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<concept id="GUID-8DF46A11-874A-52E5-9298-C083EA633BA0" xml:lang="en"><title>Implementing

Dynamic DSA Allocation</title><shortdesc>This topic describes how device creators can allocate the Direct

Screen Access (DSA) buffer dynamically. This feature is new in Symbian^3 (S^3). </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p> <b>Target audience</b>: Device creators. </p>

<section><title>Overview</title> <p>Before the introduction of ScreenPlay,

applications (such as video and EGL games) that required high frame rates

used Direct Screen Access (DSA) to bypass the Window Server and draw directly

to the frame buffer (video memory). However, some interaction with the Window

Server was necessary in order to prevent the application from drawing over

other applications' data. The frame buffer was allocated at device startup

and was never freed. </p> <p>In ScreenPlay, applications that require a high

frame rate can render directly to <xref href="GUID-ADA8CECB-0E70-5B9C-8F36-0714AAF0CD13.dita">composition

surfaces</xref>. In ScreenPlay the Window Server's content is also rendered

to a composition surface, which is known as the <b>UI surface</b>. This means

that the Window Server and its render stages do not render directly to the

frame buffer. However, DSA is still supported in order to provide backwards

compatibility. </p> <p>The DSA buffer is now just another buffer (known as

the <b>DSA buffer</b>), which the Window Server process makes use of only

when a client uses DSA on a visible window. This means that it is no longer

necessary to allocate the DSA buffer at startup and not free it. In fact to

do so in ScreenPlay is wasteful of resources. </p> <p>For this reason, it

is now possible to allocate the DSA buffer as a <xref href="GUID-51514A4B-0220-557B-9F7A-FB110CEFEF10.dita">shared

chunk</xref> and free it when it is not in use. This requires changes in the

implementation of the LCD Driver (also known as the <xref href="GUID-8C22AF20-EE0E-5AD2-BEFD-FED5A7DBB09B.dita">LCD

Extension</xref>). For example, in order to free the DSA buffer when it is

no longer in use, the LCD Driver must keep track of the processes that use

the buffer and when they finish with it. These changes can be implemented

regardless whether ScreenPlay is in use. </p> <p> <i>Note</i>: Semi-transparent

UI content over content provided by Direct Screen Access (DSA) has never been

supported and ScreenPlay does not change this. </p> </section>

<section><title>DSA client-side API changes</title> <p>To use DSA, the client

creates an instance of <xref href="GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129.dita"><apiname>CDirectScreenAccess</apiname></xref>. This allows

the client to draw directly to the DSA buffer and to determine the visible

area of its window and be notified when the visible area changes. DSA is typically

performed by using the <xref href="GUID-B229156F-2344-3F46-8542-AC65882D80DE.dita"><apiname>CFbsScreenDevice</apiname></xref> returned by <xref href="GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129.dita#GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129/GUID-9528CBFF-7D01-3B62-81B1-7F9B2C799C14"><apiname>CDirectScreenAccess::ScreenDevice()</apiname></xref>. </p> <p>However,

applications can call the HAL user-side API (described below) to allocate

the DSA buffer (if not already allocated) and obtain the DSA buffer's address

directly. Clients that do this can now use DSA simply in order to be notified

when the visible area changes and not for accessing the DSA buffer itself.

This is called <b>region-tracking DSA</b>. (<xref href="GUID-B229156F-2344-3F46-8542-AC65882D80DE.dita"><apiname>CFbsScreenDevice</apiname></xref> also

obtains the DSA address through the use of the HAL user-side API.) </p> <p>To

use region-tracking DSA, call the following overload of the <codeph>CDirectScreenAccess::NewL()</codeph> function

and pass <codeph>ETrue</codeph> as the final argument: </p> <p> <xref href="GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129.dita#GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129/GUID-6C26D5CD-0B55-3D30-B365-9C431BDD3721"><apiname>CDirectScreenAccess::NewL(RWsSession&amp;,CWsScreenDevice&amp;,RWindowBase&amp;,MDirectScreenAccess&amp;,TBool)</apiname></xref> </p> <p>This does not allocate a screen device and graphics context. It

therefore does not cause the DSA buffer to be allocated. </p> <p>Here is an

example that shows how to create a <xref href="GUID-24C7AE25-B44A-3B6F-AA05-EA90A8D36129.dita"><apiname>CDirectScreenAccess</apiname></xref> object

to track visible regions only: </p> <codeblock id="GUID-E378E1D5-C700-5AC6-AE3B-6CF9DA993671" xml:space="preserve">CDirectScreenAccess * cDsa = NULL;

// Specify additional parameter to NewL to indicate region tracking only.

cDsa = CDirectScreenAccess::NewL(TheClient->iWs,*TheClient->iScreen,*win,

           *this, ETrue); 

// In this mode, the client cannot get a screen device or graphics 

// context from the DSA object.

CFbsScreenDevice sd = cDsa->ScreenDevice();

CFbsBitGc gc = cDsa->Gc();

ASSERT(sd == NULL)

ASSERT(gc == NULL)

</codeblock> </section>

<section> <title>User-side HAL API changes</title> <p>The user-side HAL API

is a set of static functions that get and set the hardware attributes of the

device. For general information about the interface to HAL, see <xref href="GUID-9AE254D4-AA60-579E-8D9D-F2797106A413.dita">User-Side

Hardware Abstraction Technology</xref>. </p> <p>The <xref href="GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8.dita#GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8/GUID-573C49D6-7763-37AE-B2B2-4C8FB1327E21"><apiname>HAL::Get()</apiname></xref> function

is used to obtain information about the DSA buffer memory. </p> <codeblock id="GUID-A50C3B6C-364C-58B8-B5FC-6472C5C8FF09" xml:space="preserve">static IMPORT_C TInt Get(TAttribute aAttribute, TInt &amp;aValue); 

static IMPORT_C TInt Get(TInt aDeviceNumber, TAttribute aAttribute, 

     TInt &amp;aValue);</codeblock> <p>In versions earlier than S^3, in order

to obtain the memory address of the DSA buffer, a client calls <xref href="GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8.dita#GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8/GUID-573C49D6-7763-37AE-B2B2-4C8FB1327E21"><apiname>HAL::Get()</apiname></xref> with

an attribute of <xref href="GUID-8BE90160-2C60-3582-82C8-4A108C7C0317.dita#GUID-8BE90160-2C60-3582-82C8-4A108C7C0317/GUID-4702237B-185B-33B1-B0B6-DE388DBDE294"><apiname>HALData::EDisplayMemoryAddress</apiname></xref>. If successful,

the call returns <codeph>KErrNone</codeph> and sets the <codeph>aValue</codeph> parameter

to the virtual address of the DSA buffer in the caller’s process. If the DSA

buffer cannot be allocated, the call returns <codeph>KErrNoMemory</codeph> and

sets the <codeph>aValue</codeph> parameter to NULL. </p> <p>From S^3 onwards,

a client can obtain a handle to the DSA buffer by calling <xref href="GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8.dita#GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8/GUID-573C49D6-7763-37AE-B2B2-4C8FB1327E21"><apiname>HAL::Get()</apiname></xref> with

an attribute of <xref href="GUID-8BE90160-2C60-3582-82C8-4A108C7C0317.dita#GUID-8BE90160-2C60-3582-82C8-4A108C7C0317/GUID-6124E485-D8A6-3399-AA6C-7021D130591D"><apiname>HALData::EDisplayMemoryHandle</apiname></xref>. If successful,

the call returns <codeph>KErrNone</codeph> and sets the <codeph>aValue</codeph> parameter

to the value of an <xref href="GUID-326A2F4D-0E99-31C0-A35D-E8BF45913F07.dita"><apiname>RChunk</apiname></xref> handle in the caller’s process.

If the DSA buffer cannot be allocated, the call returns <codeph>KErrNoMemory</codeph> and

sets the <codeph>aValue</codeph> parameter to 0. </p> <p>The advantage of

using a handle is twofold: </p> <ul>

<li id="GUID-187317E0-0EED-5C8F-AB31-542D6EA0CBF2"><p>The memory chunk exists

for as long as the handle is open. </p> </li>

<li id="GUID-7984A11D-F7A2-5C1A-AB11-628911BCA292"><p>The handle is automatically

closed by the kernel if the process holding it exits prematurely. </p> </li>

</ul> <p>When possible, legacy clients should be changed to take advantage

of the new <xref href="GUID-8BE90160-2C60-3582-82C8-4A108C7C0317.dita#GUID-8BE90160-2C60-3582-82C8-4A108C7C0317/GUID-6124E485-D8A6-3399-AA6C-7021D130591D"><apiname>HALData::EDisplayMemoryHandle</apiname></xref> attribute. This

means that you need to modify <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> calls

to use <xref href="GUID-8BE90160-2C60-3582-82C8-4A108C7C0317.dita#GUID-8BE90160-2C60-3582-82C8-4A108C7C0317/GUID-6124E485-D8A6-3399-AA6C-7021D130591D"><apiname>HALData::EDisplayMemoryHandle</apiname></xref> to obtain a handle

to the DSA's shared chunk. You also need to add code to close this handle

when the memory is no longer required. </p> <p>For example, replace the following

code: </p> <codeblock id="GUID-DAB0136B-A5A9-5B15-9C88-1A37B04804AB" xml:space="preserve">err = HAL::Get(screenId, HALData::EDisplayMemoryAddress, dsaAddr);</codeblock> <p>With something like this: </p> <codeblock id="GUID-BD8049BB-378C-5C56-A19B-96227C71A325" xml:space="preserve">err = HAL::Get(screenId, HALData::EDisplayMemoryHandle, val);

if (err == KErrNone)

    {

    RChunk chunk;

    err = chunk.SetReturnedHandle(val);

    if (err == KErrNone)

        {

        iDSAChunk = chunk;

        err = iDSAChunk.Duplicate(RThread(), EOwnerProcess);

        chunk.Close();

        if (err == KErrNone)

            {

            dsaAddr = (TInt) iDSAChunk.Base();

            }

else if (err == KErrNotSupported)

    {

    err = HAL::Get(screenId, HALData::EDisplayMemoryAddress, dsaAddr);

    }

</codeblock> </section>

<section><title>Kernel-side HAL changes</title> <p>HAL requests are handled

on the kernel side by HAL handlers. A single HAL handler deals with all the

HAL attributes defined in a particular HAL group. Each HAL group has a one-to-one

relationship with a HAL handler. <codeph>EHalGroupDisplay</codeph> identifies

the group associated with the HAL display attributes. </p> <p>In S^3 the <xref href="GUID-BB011D9B-105F-345C-9FC0-39B0BA509394.dita"><apiname>TDisplayHalFunction</apiname></xref> enum

has two new attributes—<codeph>EDisplayHalGetDisplayMemoryAddress</codeph> and <codeph>EDisplayHalGetDisplayMemoryHandle</codeph>.

These allow the generic user-side HAL code to obtain the DSA buffer's address

and handle independently of all of the other display attributes. </p> </section>

<section><title>Handling HAL::Get(EDisplayMemoryAddress)</title> <p>In versions

earlier than S^3, the DSA buffer address is returned from the HAL handler

as part of a <xref href="GUID-C4712A78-6C58-39ED-AF84-11038DB8571D.dita"><apiname>TVideoInfoV01</apiname></xref> class in response to an <codeph>EDisplayHalCurrentModeInfo</codeph> request.

The user-side HAL code then extracts the address from the <xref href="GUID-C4712A78-6C58-39ED-AF84-11038DB8571D.dita"><apiname>TVideoInfoV01</apiname></xref> structure

and returns it. </p> <p><b>Configuration file setting </b> </p> <p>The HAL

configuration file <filepath>config.hcf</filepath> therefore includes the

following: </p> <codeblock id="GUID-F496B803-B7B8-57D4-B083-9219D4F849DB" xml:space="preserve">EDisplayMemoryAddress=ProcessDisplayCurrentModeInfo</codeblock> <p><b>Modifications

to generic HAL code </b> </p> <p>In S^3 the switch case for <codeph>EDisplayMemoryAddress</codeph> in <codeph>ProcessDisplayCurrentModeInfo()</codeph> is as follows: </p> <codeblock id="GUID-7E2FC07B-670D-57BA-BF3E-A4432BF83E81" xml:space="preserve">case HAL::EDisplayMemoryAddress:

// If the address is zero, the actual address

// is returned by EDisplayHalGetDisplayMemoryAddress

if (info.iVideoAddress == 0)  

    {

    r = UserSvr::HalFunction(EHalGroupDisplay, 

                             EDisplayHalGetDisplayMemoryAddress, 

                             aInOut, (TAny*)EFalse, aDeviceNumber);

    }

else

    {

    *(TInt*)aInOut = info.iVideoAddress;

    }

break;

</codeblock> <p><b>Dealing with HAL::GetAll() </b> </p> <p> <xref href="GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8.dita#GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8/GUID-EF03B832-E9AA-32CF-903F-DFFA63A3E9AB"><apiname>HAL::GetAll()</apiname></xref> sets

the input parameters passed to the HAL handler to -1. You can use this in

the LCD Driver to determine whether the call for the DSA buffer's address

is from <codeph>HAL::GetAll()</codeph> and if it is, to not allocate the memory.

For example: </p> <codeblock id="GUID-8734B0EF-A2A2-5929-972F-56F7E0195940" xml:space="preserve">case EDisplayHalGetDisplayMemoryAddress:

    {

    TInt val = 0;

    TInt passedIn = 0;

    kumemget32(&passedIn, a1, sizeof(TInt));

    // Not from a GetAll().

    if (passedIn != -1)    

        {

        r = DisplayMemoryAddress(aDeviceNumber, val);

        }

    else

        {

        r = KErrNone;

        }

    // Write the value returned back to the user side.

    kumemput32(a1, &val, sizeof(TInt));

    }

    break;

</codeblock> <p><b>Keeping track of processes that call HAL::Get(EDisplayMemoryAddress) </b> </p> <p>Legacy

DSA clients request the DSA buffer address rather than the handle and assume

that the address will remain valid for the lifetime of the process. The LCD

Driver needs to keep track of processes that call <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> and

free the DSA buffer when the last of these processes has exited. </p> <p>The

LCD Driver must therefore maintain a list of the relevant processes and the

address of the DSA buffer for each one. When the number of processes using

a buffer address returns to zero, the buffer is freed. </p> <p>To track the

usage, the LCD Driver creates a new shared chunk for each process that calls <codeph>HAL::Get(EDisplayMemoryAddress)</codeph>.

The LCD Driver then calls <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-BA56B205-60D8-326E-8340-3F78CCEA3DD1"><apiname>Kern::MakeHandleAndOpen()</apiname></xref> to

create a user handle to the shared chunk for the calling process. The LCD

Driver closes the kernel-side handle to the chunk. </p> <p>There is then a

handle to the shared chunk in the user process, which means that the shared

chunk stays open for as long as the process exists. When the user process

exits, the kernel closes any open handles that the process owns. This causes

the deletion of the shared chunk and its Deferred Function Call (DFC) to run.

The DFC removes the process from the list and decrements the reference count

of users of the DSA memory. </p> <p>Here is the sequence when a process calls <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> for

the first time: </p> <ol id="GUID-DA7B9F91-6F52-532C-8E67-8DE89B1EFD42">

<li id="GUID-2055954D-2B13-5307-93A6-78936172416C"><p>Legacy process <i>X</i> implicitly

calls <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> for the first time

by creating a DSA session that creates a <xref href="GUID-B229156F-2344-3F46-8542-AC65882D80DE.dita"><apiname>CFbsScreenDevice</apiname></xref>. </p> </li>

<li id="GUID-33A00E2F-B04C-51CE-9993-661DFF5BE00B"><p>The driver looks for

process <i>X</i> in the list of processes and addresses and does not find

it. </p> </li>

<li id="GUID-A8E33530-C089-5C85-9563-A39BBA9259F2"><p>The driver creates a

shared chunk for process <i>X</i> with a destruction DFC. </p> </li>

<li id="GUID-19CDF330-315D-5806-AB3D-9AB8857A02A4"><p>The driver creates a

user-side handle to the shared chunk. </p> </li>

<li id="GUID-E13883F3-6418-5920-A16A-8327744E6AF1"><p>The driver closes its <xref href="GUID-85454082-6734-3F1D-983F-734D4C2AB12D.dita"><apiname>DChunk</apiname></xref> handle

to the shared chunk. </p> </li>

<li id="GUID-8CF22900-AD3C-528D-B88A-74AC59BAAA20"><p>The driver calls <xref href="GUID-AC098719-7AD6-3B48-9BE3-7DCEADA3530B.dita#GUID-AC098719-7AD6-3B48-9BE3-7DCEADA3530B/GUID-0632CC60-AF0C-36F5-8E8F-3D8AD2568E31"><apiname>TUint8*

Kern::ChunkUserBase(DChunk*                 aChunk, DThread* aThread)</apiname></xref> to

get the address of the shared chunk in the user process . </p> </li>

<li id="GUID-0180A075-BF6B-53E4-8B6F-38BE083C58DA"><p>The driver stores the

calling process's ID and the address of the shared chunk in a table. </p> </li>

<li id="GUID-3663A069-09FA-55DF-9EA7-F1875B379569"><p>The driver returns the

address of the shared chunk to the calling process. </p> </li>

</ol> <p>Here is the sequence when the same process calls <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> again: </p> <ol id="GUID-E7D5E120-364C-5CA6-BE4E-56563F58A293">

<li id="GUID-57947C06-0BA1-5CB0-AB3F-493D71510A95"><p>Legacy process <i>X</i> implicitly

calls <codeph>HAL::Get(EDisplayMemoryAddress)</codeph> again by creating a

DSA session that creates a <xref href="GUID-B229156F-2344-3F46-8542-AC65882D80DE.dita"><apiname>CFbsScreenDevice</apiname></xref>. </p> </li>

<li id="GUID-493E5ABC-5032-5653-A87B-27441F0AF88E"><p>The driver looks for

the process in the list of processes and addresses. </p> </li>

<li id="GUID-6CF1A43C-966E-59EF-BA68-BB1D5CA5A4F0"><p>The driver returns the

address previously given to the process. </p> </li>

</ol> <p>Here is the sequence when the same process exits: </p> <ol id="GUID-388DC71E-FD9B-5A56-8B3D-520159FA8939">

<li id="GUID-B95DB340-C730-5419-ADB8-0B5B10F46556"><p>The kernel cleans up

any open handles for the exiting process. </p> </li>

<li id="GUID-E8654886-77E4-5748-A0BE-A0E3CCCC752A"><p>The shared chunk destruction

DFC executes and removes the process from the list and decrements the DSA

usage count. </p> </li>

<li id="GUID-68624448-1DCF-591E-97EF-F0B46F656FC8"><p>The DFC checks the DSA

usage count and if it is zero, the DFC frees the DSA memory. </p> </li>

</ol> </section>

<section><title>Handling HAL::Get(EDisplayMemoryHandle)</title> <p>In S^3

there is a new <codeph>GetDisplayMemoryHandle()</codeph> function in the user-side

HAL implementation. This function is the handler for the new <codeph>EDisplayMemoryHandle</codeph> attribute. </p> <p><b>Configuration

file setting </b> </p> <p>The following line therefore needs to be added to

the <filepath>config.hcf</filepath> file: </p> <codeblock id="GUID-90AEC511-D592-5755-A569-7703A09EE40C" xml:space="preserve">EDisplayMemoryHandle=GetDisplayMemoryHandle</codeblock> <p><b>Dealing with HAL::GetAll() </b> </p> <p> <xref href="GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8.dita#GUID-BD00E7FC-C234-3111-87A5-10F79EB0F2B8/GUID-EF03B832-E9AA-32CF-903F-DFFA63A3E9AB"><apiname>HAL::GetAll()</apiname></xref> sets

the input parameters passed to the HAL handler to -1. You can use this in

the LCD Driver to determine whether the call for the DSA buffer's address

handle is from <codeph>HAL::GetAll()</codeph> and to take appropriate action.

For example: </p> <codeblock id="GUID-22A22111-1B29-59F4-B3F7-83E43077E0FD" xml:space="preserve">case EDisplayHalGetDisplayMemoryHandle:

    {

    TInt val = 0;

    TInt passedIn = 0;

    kumemget32(&passedIn, a1, sizeof(TInt));

    // Not from a GetAll().

    if (passedIn != -1)    

        {

        r = CreateNewChunkToRepresentDSABuffer(aDeviceNumber,  val);

        }

    else

        {

        r = KErrNone;

        }

    kumemput32(a1, &val, sizeof(TInt));

    }

    break;

</codeblock> <p><b>Keeping track of processes that call HAL::Get(EDisplayMemoryHandle) </b> </p> <p>New

DSA clients request the DSA buffer handle rather than the address. After a

process obtains the handle, the DSA buffer is in use for this process until

the handle is closed or the process exits (which implicitly closes the handle). </p> <p>To

track the usage, the LCD Driver creates a new shared chunk every time a process

calls <codeph>HAL::Get(EDisplayMemoryHandle)</codeph>. The LCD Driver then

calls <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-BA56B205-60D8-326E-8340-3F78CCEA3DD1"><apiname>Kern::MakeHandleAndOpen()</apiname></xref> to create a user handle

to the shared chunk for the calling process. The LCD Driver closes the kernel-side

handle to the shared chunk and returns the user-side handle to the calling

process. </p> <p>When the user process closes the handle, the shared chunk

is deleted and its deletion DFC runs. The DFC decrements the reference count

of users of the DSA memory. </p> <p>Here is the sequence when a process calls <codeph>HAL::Get(EDisplayMemoryHandle)</codeph> for

the first time: </p> <ol id="GUID-A92B09E9-5E76-5B6C-B281-64A4F4A65467">

<li id="GUID-0CF94725-F9BA-5C92-9429-4206D38040CF"><p>The driver creates a

new shared chunk for this process with a destruction DFC. </p> </li>

<li id="GUID-8521F99F-A890-5F82-9EB0-C0C1E9ABDE5B"><p>The driver creates a

user-side handle to the shared chunk. </p> </li>

<li id="GUID-876EECF6-BC1C-5E57-8555-55D3F5AD01E3"><p>The driver closes its <xref href="GUID-85454082-6734-3F1D-983F-734D4C2AB12D.dita"><apiname>DChunk</apiname></xref> handle

to the shared chunk. </p> </li>

<li id="GUID-F705B1B9-F9F4-56F2-800B-93E4B8FBE75B"><p>The driver returns the

user-side handle to the calling process. </p> </li>

</ol> <p>Here is the sequence when a process closes the DSA buffer: </p> <ol id="GUID-2310D2DC-76BA-5B87-A964-DCA01051CB3E">

<li id="GUID-65CE3E41-D418-59C5-A0FA-A90195A7FE23"><p>The shared chunk destruction

DFC executes and decrements the DSA usage count. </p> </li>

<li id="GUID-96386F96-FF1E-5BD3-B98A-E0B180463AFB"><p>The DFC checks the DSA

usage count and if it is zero, the DFC frees the DSA memory. </p> </li>

</ol> </section>

</conbody><related-links>

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