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     7     Nokia Corporation - initial contribution.

     8 Contributors: 

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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-F09740DA-015B-449D-A124-0BEABBDDCB52" xml:lang="en"><title>Enabling

    13 and Disabling</title><shortdesc>This document describes how device drivers enable and disable interrupts.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>Drivers can enable and disable the interrupts on the relevant interrupt

    15 sources while handling the interrupts using the <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita"><apiname>Interrupt</apiname></xref> class.

    16 Typically, handling an interrupt is done with disabled interrupts. The interrupt

    17 is then enabled after the completion of interrupt handling. </p>

    18 <codeblock id="GUID-7805933A-51F6-5169-81D0-C1A8E9647C0D" xml:space="preserve">Interrupt::Disable(aIntId);

    19 ...

    20 // handle interrupt

    21 ...

    22 Interrupt::Enable(aIntId);

    23 </codeblock>

    24 <codeblock id="GUID-87C0A4CF-BA67-58DE-A180-EA30189BEDF4" xml:space="preserve">/**

    25  Interrupt Service Routine (ISR) for the UART driver. This function

    26  is associated with the UART interrupt, EUartIrq (value obtained from 

    27  TRM). This function is called whenever there is an interrupt on this 

    28  interrupt line and the interrupt source is enabled. The ISR must be a 

    29  static void function, taking a single TAny* parameter, which is an 

    30  object passed at the time of binding, usually a pointer to the owning

    31  class. Interrupt::Enable() must be called to start receiving 

    32  interrupts.

    33  

    34  The ISR implementation is hardware and driver specific. The ISR must

    35  be self-contained as the state of the system is indeterminate. It

    36  should not signal threads, allocate or de-allocate memory, copy

    37  memory to or from user processes, or call most Kernel functions.

    38  Also the whole system is blocked while the ISR is running, so it must be

    39  kept as short as possible. Any lengthy processing must be done in a

    40  delayed function call (DFC).

    41  

    42  A typical ISR would,

    43    Clear the interrupt in the peripheral.

    44    Disable the interrupt.

    45    Read or write peripheral registers, e.g. to determine errors, or 

    46    to start a new task.

    47    Read data from, or write data to, the peripheral.

    48    Start or stop NTimers or DMA transfer operations.

    49    Queue DFCs.

    50  

    51  @param     aParam

    52          pointer to object passed as parameter while binding

    53  */    

    54 

    55 void DExUartPhysicalChannelH4::UartIsr(TAny* aParam)

    56     {

    57     // Do the high priority device specific interrupt handling

    58     // i.e. read the interrupt type from device, clear the 

    59     // interrupt.

    60     ...

    61     // Defer the remaining processing by queuing the respective 

    62     // DFC based on the interrupt type

    63     ...

    64     }</codeblock>

    65 <p>Some operations may need to be performed with all the interrupts, not just

    66 the particular interrupt source, disabled. In this case, the <xref href="GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02.dita"><apiname>NKern</apiname></xref> class

    67 must be used to disable and enable all interrupts globally at the Interrupt

    68 controller. <xref href="GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02.dita#GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02/GUID-8C251C65-FDE7-3161-8D2B-61401FB6487F"><apiname>NKern::DisableAllInterrupts()</apiname></xref> can disable all

    69 IRQs, or IRQs and FIQs. <xref href="GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02.dita#GUID-3A3C08F3-3D33-3D9E-80E7-7855C7B21E02/GUID-2D328082-3A9F-3467-81CF-B1C68866E163"><apiname>NKern::RestoreInterrupts()</apiname></xref> can

    70 restore the interrupts to their state before disabling. </p>

    71 <codeblock id="GUID-A34A1AEB-9B00-556D-A280-9BFF7A6BAA8C" xml:space="preserve">alevel = 1; // 1 to disable IRQs and 2 to disable IRQs and FIQs

    72 state = NKern::DisableAllInterrupts(aLevel);

    73 ...

    74 // perform the required operations

    75 ...

    76 NKern::RestoreInterrupts(state);

    77 </codeblock>

    78 </conbody></concept>