Hi Glen,

Am 18.09.2016 um 05:22 schrieb glen english:
> Hi Danilo
>
> Yes, there is most-certainly a penalty for const access from flash, at
> least on the M4.
>
> and of course instruction cache is no use, is just that, only for
> instructions
Hence the name :-)
> I wonder what the bus matrix penalty is for data fetches from flash.
> There is an app-note about it somewhere I once read. It depends on what
> else is going on. The processor is pretty smart and interleaving the
> accesses as not to stall the pipeline or bus matrix.
>
> As Danilo I am sure you know : (pointed out for others)
> You can force variables into sections (like forcing a static const ) by
> using
>
> __attribute__((section("name"))) to assign say into data.
Yes,  indeed. We use that extensively at the mcHF to move certain parts 
to the CCM memory which is otherwise not so easily accessible (and 
should be used with care as it does not support DMA to/from peripherals).
Works great.
> I will some time run up the code on an M4 and see what kiss-fft does.
>
> I am very very very surprised , and do not really believe that kissFFT
> is as fast as the arm assembler  on the M4 -  my immediate thoughts are
> "you are doing it wrong". so, I will investigate.
Would be happy if I am wrong. Which would mean we get even faster fft on 
the M4 almost for free (minus investigation time that is). No problem at 
all with me :-)


Danilo



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