Re: does beaglebone black device tree need to specify amount of eMMC flash?

[Alexander Aring <alex.aring@gmail.com>]

Hi Lucas,

On Mon, Jul 07, 2014 at 10:53:20AM +0200, Lucas Stach wrote:
> This has nothing to do with barebox, but I feel this needs an answer as
> a lot of misinformation is spread here.
> 

Thanks for your answer.

Yes, it's off-topic but I always feel bad when I heard "eMMC".

> Am Montag, den 07.07.2014, 09:06 +0200 schrieb Alexander Aring:
> [...]
> > 
> > btw.: that's why eMMC is evil.
> > 
> > Raw-Flash:
> > 
> > Disadvantage:
> >  - you can't replace it.
> > 
> > Advantage:
> >  - no mcu in the middle, access the raw Flash.
> 
> This isn't an advantage. If your not working for the NAND flash
> manufacturer you will have an extremely hard time getting the wear
> leveling parameters right. Having this abstracted behind an MCU that
> actually know about the flash chip behind it is a good thing.
> 

yes, but I think that a mtd filesystem can do a better scheduling of
erase/write/read cycles than the integrated mcu with an abstracted block
device.

I need to test it myself, to see what the mcu exactly do and this
depends on manufacturer.

> > 
> > 
> > - MMC/SD:
> > 
> > Disadvantage:
> >  - mcu in the middle, abstract block device. OS doesn't know about this.
> 
> No disadvantage, see above.
> 
> > 
> > Advantage:
> >  - you can replace it.
> > 
> > 
> > Combines these Disadvantage and Advantage you will get:
> > 
> > Disadvantage:
> >  - mcu in the middle, abstract block device. OS doesn't know about this.
> >  - you can't replace it.
> > 
> > Advantage:
> >  - maybe a little bit cheaper...
> >  - maybe avoid some bad connections (never expired by using sd cards)
> > 
> You are neglecting the fact that the eMMC interface can be driven with a
> lot higher clock speeds compared to an SD card. Also most eMMCs have an
> interface width of 8 bits, which is double the SDs 4 bit.
> 

okay, I didn't know that. Does barebox use the 8 bit interface at the
moment?

> This amounts to a lot more raw speed on the interface side and most
> eMMCs are actually capable of supplying data at those rates.
> 
> Also eMMC provides some really useful additional features like the boot
> partitions and health checks.
> 
> While SD cards may be convenient for the casual hobbyist user when it
> comes to real embedded devices, where speed and reliability matters,
> eMMC has a huge lead.
> 
> Raw NAND is only an option if your device manufacturing runs are big
> enough that the lower price for NAND stacks up enough to make up the
> additional development time (cost) you need to get things right. Note
> there is a big difference here between getting it working and getting it
> right.
> 

So now I have the question about "Why they don't make a new sd/mmc card
holder standard and sells replaceable cards". I could say that to every
electronic device, but maybe it's better that there is no new standard.
:-)

- Alex

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