RE: [ECOS] USB MSD stack

["Doyle, Patrick" <Patrick_Doyle at dtccom dot com>]

> -----Original Message-----
> From: Andrew Lunn [mailto:andrew@lunn.ch] 
> 
> Arn't these commands actually SCSI commands? And i assume these
> commands are for read block/write block. How are you thinking about
> mapping this onto a disk in eCos? How do you think you will solve the
> concurency problems? I guess you will have FATFS running locally on
> eCos and your host also using FAT, both accessing the same block
> storage device.
> 
> If you fancy going down the closed source, commercial path,
> eCosCentric have the Jungo USB protocol stack. I expect this includes
> MSD.
> 
> I think i also remember a press release from ATMEL about a free MSD
> implementation, but i don't remember the details. 
> 
>       Andrew
> 
Hi Andrew,
Thanks for your thoughts...

>From what I've learned so far, yes the commands are actually SCSI commands.
They are transferred from the host to the device via one of the "Bulk"
transport pipes define by USB.  The device responds with standard SCSI
status messages via a Bulk transport pipe.  Data sectors are also exchanged
between the host and the device via these pipes.

That's what I know so far.  The SCSI packets include things like "write the
following block of data to sector 123 on your disk" and "please read sector
456 from your disk" (as well as things like "please tell me how many disks
you have and how big they are).

So, at that level, I wouldn't have FATFS running locally on eCos (not when
I'm allowing the host to access the local storage), I will interact directly
at the block level with the storage device (RAM, flash, SD-Card, etc...).

As I said, it strikes me as a pretty simple protocol (read a block, write a
block) and probably will take me longer to think about it than to write the
code, but if somebody else has already thought about it and written the eCos
code, I figured I would stand upon the shoulders of the giants who have
preceeded me.

As I look at this, I am becoming more concerned about the HID requirements
for our product because a) I still don't know what I don't know about it; b)
I've read that it requires "interrupt" transfers; c) I've read that the eCos
stack in the CVS tree doesn't include "interrupt" transfers; d) I know more
about SCSI than I do about HID; and e) I don't know very much at all about
SCSI :-)

All of that being said, I still need to finish the low level driver... off
to coding land!

--wpd

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