Re: [v5 PATCH 7/7] crypto: stm32 - Save and restore between each request

[Linus Walleij]

On Thu, Mar 9, 2023 at 11:19 PM David Laight <David.Laight@xxxxxxxxxx> wrote:

> > But actually I think the bug will never trigger, because the datasheet
> > for the DB8500 (Ux500) says this:
> >
> > "Then the message can be sent, by writing it word per word into the
> > HASH_DIN register.
> > When a block of 512 bits, i.e. 16 words have been written, a partial
> > digest computation will
> > start upon writing the first data of the next block. The AHB bus will
> > be busy for 82 cycles for
> > SHA-1 algorithm (66 cycles for SHA-256 algorithm)."
>
> What speed clock is that?

133 MHz.

> 4 or 5 extra clocks/word may (or may not) be significant.
>
> In terms of latency it may be noise compared to some PCIe
> reads done by hardware interrupt handlers.
> Some slow PCIe targets (like the fpga one we use) pretty
> much take 1us to handle a read cycle.

So in this case it's 1/133M s = 8ns cycle time, 82 in worst case,
so 82*8 = 656 ns < 1 us.

Yours,
Linus Walleij