Re: Closing devices

[Frank Pagliughi <fpagliughi@mindspring.com>]

Hello All,

I'm looking for ideas on how to close and re-open devices on eCos. The 
needs for this are (1) to support swappable/removable devices, (2) to 
have a consistent way to put devices into a low-power state when they 
are not being used, and (3) to prevent devices from using the CPU when 
they are not needed.

On a current project for a battery-powered device, I have a need for all 
of this: a CompactFlash that can be removed; a GPS that sends data 
constantly, but only needs to be read once every few hours; and many of 
the peripherals need to be put in a consistent state prior to going into 
a low power mode.

I've been able to accomplish all of this with ugly application-level 
code, but thought that a much better solution would be to propagate the 
close() call of a devfs device down to the driver, so you could do this:

    while (true) {
         int fd = open("/dev/ser0", O_RDONLY);
         read(fd, buf, BUF_LEN);

         // power down the port
         close(fd);

         sleep(60);
    }

I think this can be done pretty easily by adding a "shutdown" function 
to the devtab entries:

    typedef struct cyg_devtab_entry {
         // ...
         Cyg_ErrNo        (*lookup)(struct cyg_devtab_entry **tab,
                                    struct cyg_devtab_entry *sub_tab,
                                    const char *name);
         Cyg_ErrNo        (*shutdown)(struct cyg_devtab_entry *tab);
         // ...
    } CYG_HAL_TABLE_TYPE cyg_devtab_entry_t;


The existing macros CHAR_DEVTAB_ENTRY, BLOCK_DEVTAB_ENTRY, etc, can just 
insert an empty function into the shutdown slot, whereas a new set of 
macros can accept the shutdown pointer:

    #define
    CHAR_CLOSABLE_DEVTAB_ENTRY(_l,_name,_dep_name,_handlers,_init,_lookup,_shutdown,_priv)
    ...
    #define
    BLOCK_CLOSABLE_DEVTAB_ENTRY(_l,_name,_dep_name,_handlers,_init,_lookup,_shutdown,_priv)
    ...

The assumption is that anything done in the shutdown would be undone in 
a subsequent lookup so that the devices could bre closed then re-opened, 
and that the shutdown would mark the entry as "unavailable":

    tab->status &= ~CYG_DEVTAB_STATUS_AVAIL;


Assuming that this is a good way to handle this I would need to know how 
to hook this into the upper-level device and file operations. What would 
be needed?

Thanks,
Frank