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MMC_sync was especially dangerous, as some cards don't like being selected and immediately deselected w/o any transmitted data. ref: https://dmitry.gr/?r=05.Projects&proj=36.%208pinLinux#_TOC_e23710bba70063c2e153d50ddbe4a51c
As per Part 1 of the SD Spec, the initial clock can be anything from 100 to 400KHz. Setting it to 400KHz may slightly improve the bootup time, and there are no downsides.
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Some minor improvements to the SD handling code, which should drastically improve the performance (~23% faster on read, ~10% faster on write; measured through USB mass storage mode with
dd conv=fsyncon a plenty fast SanDisk 8GB SDHC card.results may vary, no guarantees :p
Biggest bottleneck seems to be USB, but I bet we could squeeze 20-40% more performance on both read and write if we migrated away from the blocking SPI calls. Increasing the SD clock beyond the default 20MHz didn't make a significant difference, which leads me to believe that the majority of the time is wasted on copying data with blocking calls.