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diff --git a/docs/overview b/docs/overview index 6528b058c2..350d53ee70 100644 --- a/docs/overview +++ b/docs/overview @@ -1,123 +1,96 @@ -Instead of heading in on another "proposal" document, I thought I'd send out -this email describing ideas I've had about udev - thanks to the comments I've -received. The idea is starting to mushroom a bit and I'd like to get people's -comments before I go further down the path. - -As I see it, we've got a couple goals for udev: +We've got a couple goals for udev: 1) dynamic replacement for /dev 2) device naming 3) API to access info about current system devices -I'd like to split these goals into separate subsystems: +Splitting these goals into separate subsystems: 1) udev - dynamic replacement for /dev 2) namedev - device naming -3) libsysfs - a standard library for accessing device information on the -system. +3) libsysfs - a standard library for accessing device information on the + system. Udev ------ -Udev will be responsible for responding to /sbin/hotplug on device events. It -will receive the device class information along with device's sysfs -directory. Udev will call the name_device function from the naming device -subsystem with that information and receive a unique device name in return. -Udev will then query sysfs through the libsysfs for specific device -information required for creating the /dev node like major and minor number. -Once it has the important information, udev will create a /dev entry for the -device, add the device to the in memory table of current devices, and send -notification of the successful event. On a remove call, udev will remove the -/dev entry, remove the device from the in memory table, and send -notification. - -Udev will consist of a command udev - to be called from /sbin/hotplug. It will -require the in memory dynamic database/table for keeping track of current -system devices, and a library of routines for accessing that database/table. -Udev will not care about "how" devices are named, that will be separated into -the device naming subsystem. It's presented a common device naming API by the -device naming subsystem to use for naming devices. +Udev will be responsible for responding to /sbin/hotplug on device +events. It will receive the device class information along with +device's sysfs directory. Udev will call the name_device function from +the naming device subsystem with that information and receive a unique +device name in return. Udev will then query sysfs through the libsysfs +for specific device information required for creating the /dev node like +major and minor number. Once it has the important information, udev +will create a /dev entry for the device, add the device to the in memory +table of current devices, and send notification of the successful event +through a D-BUS message. On a remove call, udev will remove the /dev +entry, remove the device from the in memory table, and send +notification. + +Udev will consist of a command udev - to be called from /sbin/hotplug. +It will require the in memory dynamic database/table for keeping track +of current system devices, and a library of routines for accessing that +database/table. Udev will not care about "how" devices are named, that +will be separated into the device naming subsystem. It's presented a +common device naming API by the device naming subsystem to use for +naming devices. + + namedev ---------- -From comments Martin has made, I've decided to push out the device naming part -of udev into its own "subsystem". The reason is to make this as flexible and -pluggable as possible. The device naming subsystem, or namedev, will present -a standard interface for udev to call for naming a particular device. Under -that interface, system administrators can plug in their own methods for -device naming. - -We would provide a default naming scheme. The first prototype implementation -could simply take the sysfs directory passed in with the device name -function, query sysfs for the major and minor numbers, and then look up in a -static device name mapping file the name of the device. The static device -naming file could look just like devices.txt in the Linux kernel's -Documentation directory. Obviously, this isn't a great implementation because -eventually we'd like major an minor numbers to be dynamic. - -The default naming scheme in the future would have a set of policies to go -through, these were given to me by Greg. The device naming subsystem would -get the sysfs directory of the to be named device and would use the following -information in order to map the device's name: +From comments people have made, the device naming part of udev has been +pushed into its own "subsystem". The reason is to make this as flexible +and pluggable as possible. The device naming subsystem, or namedev, will +present a standard interface for udev to call for naming a particular +device. Under that interface, system administrators can plug in their +own methods for device naming. + +We would provide a default naming scheme. The first prototype +implementation could simply take the sysfs directory passed in with the +device name function, query sysfs for the major and minor numbers, and +then look up in a static device name mapping file the name of the +device. The static device naming file could look just like devices.txt +in the Linux kernel's Documentation directory. Obviously, this isn't a +great implementation because eventually we'd like major an minor numbers +to be dynamic. + +The default naming scheme in the future would have a set of policies to +go through in order to determine the name of the device. The device +naming subsystem would get the sysfs directory of the to be named device +and would use the following information in order to map the device's +name: 1) Label info - like SCSI's UUID 2) Bus Device Number 3) Topology on Bus 4) Kernel Name - DEFAULT -System administrators could use the default naming system or enterprise -computing environments could plug in their Universal Unique Identifier (UUID) -policies. The idea is to make the device naming as flexible and pluggable as -possible. +System administrators could use the default naming system or enterprise +computing environments could plug in their Universal Unique Identifier +(UUID) policies. The idea is to make the device naming as flexible and +pluggable as possible. + +The device naming subsystem would require accessing sysfs for device +information. It will receive the device's sysfs directory in the call +from udev and use it to get more information to determine naming. The +namedev subsystem will include a standard naming API for udev to use. +The default naming scheme will include a set of functions and a static +device naming file, which will reside in /etc or /var. + -The device naming subsystem would require accessing sysfs for device -information. It will receive the device's sysfs directory in the call from -udev and use it to get more information to determine naming. The namedev -subsystem will include a standard naming API for udev to use. The default -naming scheme will include a set of functions and a static device naming -file, which will reside in /etc or /var. libsysfs -------- -Greg may object, but I believe there's a need for a common API to access -device information in sysfs. The device naming subsystem and the udev -subsystem need to take the sysfs directory path and query device information. -Instead of copying code so each one will have to readdir, etc., I've decided -to split out the sysfs calls into a separate library that will sit atop -sysfs. Sysfs callbacks aren't standard across devices, I beleive this is -another reason for creating a common and standard library interface for +There is a need for a common API to access device information in sysfs. +The device naming subsystem and the udev subsystem need to take the +sysfs directory path and query device information. Instead of copying +code so each one will have to readdir, etc., splitting this logic of +sysfs calls into a separate library that will sit atop sysfs makes more +sense. Sysfs callbacks aren't standard across devices, so this is +another reason for creating a common and standard library interface for querying device information. -Another reason for libsysfs is it satisfies requirements the LTC RAS team has -for getting current system device information. Rather than keeping tons of -information in udev's in memory database, or even querying that database for -the sysfs directory that will require storing extra reference info in memory, -I've decided the RAS requirements can be fulfilled with a library atop sysfs. -Sysfs contains devices currently on the system. - -Applications like the Error Log Analysis piece, for example, can query the -sysfs library for device information. ELA gets specific information in an -error message thanks to the dev_* and soon to be proposed netdev_* macros. -One goal of the ELA is to gather as much information about an erroring device -so service engineers and administrators can diagnose the problem. The ELA -will get an error message with the bus id and driver name of the device. It -will then need to query sysfs for other VPD information. - -I've used syfs in the name of libsysfs for a reason, I believe sysfs will be -the device tree to use in the future. Until all VPD info is in sysfs, the -library could also make use of /proc, sginfo, and other sources for device -information under the covers so ELA and other applications don' t need to all -have that knowledge. - - -I'd like to know what everyone thinks about my proposal to split this all up -into three separate subsystems. All comments are welcome. - -Thanks, - -Dan - - |