[ECOS] DSRs and scheduling thread-context work

Sergei Gavrikov sergei.gavrikov@gmail.com
Thu Oct 28 19:09:00 GMT 2010


On Thu, 28 Oct 2010, Tomas Whitlock wrote:
> Hi Sergei,
> 
> Apologies for not replying for a few days. Thank you for your reply,
> particularly for the information about ISRs.
> 
> Do you mean that a general mechanism (i.e. not tied to ISRs) for
> scheduling thread-context work is planned for a future release, and
> will be part of the driver API?

Hi

I'm afraid I do not understand why this issue has arisen. In my opinion
eCos has neat and complete driver API. Well, perhaps, I misunderstood
the question.

> I had read most of the documentation that you pointed to. One thing
> that is not explained by the docs or source code (as far as I could
> tell) is what eCOS functions drivers are supposed to use. Are they
> supposed to restrict themselves to just the driver API as described in
> chapter 13? Or are they supposed to use the driver API plus any other
> eCOS functions as necessary (e.g. for starting a worker thread)?

I would rather recalled chapter 11 from 5th section: V. I/O Package
(Device Drivers), those cyg_io_* API. Be sure that the set of functions
for a driver is not limited by cyg_io_ "namespace".

Regarding the driver's thread (if I understand the question). A driver
can call own service thread on its start. Wait a minute... well that's
here, for example: devs/kbd/arm/aaed2000/current/src/aaed2000_kbd.c.

Generally speaking, I would not refer to specific chapter of the
documentation. I would say, at first, read all the documentation, well,
at least across and you begin understand how to implement own driver.
Because, the eCos watchdog driver is not similar to the eCos serial
driver and so on.  But keep it simple, do not develop /dev/ledX device
driver, if you can manage the LEDs directly.

IMO, driver for mesuaring temperature and voltage is a good example of
how to develop a specific driver on top of generic eCos I/O driver. Of
course, that interface to the driver can not complete a set of the
cyg_io_* fuctions (though this is a good start and in common cases
that's enough), for higher-level aplications it can be SNMP interface to
the its OID values, but nevertheless, the interface must be implemented
on top of the eCos I/O driver model. In short, after reading the
documentation it would be good then look at the different approaches and
implementations of the eCos drivers under eCos packages directory (see
sources: io/* and devs/*).

Sergei

> thanks
> Tomas
> 
> -----Original Message-----
> From: Sergei Gavrikov [mailto:sergei.gavrikov@gmail.com]
> Sent: Fri 10/22/2010 21:03
> To: Tomas Whitlock
> Cc: ecos-discuss@ecos.sourceware.org
> Subject: Re: [ECOS] DSRs and scheduling thread-context work
> 
> On Fri, 22 Oct 2010, Tomas Whitlock wrote:
> > Dear all,
> >
> > I have been tasked with porting a driver for a PCI device from VxWorks /
> > Linux to eCOS 3.0. While most things look reasonably straight-forward, I
> > have a few queries relating to how a driver can schedule work to be done
> > in a thread context.
> 
> Hi Tomas,
> 
> Only in brief
> 
> > The ISR of this driver must handle interrupts that originate from
> > several distinct sources inside the device being controlled. These
> > sources are in reality DMA channels that can run independently of one
> > another, but share the same physical interrupt pin for signalling
> > completion of DMA. I'd like to be sure that my proposed interrupt
> > handling scheme won't lose interrupts.
> >
> > First:
> >
> > Will the following logic work?
> >
> > ISR:
> > 1. Read interrupt status register in device to sample what interrupt
> > sources are currently active. If nothing active, call
> > cyg_drv_interrupt_acknowledge and return 0.
> > 2. For each active interrupt source set an "active flag" somewhere in
> > the device context structure (for use in the DSR).
> > 3. Write to the device's interrupt status register to clear the
> > interrupt sources that we sampled in step 1.
> > 4. Acknowledge the hardware interrupt using
> > cyg_drv_interrupt_acknowledge.
> > 5. Return CYG_ISR_HANDLED | CYG_ISR_CALL_DSR.
> >
> > DSR:
> > 1. If active flag 0 is set,
> >     clear active flag 0
> >     do any necessary processing
> > 2. If active flag 1 is set,
> >     clear active flag 1
> >     do any necessary processing
> > 3. ... other sources ...
> > n. Return.
> >
> > (assume that by design of the driver and hardware, an active flag can
> > never transition to "set" while the corresponding DSR processing is
> > happening)
> >
> > Now my question is: suppose the DSR is executing any of steps 2 to n-1,
> > interrupt source 0 becomes active and the ISR runs to completion. Is the
> > DSR guaranteed to be rescheduled even though it is still executing?
> >
> > If the answer is 'yes' then everything should be fine.
> 
> - Yes
> 
> Of course, if that won't be some king of ISR storm, although DSRs have
> been planned (=pended) at the least.
> 
> See, please,
> 
>    http://ecos.sourceware.org/docs-latest/ref/kernel-overview.html
> 
> Sections:
> 
>    * Threads and Interrupt Handling
>    * Calling Contexts
> 
> > If the answer is 'no', then it seems that even masking the hardware
> > interrupt in the ISR and unmasking it right at the end of the DSR isn't
> > 100% guaranteed to work correctly...
> >
> > Alternatively, maybe I could create multiple interrupt objects using the
> > same vector / priority parameters, so that we have one ISR and one DSR
> > for each interrupt source in the device. That seems like an odd way to
> > do things, though. Is it legal?
> >
> > Second:
> >
> > I'm a bit confused about the need for cyg_drv_interrupt_acknowledge. If
> > my ISR is called as part of a chain of ISRs because our interrupt vector
> > is shared by other devices, shouldn't whoever traverses the chain (i.e.
> > not me) call cyg_drv_interrupt_acknowledge? If everyone in the chain
> > including me calls it, then the interrupt gets acknowledged multiple
> > times, which may be a bad thing. Or is cyg_drv_interrupt_acknowledge
> > smart enough to be a no-op in that situation?
> 
> This depends on the implementation details in a HAL as the
> driver's/kernel's interrupt acknowledge functions in fact call
> HAL_INTERRUPT_ACKNOWLEDGE macro.
> 
>    hal/common/v3_0/src/drv_api.c:
>    externC void cyg_drv_interrupt_acknowledge
> 
>    kernel/v3_0/src/intr/intr.cxx:
>    Cyg_Interrupt::acknowledge_interrupt
> 
> > Third:
> 
> > It's quite common that a driver has to perform some periodic work in a
> > thread context, much like a DSR function. In our case, we want to read a
> > hardware monitoring chip periodically, to get temperatures and voltages
> > etc.
> >
> > It looks like there has been some effort to "wall off" drivers from
> > everything else with their own API, as can be seen from the cyg_drv_*
> > functions. However, that API doesn't seem to have generalised timer and
> > work-scheduling functionality. So a driver has to use non-cyg_drv_*
> > functions. For example, the driver could call cyg_thread_create to
> > create a worker thread and have that thread wait on something in a loop.
> >
> > If one were coming from Linux or Windows land, one might argue that it's
> > a job for a user-mode daemon that periodically makes a special call to
> > the driver, but that adds the complexity of having to create a special
> > user-mode interface.
> 
> From eCos islet that is a job for an eCos thread.
> 
> > I guess I'm wondering why there is this set of cyg_drv_* functions that
> > seem to be insufficient for nontrivial drivers. Are there some
> > guidelines on what non-cyg_drv_* functions a driver can/should use? Is
> > this API going to be expanded in the future? In any case, the
> > distinction between a "driver" and any other eCOS module seems a bit
> > artificial to me...
> 
> I hope you will find answers here as well:
> 
>    http://ecos.sourceware.org/docs-latest/ref/kernel-overview.html
> 
>    * Calling Contexts
> 
> and here (eCos driver "Big picture"):
> 
>   http://ecos.sourceware.org/docs-latest/ref/devapi-device-driver-interface-t
> o-the-kernel.html
> 
>    * Synchronization Levels
> 
> I hope my "come-off" will help a bit.
> 
> Sergei
> 
> > Thanks in advance for any answers / advice
> > Tom
> 
> This message has been delivered to Alpha Data by MessageLabs Virus Scanning
> Service.
> 
> 
>
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