Hi Greg, Thanks for taking the time to answer my email. On 04.09.2010 02:58, Greg KH wrote: > On Tue, Aug 24, 2010 at 11:52:07AM +0200, Christophe Aeschlimann wrote: >> Hi, >> >> I'm implementing a user-space API that will give access to >> custom hardware features through sysfs. This library is here >> to hide the sysfs specifics. > > Ick, don't. > > What's wrong with the existing sysfs interfaces we have today, open(), > read(), close()? > > And what type of hardware features are you wanting to export through > sysfs? Why sysfs? Well we have to offer a board-support-package to a customer. When I told them about sysfs to access LEDS / GPIOS they told me they wanted some layer above that so they can just call a "switch_led_on" function and they wouldn't have to know the way sysfs works. E.G. Know the sysfs path name, attributes names and values to write to the attributes to control the device. >> This library must support multi-threaded accesses. > > sysfs handles that just fine. Thanks for confirming that. >> I would like to know if something special must be done in >> this library to assure concurrent accesses. >> >> Here is an example : >> >> I want the users of my library to be able to switch-on/-off a LED using >> a simple function like the following : >> >> platform_led_on(0); //switches on led '0' >> platform_led_off(0); //switches on led '0' >> >> This led is declared in my platform_data in the board init file as a >> gpio_led and can be seen in sysfs under /sys/class/leds/led0 >> >> the platform_led_on/off(0) functions will : >> >> - build the sysfs path to the led0 based on the argument (0) >> - open the brightness file in the sysfs path >> - write to the brightness file a value != 0 to switch the led on or 0 to switch it off >> - close the brightness file >> - return >> >> Now what will happen if a thread tries to switch on the led while another >> thread tries to switch it off ? > > Who ever writes to the file first will cause their action to happen, > followed by the action of the second process/thread. What happens with the open() on sysfs pseudo files ? Can the file be opened twice ? Or does it requires a close first ? > If you need locking, do it in your library, but again, this really looks > like overkill, why do you need to do all of this? Because it's a customer request :) Is it possible to do the locking with the open() or flock() ? Or should I use pthread mutex ? > Oh, and you have looked at libudev, right? No I wasn't aware of that lib but I guess : udev_device_get_sysattr_value () does the open/read/close for me in a thread-safe way ? > thanks, > > greg k-h Thank you, Christophe -- Christophe Aeschlimann Embedded Software Engineer Advanced Communications Networks S.A. Rue du Puits-Godet 8a 2000 Neuchâtel, Switzerland Tél. +41 32 724 74 31 c.aeschlimann@xxxxxxxxxxxx -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
