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<concept id="GUID-FBE6E61F-5A2E-5A7A-BC59-51F072EBED7D" xml:lang="en"><title>Media
Driver Migration Guide</title><shortdesc>Describes the issues that need to be considered, when migrating
media drivers to a writable data demand paging environment. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-AB10DA2D-B7CC-47FD-B68A-1610B6BEAAE6"><title>Purpose</title> <p>This
document explains the points that have to be considered when migrating existing
media drivers to a writable data paging environment. </p>  </section>
<section id="GUID-EB26A48A-3A34-44E8-9651-C7ED658CED51"><title>Issues Involved</title> <p>The
two main issues that have to be addressed when migrating existing media drivers
are: </p> <ul>
<li id="GUID-68A44C67-6D4E-5D7E-A13B-0A82154073B2"><p>A deadlock condition
can occur between the driver and the client process, where both are trying
to access the same page of paged memory</p> </li>
<li><p>A deadlock condition can occur between the driver and the client process,
where the client is allocating on the kernel heap, the paging system tries
to page out some data and the media driver also tries to allocate on the kernel
heap while servicing the resulting request.</p></li>
<li id="GUID-56A3815D-E07B-5195-85C3-0B93C08745C3"><p>Unpredictable delays
in servicing a DFC can occur, since memory might need to be paged in.</p> </li>
</ul> <p>To address the above issues, the following points will have to be
considered/undertaken: </p> <ul>
<li id="GUID-A15DB387-AB22-51BB-A4AD-0353932B788C"><p>Pass data by value to
DoRequest/DoControl </p> </li>
<li id="GUID-0FB5B141-94FE-54E4-9915-2DD50F840879"><p>Return results by using
a return code </p> </li>
<li id="GUID-9B3D3D25-010E-58C3-9C48-13AF494C3E62"><p>Use of a dedicated DFC
queue </p> </li>
<li id="GUID-AADA0BD7-75E7-5A11-98EB-F7EF3E1B5139"><p>Determine if unpredictable
delays in servicing DFCs are acceptable </p> </li>
<li id="GUID-E5B45EC9-DF21-51A9-814C-17CF3C13DFAE"><p>Validate the arguments
in the client context as far as possible </p> </li>
<li id="GUID-E6E4F7E2-1BE4-5590-AB05-777637C9B32B"><p>Move user memory accesses
to client context where possible </p> </li>
<li id="GUID-F4B44B27-9AE6-5A8C-B88C-C1809DAF6B89"><p>Replace the use of small
fixed-size structures with the use of the TClientDataRequest object </p> </li>
<li id="GUID-990D43F2-21C3-5C55-AC13-89C7153EA83B"><p>Pin user memory accessed
from DFCs </p><p>Do not allocate on the kernel heap while handling page out
requests.</p> <p>The paging algorithm can be over ridden and pages of memory
can be kept in memory (pinned). This situation can be reversed by 'unpinning'
the page. The TClientBufferRequest API provides this functionality. </p> </li>
<li id="GUID-67DB522A-DEB2-5FEA-ACA4-D80D09D80926"><p>Re-write the driver
to use shared chunks </p> </li>
<li id="GUID-A298C045-2260-55B5-89B9-595E3AF5472F"><p>No fast mutex may be
held when calling APIs that access user memory </p> </li>
<li id="GUID-F9F79078-0CBC-5E8F-899F-117D148C605A"><p>No kernel mutex may
be held </p> </li>
<li id="GUID-D378D276-CAF0-5B8B-BDA2-4D18EB87F96A"><p>Use the new APIs, (See
the <xref href="GUID-592B6D20-4ABA-5C79-9734-D18E2CE4A86C.dita">Writable Data Paging
Overview</xref> ). </p> </li>
</ul> </section>
<section id="GUID-32E564F3-D66A-472B-A3C1-D2D61807A5F2"><title>See
also</title> <ul>
<li id="GUID-77417650-C1F4-5ABC-9E75-54D274556FCD"><p> <xref href="GUID-E7C55048-5B7A-5BF2-B7F4-4D731659B88C.dita">Device
Driver Writing and Migration Guide</xref>  </p> <p>This document explains
techniques for writing device drivers on data paged systems and migrating
device drivers to data paged systems. </p> </li>
</ul> </section>
</conbody></concept>