ucb-bar/chipyard

Try to speed up VCU118 SDCard loading

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#854 opened on Apr 14, 2021

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enhancementhelp wanted

Description

Impact: software

Description In order to get SDCard loading on the VCU118 working, we need to have a slow SPI divisor.

https://github.com/ucb-bar/chipyard/blob/45684f1b875f018a1945476cec9269068e1618ca/fpga/src/main/resources/vcu118/sdboot/sd.c#L175

However, it looks like the Freedom platform uses a faster div.

https://github.com/sifive/freedom/blob/b348a50d5d10ffa94ab2a07a33622b1c634a01ea/bootrom/sdboot/sd.c#L169

Why does this div not work? Can we find a middle ground?

What is a motivating example for changing the behavior?

Speed up Linux loading!

Contributor guide

Tech stack
c
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
C programmingSPI protocolRISC V boot
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Compare the SPI divisor configuration in the two referenced sd.c files (VCU118 vs Freedom platform). Analyze the timing constraints of the SD card and the VCU118 SPI controller. Experiment with intermediate divisor values, possibly by adjusting the delay loops or clock gating. Consider measuring actual SPI clock frequency on the hardware to validate.