Re: Self modifying ms device

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

On Mon, 3 Dec 2007, Brad Schick wrote:

> Looking for some input on the following problem that I realized isn't
> really usb specific:
> 
> My goal is to display an html page that resides on a usb mass-storage
> device and changes over time. The mass-storage device will be
> read-only from the host OS, but needs to self modify its own data. The
> end-user will view an html page that periodically refreshes itself and
> hopefully changes over time. My concern is how caching in the host OS
> may hide these changes.

Sounds more like you're concerned about preventing caching in the host 
from hiding the changes.

> The usb device's MCU is very small and doesn't have space for a FAT
> driver. To modify its data I was planning to just poke changes
> directly to hard-coded offsets (the files can be pre-allocated since
> data is not created, just modified). Perhaps I could also poke the FAT
> file date, but would that be enough to cause Linux to reload the file?

No.

> Or will the kernel assume all changes to local file-systems must pass
> through the kernel and miss even that?

It will.  So will Windows or any other OS.

> Maybe there is a better approach? One big limitation is that nothing
> can be installed on the host. And I'll have to figure out Windows
> eventually as well.

If you want to use the mass-storage protocols then you're stuck.  
The only choice is to make the peripheral read-only and to go through a 
disconnect/reconnect cycle whenever the data changes.  Or else you 
could have a program on the host that flushes the cached pages.

A less clumsy approach might be to avoid being a mass-storage device.  
But I don't know what other protocol would be more suitable.  Does PTP 
support dynamic updating?

Alan Stern


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