[Bug 1580] New: spi-sd-card: use bigger chunks and yield processor while waiting for read data

[bugzilla-daemon at rtems.org]

https://www.rtems.org/bugzilla/show_bug.cgi?id=1580

           Summary: spi-sd-card: use bigger chunks and yield processor
                    while waiting for read data
           Product: RTEMS
           Version: HEAD
          Platform: All
        OS/Version: RTEMS
            Status: NEW
          Severity: enhancement
          Priority: P3
         Component: misc
        AssignedTo: sebastian.huber at embedded-brains.de
        ReportedBy: arnout at mind.be
                CC: joel.sherrill at oarcorp.com
        Depends on: 1567


Created an attachment (id=811)
 --> (https://www.rtems.org/bugzilla/attachment.cgi?id=811)
spi-sd-card.c: do read polling in bigger blocks and yield the CPU

The function sd_card_read() polls the SPI bus until the data is available.  On
various SD cards, I've seen latencies between 5 and 200 bytes.

First of all, reading in chunks of 18 bytes is not efficient since the DMA of
the SPI remains under-used.

Second, while we wait, other tasks should be allowed to take over.

Attached patch uses bigger chunks while waiting for the read data and yields
the processor.  It depends on attachment 792 to bug 1567.

Note that sleeping like in attachment 810 to bug 1579 is not efficient since
the waiting time tends to be far below 1 tick.  Sleeping reduces the read
throughput significantly.

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