fixes for boot and (single-block, cmd17-only) Linux driver #820
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This allows the Linux driver in single-block mode (cmd17-only) to operate solidly, without running into timeouts from LiteSDCard FSMs. FIXME: multi-block (cmd18) transfers still time out, so revisit this after some additional debugging.
When the system/bus clock frequency is an exact power-of-2 multiple of the desired sdcard frequency, we can drive the latter at the "maximum" speed via the "perfect" divider. That sometimes turns out too fast, so in order to be conservative, we double the divider, thus halving the resulting sdclock.
enjoy-digital
approved these changes
Feb 15, 2021
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Thanks @gsomlo, this looks fine and will indeed help stabilizing the Linux driver.
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With the current sdcard clocking, when building a rocket-litex system at exactly 50 MHz, when 12.5 or 25 MHz are requested, it is possible to obtain those frequencies by using an exact power-of-two divisor for the sdclock (4 and 2, respectively). However, when driven at the full 25 MHz during
sdcardboot, data transfer from the sdcard hangs. The first patch adds an additional halving of the sdclock frequency to ensure the sdclock frequency is never too fast forsdcardboot.The second patch allows the current single-block-at-a-time (cmd17-only) Linux driver to avoid running into command timeouts when e.g.,
dd-ing a large file to/dev/nullfrom the mounted sdcard. This doesn't help enable multi-block (cmd18) transfers without timeouts, but I will open a separate issue to keep track of those debugging efforts.