4740;
 
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}
 
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}}}
 
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########
 
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And here is how the Letux 400's framebuffer is supported:
 
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######## A graph showing the relationship between components.
 
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{{{#!graphviz
 
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#format svg
 
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#transform notugly
 
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digraph letux400_framebuffer {
 
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  node [shape=rectangle,fontsize="13.0",fontname="Helvetica"];
 
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  fb_drv [label="fb-drv"];
 
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  dev_cpm_jz4730 [label="dev_cpm_jz4730\nClock configuration"];
 
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  dev_display_letux400 [label="dev_display_letux400\nDisplay control"];
 
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  dev_backlight_pwm [label="dev_backlight_pwm\nBacklight control"];
 
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  dev_pwm_jz4730 [label="dev_pwm_jz4730\nBacklight communication"];
 
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  fb_drv -> dev_cpm_jz4730;
 
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  fb_drv -> dev_display_letux400 -> dev_backlight_pwm -> dev_pwm_jz4730;
 
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}
 
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}}}
 
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########
 
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Note that fb-drv links to the same generic JZ4740 LCD controller library in
 
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both cases
 
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Here, the CPM device provides access to the clock and power management
 
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functionality, the display device provides access to the backlight and is
 
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responsible for configuring pins for the display. Device servers are connected
 
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using capabilities (object references).
 
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Meanwhile, panel information is provided via a dynamically-loaded library:
 
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######## A graph showing the acquisition of panel information.
 
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{{{#!graphviz
 
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#format svg
 
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#transform notugly
 
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digraph panel_information {
 
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  node [shape=rectangle,fontsize="13.0",fontname="Helvetica"];
 
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  subgraph {
 
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    rank=min;
 
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    fb_drv [label="fb-drv"];
 
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  }
 
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  paneldata [label="mips-jz4740-panel.txt\nLibrary details"];
 
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  library [label="...\nLibrary with panel data"];
 
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  paneldata -> fb_drv -> library;
 
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}
 
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}}}
 
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########
 
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This method of obtaining a configured library name in order to load it
 
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dynamically is effectively equivalent to having a symbolic link identifying
 
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the library referencing the desired, specific library to be used. Since the
 
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"rom" virtual filesystem does not appear to support symbolic links, this
 
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method is used instead.
 
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By providing well-defined interfacing mechanisms, the behaviour of driver code
 
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can be parameterised using external components, and a proliferation of
 
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specially-constructed device drivers is hopefully avoided.