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Re: drivers for l4 (2)

From: Maurizio Boriani
Subject: Re: drivers for l4 (2)
Date: 24 Mar 2003 14:24:48 +0100
User-agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/20.7

>>>>> "Daniel" == Daniel Wagner <address@hidden> writes:

I think all DDE could be well designed and implemented using OOP (also new
L4, pistachio should be implemented using c++). Also, because
it run in user space, could use shared objects as device driver. A design
like ACE (from smidth) could be a good example. 

    Daniel> The driver framework basically has to worry about IRQ
    Daniel> Handling, bus management and access to hardware and
    Daniel> hotplugging (driver loading,...).

    Daniel> + IRQ handling is basically done by L4 for us. The only
    Daniel> thing left is handling mapping bus IRQ to a system
    Daniel> interrupt. This can be easily done by the bus managers.

    Daniel>   How to install a interrupt handler: Precondition(s): 
    Daniel>   - All interrupt handler code should be relocable.
    Daniel>   + The code is mapped into the interrupt thread's
    Daniel>     address space.  
    Daniel>   + The interrupt thread's address space loader thread copies and 
    Daniel>     relocates the code and adds it to
    Daniel>     the chain of handlers (linked list) to be executed for the
    Daniel>     specific IRQ.

the above should be the basic IRQ handling on L4, right?

    Daniel> + A bus manager has to offer the following API:

    Daniel>     + enumerate children: lists all children of this
    Daniel>       bus. Obviously children can be bus managers themselves.  
    Daniel>     + alloc resource 
    Daniel>     + free resource 
    Daniel>     + map resource: provides access to a resource in the 
    Daniel>       requesting AS 
    Daniel>     + unmap resource
    Daniel>     + activate resource 
    Daniel>     + desactivate resource 
    Daniel>     + get interrupt

    Daniel> resources can be: 
    Daniel>     I/O space.  prefetcheable memory
    Daniel>     address space: address space which is accessible using
    Daniel>         normal CPU load/store instructions and where the CPU may
    Daniel>         assume that reading a memory location will not have side
    Daniel>         effects.  
    Daniel>     non-prefetcheable memory address space: address
    Daniel>         space which is accessible using normal CPU load/store
    Daniel>         instructions.  Reading a memory location from this space
    Daniel>         may have side effects.  
    Daniel>         DMAable memory: Physical memory which can be accessed by 
    Daniel>         the device. (More thoughts on DMA at the end of the email)

    Daniel> for PCI this API can be extended with:
    Daniel>     + enable i/o 
    Daniel>     + disable i/o 
    DAniel>     + enable mem 
    Daniel>     + disable mem
    Daniel>     + enable busmastering 
    Daniel>     + disable busmastering 
    Daniel>     + read configspace + write configspace

    Daniel>     (other busses probably need their own extensions)

As said above, a base class bus manager could have inherited classes which 
implement extended interfaces for a particular type of bus. 

    Daniel> + The hotplug manager implements the following API: 
    Daniel>     + add device: Announces a new device in the system.  
    Daniel>     + remove device: Device has gone

    Daniel> The hotplug manager will load the appropriate device
    Daniel> driver into one of the device driver AS's. The hotplug
    Daniel> manager create or deletes these AS's as necessary. Whithin
    Daniel> each AS a device driver management thread runs which
    Daniel> handles the loading and unloading of the drivers.

The above could be an inherited class from base bus manager.

    Daniel> Device drivers themselves are modules which are
    Daniel> relocateable so they can be loaded anywhere into the
    Daniel> AS. This can be done by building them as Position
    Daniel> Independent Code or having explicit relocation tables. The
    Daniel> latter is probably the best choice, except if we would
    Daniel> load a driver multiple times for multiple devices of the
    Daniel> same type. They are loaded and started by the management
    Daniel> thread.  They also get a reference to a port of their
    Daniel> parent bus manager so they can ask for resources, etc.

IMHO, device driver code could be builded as shared object and (if vm permit)
mapped in thread which request it (an interrupt handler for example). Like
bus manager, device drivers of the same type could be inherited from 
a base common interface and implement only method useful for interaction
with device (read, write, and so on...). The interface between device driver
and other procs could be implemented by a generic thread which manipulate
the well known interface to driver   

    Daniel> + Bootup: 
    Daniel>   + All drivers are loaded in their own address
    Daniel>     space for now 

as shared object

    Daniel>   + Drivers which are necessary for system
    Daniel>     bootup (HD driver, console driver, ...) are loaded by
    Daniel>     grub.  
    Daniel>   + The hotplug manager and a config file is also
    Daniel>     loaded by grub.  The config file has also a list of device
    Daniel>     names/identifier strings which tells wich driver can
    Daniel>     handle which device.  This list will be discared after the
    Daniel>     system is able to load an editable list from the harddisk.

As said above, device driver shouldn't be started as device manipulator 
directly but be used from another task as a dynamic shared object. So the
config file tell to root bus manager which how many device driver task should
be started up and which object (loaded previously as shared object by grub) them
instantiate and use. So all device driver task implement same code but use
different object implementation as specified in config file. 
This kind of design is well explainated in Service definitions bye smidth's 


Obviously the above describe particularly ACE framework but the generic idea
and design could be used in DDE context.

    Daniel> + The hotplug manager gets a list of the driver loaded by
    Daniel>   grub with their thread ID's.  
    Daniel> + All drivers are loaded as
    Daniel>   separate process by the resource manager.  
    Daniel> + The root bus  manager starts as first driver.  
    Daniel>   The root driver reads possible BIOS data (or the needed 
    Daniel>   information which driver should be loaded is compiled in).  
    Daniel>   Then it asks the hotplug manager for the driver which should 
    Daniel>   be inserted into the driver tree.  

I think hotplug could be skipped out make reading directly config file to 
root bus manager and making it starting out threads as needing.

    Daniel> + All output to stdout and stderr
    Daniel>   will be buffered in memory untill the video driver is
    Daniel>   ready to dump it on the screen.  It might be usefull to
    Daniel>   write it to a serial port.

    Daniel> For actually bootstrapping the resource manager (rmgr)
    Daniel> with some small additions could be used: 
    Daniel>   + keeps a list of the modules started, their name, their grub 
    Daniel>     commandline and their thread id.  
    Daniel>   + no reallocation needed for modules

couldn't be rmgr the root bus manager? 

    Daniel> (Why is this done anyway?  Is it to keep sigma0 simpler
    Daniel> (one-to-one map?))


    Daniel> And here now some question or thoughts we haven't solved
    Daniel> yet: 
    Daniel>   + In which address space is the resource manager started?  

 user space I think. And get initial resources by sigma0

    Daniel>   + We can take control of the interrupt threads by migrating 
    Daniel>     them to our own address space.  How does
    Daniel>     thread migration work?  Restrictions?

as said above if rmgr is the "root bus manager" there isn't need of
threads migration
    Daniel> TODO :

    Daniel> think about API's for other busses (ISA, Nubus, Zorro,
    Daniel> ...)  think about API's for message busses such as USB,
    Daniel> firewire, scsi, fibrechannel ADB,...

a well designed hierarchy of classes :)

    Daniel> and all things we forgot.

Now I'm working on port UVM to L4 and I think this could be finished in next
3 months. Next I'll apply full time in DDE (in coworking with you all :) )

What do you think about this design? Are only few ideas, I've not try to 
implement them before.


Maurizio Boriani 
GPG key: 0xCC0FBF8F
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