flash read performance

[Trent Piepho]

On Fri, 7 Nov 2008, Jamie Lokier wrote:
> Based on Andre's observation, I will soon try enabling cache for my
> NOR, and see if it makes a difference to cold-cache read performance.
> I don't expect it, but it's worth a try.

It possible it could help by efficiently doing the reads back-to-back with
no wasted cycles between them.  I think it's necessary if you want to
benefit from page mode, but that's something I haven't tried yet.

> You might also find the write operation to be unreliable, if the
> caching mode is write-back rather than write-through.

Proper use of "sync" instructions, or whatever the arch uses to insure that
writel() is strictly ordered should fix that.

> Really, you should use an uncached mapping to write commands to the
> flash, flush the cached mapping (for reads) when commands are written,
> and prevent any access during the writes (this is in MTD normally).
> You could optimise by flushing only the cached read regions which are
> affected by write and erase commands.

Yes, that is probably the best.  Most NOR flash writing is so slow that the
cache flushes shouldn't be too expensive.

>> Mapping				Speed (MB/sec) (MB = 1048576 bytes)
>> un-cached and guarded		12.30
>> cached and gaurded		14.24
>> cached and un-guarded		14.31
>> un-cached and un-guarded	14.66
>>
>> I measured by reading flash linearly from beginning to end 32-bits at a time.
>> Since the flash is bigger than the cache, ever read should have come from the
>> flash.  If I just read the same 1k over and over that would obviously be much
>> faster if it could come from the cache.
>
> That's nice to see that cache helps cold-read performance too, not
> just cached reads.  Thanks :-)

Though if the mapping is not in guarded mode, turning cache on hurts
performance.  That surprises me.

>> The biggest bootup delay I have now is waiting for the ethernet phy to get
>> online, which takes almost 3 seconds.
>
> Do you need to delay everything else for that, or can you parallelise?

It's parallel.  I start the phy very early in the boot loader, and linux is
done booting and sitting in userspace waiting for it for a few hundred ms
before it's ready.