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AIX Version 4.3 Kernel Extensions and Device Support Programming Concepts

Execution of I/O Requests

During normal processing, many transactions are queued in the IDE device driver. As the IDE device driver processes these transactions and passes them to the IDE adapter device driver, the IDE device driver moves them to the in-process queue. When the IDE adapter device driver returns through the iodone service with one of these transactions, the IDE device driver either recovers any errors on the transaction or returns using the iodone kernel service to the calling level.

The IDE device driver can send only one ataide_buf structure per call to the IDE adapter device driver. Thus, the ataide_buf.bufstruct.av_forw pointer should be null when given to the IDE adapter device driver, which indicates that this is the only request. The IDE adapter driver does not support queuing multiple requests to the same device.

Spanned (Consolidated) Commands

Some kernel operations may be composed of sequential operations to a device. For example, if consecutive blocks are written to disk, blocks may or may not be in physically consecutive buffer pool blocks.

To enhance IDE bus performance, the IDE device driver should consolidate multiple queued requests when possible into a single IDE command. To allow the IDE adapter device driver the ability to handle the scatter and gather operations required, the ataide_buf.bp should always point to the first buf structure entry for the spanned transaction. A null-terminated list of additional struct buf entries should be chained from the first field through the buf.av_forw field to give the IDE adapter device driver enough information to perform the DMA scatter and gather operations required. This information must include at least the buffer's starting address, length, and cross-memory descriptor.

The spanned requests should always be for requests in either the read or write direction but not both, since the IDE adapter device driver must be given a single IDE command to handle the requests. The spanned request should always consist of complete I/O requests (including the additional struct buf entries). The IDE device driver should not attempt to use partial requests to reach the maximum transfer size.

The maximum transfer size is actually adapter-dependent. The IOCINFO ioctl operation can be used to discover the IDE adapter device driver's maximum allowable transfer size. To ease the design, implementation, and testing of components that may need to interact with multiple IDE-adapter device drivers, a required minimum size has been established that all IDE adapter device drivers must be capable of supporting. The value of this minimum/maximum transfer size is defined as the following value in the /usr/include/sys/ide.h file:

IDE_MAXREQUEST /* maximum transfer request for a single IDE command (in bytes) */

If a transfer size larger than the supported maximum is attempted, the IDE adapter device driver returns a value of EINVAL in the ataide_buf.bufstruct.b_error field.

Due to system hardware requirements, the IDE device driver must consolidate only commands that are memory page-aligned at both their starting and ending addresses. Specifically, this applies to the consolidation of inner memory buffers. The ending address of the first buffer and the starting address of all subsequent buffers should be memory page-aligned. However, the starting address of the first memory buffer and the ending address of the last do not need to be aligned.

The purpose of consolidating transactions is to decrease the number of IDE commands and bus phases required to perform the required operation. The time required to maintain the simple chain of buf structure entries is significantly less than the overhead of multiple (even two) IDE bus transactions.

Fragmented Commands

Single I/O requests larger than the maximum transfer size must be divided into smaller requests by the IDE device driver. For calls to an IDE device driver's character I/O (read/write) entry points, the uphysio kernel service can be used to break up these requests. For a fragmented command such as this, the ataide_buf.bp field should be NULL so that the IDE adapter device driver uses only the information in the ataide_buf structure to prepare for the DMA operation.

Gathered Write Commands

The gathered write commands facilitate communications applications that are required to send header and trailer messages with data buffers. These headers and trailers are typically the same or similar for each transfer. Therefore, there may be a single copy of these messages but multiple data buffers.

The gathered write commands, accessed through the ataide_buf.sg_ptr field, differ from the spanned commands, accessed through the ataide_buf.bp field, in several ways:

To execute a gathered write command, the IDE device driver must:

If any of these conditions are not met, the gather write commands do not succeed and the ataide_buf.bufstruct.b_error is set to EINVAL.

This interface allows the IDE adapter device driver to perform the gathered write commands in both software or hardware as long as the adapter supports this capability. Because the gathered write commands can be performed in software (by using such kernel services as uiomove), the contents of the sg_ptr field and the uio struct can be altered. Therefore, the caller must restore the contents of both the sg_ptr field and the uio struct before attempting a retry. Also, the retry must occur from the process level; it must not be performed from the caller's iodone subroutine.

To support IDE adapter device drivers that perform the gathered write commands in software, additional return values in the ataide_buf.bufstruct.b_error field are possible when gathered write commands are unsuccessful.

ENOMEM Error due to lack of system memory to perform copy.
EFAULT Error due to memory copy problem.
Note: The gathered write command facility is optional for both the IDE device driver and the IDE adapter device driver. Attempting a gathered write command to a IDE adapter device driver that does not support gathered write can cause a system crash. Therefore, any IDE device driver must issue an IDEIOGTHW ioctl operation to the IDE adapter device driver before using gathered writes. An IDE adapter device driver that supports gathered writes must support the IDEIOGTHW ioctl as well. The ioctl returns a successful return code if gathered writes are supported. If the ioctl fails, the IDE device driver must not attempt a gathered write. Typically, an IDE device driver places the IDEIOGTHW call in its open routine for device instances that it will send gathered writes to.

Related Information

Execution of I/O Requests.

IDE Error Recovery.

ataide_buf Structure.

ataide_buf Structure.

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