[Cryptech Tech] keystore flash performance

[Joachim Strömbergson]

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Aloha!

Wow, pretty impressive improvement.


Paul Selkirk wrote:
> In the course of reviewing Rob's new keystore architecture (ksng 
> branches in the libhal, pkcs11, and stm32 repos), I've been looking
> at performance of the SPI flash chip. In short, most of the poor 
> performance is self-inflicted.
> 
> As originally coded, there were 1ms delays after every SPI operation 
> (both transmit and receive), and an additional 10ms delay in the 
> _wait_while_wip() loop. I was able to significantly speed up flash 
> operations just by removing these delays, with no loss of stability
> or functionality.
> 
> I'm not an expert, and I haven't fully re-read the 84 page data
> sheet for the chip, but looking at the timing diagrams, there doesn't
> seem to be any need for a delay between transmitting the command and
> receiving the result, or for a delay before de-selecting the chip,
> especially since the low-level SPI code has its own timeout
> mechanism. Maybe Fredrik can comment?
> 
> For my test program, I read the entire flash chip, erase it in
> different ways, read again to verify erasure, write out a pattern,
> and read again to verify the write. Results are shown.
> 
> Before:
> 
> read page       131.072 sec for 65536 rounds, 2.00 ms each erase
> subsector 290.548 sec for 4096 rounds, 70.93 ms each erase sector
> 68.652 sec for 256 rounds, 268.17 ms each erase bulk      45.907 sec 
> verify erase    131.072 sec for 65536 rounds, 2.00 ms each write page
> 1048.576 sec for 65536 rounds, 16.00 ms each verify write    131.072
> sec for 65536 rounds, 2.00 ms each
> 
> After removing delays (with speed-up factor):
> 
> read page       15.681 sec for 65536 rounds, 0.23 ms each       8.4 
> erase subsector 238.017 sec for 4096 rounds, 58.10 ms each      1.2 
> erase sector    66.339 sec for 256 rounds, 259.13 ms each       1.0 
> erase bulk      45.907 sec                                      1.0 
> verify erase    16.073 sec for 65536 rounds, 0.24 ms each       8.2 
> write page      24.327 sec for 65536 rounds, 0.37 ms each       43.1 
> verify write    16.073 sec for 65536 rounds, 0.24 ms each       8.2
> 
> How did writing 16MB go from 17.5 min to 24 sec? It turns out that 
> almost all the time was spent in delays. The sequence of events is
> like this:
> 
> send WRITE_ENABLE, delay 1ms send READ_STATUS (write enable), delay
> 1ms send PROGRAM_PAGE, delay 1ms send data, delay 1ms send
> READ_STATUS (write in progress), delay 1ms WIP flag is asserted, so
> delay 10ms send READ_STATUS (write in progress), delay 1ms
> 
> Erase shows more modest gains, because each operation takes a lot 
> longer, so the delays are a smaller percentage. There is also some 
> jitter in the erase timing, so there's only a 9ms difference in the 
> sector erase in these examples.
> 
> paul _______________________________________________ Tech mailing
> list Tech at cryptech.is https://lists.cryptech.is/listinfo/tech


- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
========================================================================
 Joachim Strömbergson          Secworks AB          joachim at secworks.se
========================================================================
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