[libre-riscv-dev] System-Wide Complexity Rant (was store computation unit)

Samuel Falvo II sam.falvo at gmail.com
Thu Jun 6 21:42:46 BST 2019

On Wed, Jun 5, 2019 at 7:09 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>  yyeah except the DRAM controller even changes the impedance and the
> timings at both ends.  what settings do you use?   DDR4 even starts to
> take temperature fluctuations into account, the speeds are getting so
> high.

I remember seeing not long ago a Xeon-based PC where the BIOS has a
configuration setting for the bloody altitude above sea level.  That
should be a clear sign that things are getting a wee bit ridiculous.

>  SPI mode is single-bit bi-directional and does not involve any of the
> voltage / current negotiation, nor does it involve going about
> 25mbit/sec.

In my personal experience trying to get SDXC and SDHC cards
working[1], it looks like even in SPI mode you have to properly
soft-configure voltage/current ratings.  :(

>  you can usually just go straight into an extremely simple SD/MMC

Simple and SD/MMC in the same sentence has been known to cause black
holes.  Be careful with those quantum singularities!  :P

In all seriousness, I've very much given up on ever wanting to write
software for the SD/MMC protocol again.  Once the card is initialized,
it seems to be fairly simple to use.[2]

1.  I never did, by the way; I've never succeeded in successfully
initializing anything /except/ plain vanilla SD cards, even after
studying other sources online which ostensibly work with different
types of SD cards and different brands thereof (I make sure to only
use SanDisk when developing.  This is why the Kestrel-3 no longer has
an integrated SD card interface.  Instead, it defers all mass storage
needs to external, intelligent storage devices over a well-known
interconnect instead (currently one of my two SIAs, driving an
RS232-to-USB interface for processing by my host/development PC;
longer term, I've been thinking about replacing that with a
RapidIO-inspired interconnect).

2.  However, my experience writing the storage driver for DX-Forth on
my Kestrel-2DX computer shows that /writing/ blocks to the SD card
requires that I completely re-initialize the card from scratch after
every block written.

Samuel A. Falvo II

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