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 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 driver. Thus, the ataide_buf.bufstruct.av_forw pointer must be null when given to the IDE adapter driver, which indicates that this is the only request. The IDE adapter driver does not support queuing multiple requests to the same device.
Some kernel operations might be composed of sequential operations to a device. For example, if consecutive blocks are written to disk, blocks might or might 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 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 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, because the IDE adapter 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 driver's maximum allowable transfer size. If a transfer size larger than the supported maximum is attempted, the IDE adapter 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 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.
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 driver uses only the information in the ataide_buf structure to prepare for the DMA operation.