I am implementing a program to be updated via GPRS channel. The total size of the firmware reaches about 256 kB which makes it somewhat costly upgrades. I considered the idea of dividing the firmware into independent parts that can be updated independently and upgrade only the changed parts, but I don't know how to begin. Does anyone has had a similar problem? How do you have solved it?
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Under windows you'd do this with dll's. Even though I use Linux, I can't remember the name of the same concept, but the file suffix is .so. At any rate, whether your OS supports it or not you'd need to have nice logical modules well separated by function (so that upgrading one thing doesn't break all the rest) and a means of dynamically linking the separate bits.
256KB on GPRS is not excessive. It will finish in less than 4 minutes worst case (2G single timeslot).
If you really need faster updates or less bandwidth usage, you could inspire with RSYNC. It improves throughput for files of which a "similar" version is already present on the other side of the link. What definition of "similar" works best for your application, depends on the details of your project.
You can even improve a lot over RSYNC, if you know exactly what's on the other side (by firmware version identifier). You could then implement a "patch processor" (interpreter with defined command set), and produce a patch instruction stream, which you then optimize, compress and transmit. On the other side of the link, it is decompressed and executed, and patches the existing firmware into the new version. Done right, this is the quickest/shortest possible way of transmitting the update (because it is a compression algorithm optimized to the specific application at hand).
However, I suspect that standard compression algorithms on the binary firmware image and possibly a recommendation of using better wireless technology (EDGE or 3G) is quicker to deploy than either of these, and the user will not notice much of a difference anyway.
Presumably, you don't have a "spare" 256KB in the device to cache the image(s) -- persistently -- until *all* of them can be downloaded?
If you try to do this piecemeal, you have potentially several hazards to avoid -- all having to do with incomplete or partially complete images being available.
First, you have to have AT LEAST enough "cache" (though *this* can be volatile) to store the largest "piece" that you will be downloading. This safeguards against the case wherein the image can't be downloaded intact in a single piece (i.e., you can't risk "updating *while* downloading" for fear that the image is corrupted or interrupted
*during* the download -- you need the previous "working" image to be available for your device to continue functioning in the event that the image isn't downloaded completely/correctly).
You also need to have some strict sense of configuration management that allows you (your device and/or the device serving up these images) to know what the device in question's *current* configuration is and which "pieces" can be applied to that configuration to yield "operational" (albeit possibly with other "known bugs") configurations.
For example, if the user is at version A.0, possible upgrade paths may be A.0->A.1->A.3->A.7->B.0, or A.0->A.2->A.7->B.0 depending on which "modules" are updated along the way. Note that the path taken will determine which functionality (and "bug-onality") the device retains *while* it transitions to the final state (recall that the process may take minutes, hours, weeks, months or YEARS! Since you are allowing it to be "interrupted" -- albeit at known points -- you have to consider the possibility of the device *staying* in one of those intermediate states indefinitely!)
This also has to address the fact that the user may *not* have applied all of the upgrades that you require. I.e., you need to account for how to get that device to a state (*the* state?) that your current set of upgrades expects/requires.
And, throughout it all, you have to make sure that the upgrade process itself continues to function flawlessly. (if you brick the user's device, he won't be happy!)
The actual mechanics of creating an updatable framework are conceptually pretty simple: finely *define* modules that are
*small* enough and "independant" enough that they can be replaced individually.
As I said above, the bigger problem is accurately tracking the device's current configuration and *knowing* what behavior you (and the user) can expect from the device in *all* of those possible states. (i.e., imagine a user who got to A.7 but never took the final step to B.0. That user calls you complaining that his device is exhibiting some "unusual behavior". You need to be able to recognize that the device is at A.7 and needs to progress to B.0 to be in a "supported" configuration)
It's either Dynamic Linking Loader (dll) or Dynamic Loading Linker (dll) for Shared Objects (so). However, there are quite a bit of run- time overheads using them. For firmware, it might be easier to just separate the library into "ROM" code and calls, traps or soft intr. into it.
Yes, anything that's dynamically linking is going to be slow -- I was trying to get some concepts and search terms to the guy as much as make specific recommendations.
Certainly if size and speed were a concern, I'd want to do this with something more statically linked (although I can't see how to avoid jump tables or the equivalent). I can think of several ways to do this right off the bat, but in the end you're going to have to sacrifice some combination of speed, versatility, or design time.
I think (not sure) the shared library approach is slow only the first time the library is demand loaded. After that, use of the library is the same as if it was statically linked. Isn't that the extent of the slowness of shard libraries?
I'm not sure of the memory resource usage at load time. I'll bet it's temporary (extra memory released after loading happens).
The 256kB is not much; looks like a monolitic application to me. Breaking it into the chunks would be difficult and inconvenient.
The good way to deal with this problem is DIFF. I.e. a compressed format which uses the current application as a dictionary for the compression. Only the references to the current dictionary and the changes have to be transmitted.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
The global offset table (GOT) and procedure linkage table (PLT) have to be generated each time a program is executed. The use of position-independent code means that relocations don't have to be performed within the actual code. The PLT can either be generated at startup or be generated lazily, i.e. each entry is generated the first time that the function is called. The GOT is always generated at startup.
Linking against static libraries results in the use of fixed addresses. This reduces startup time, as no GOT/PLT entries are required, and increases execution speed slightly (a typical estimate is 4%) as a level of indirection is removed. The main performance downside is that, if multiple processes are running different programs which use the same library, each program will have its own copy of the library, which increases the demand for memory.
I wish this was true. In real world conditions I can tell you from bitter personal experience that it is more like two hours "worst" [observed] case with an average more like 30-45 minutes. That's for a gzipped image that was originally 256K pre-compression, too. Tested in many many sites.
The thread starter was talking about a firmware image of 256 kByte total. I doubt he's using a full off-the-book System V shared object implementation. There's plenty of room for optimisation.
For example, the GOT and PLT can be generated at the time of the firmware update. The shared object can be stored in fully-relocated form. It doesn't have to use position-independant code if we can patch the binary code. Essentially, this is what MS Windows does. It's called "binding":
This is what happens when you load a Linux kernel module. Up to kernel
2.4.x, a module is a standard .o file, which is statically linked against the running kernel (in 2.6 it's slightly different but the basic process, as far as I know, is the same).
Sure, it is per-byte payment here in the USA also (except for lucky people like me who kept some old iPhone account SIMs active to be grandfathered into unlimited data ;) but the problem is downtime, and sequencing.
Downtime can be removed if you have enough space to store a redundant image, but the sequencing/scheduling issue is very important. Our equipment for instance may need for legal reasons to have a technician must be on site when the upgrade is rebooted to test the system and certify it operation. The ideal scenario would be for tech to arrive, push "upgrade", wait for the code to arrive (a few minutes is ok), then test the system and leave. Since the time is so unpredictable and always LONG, this workflow doesn't work.
Also bear in mind we are deploying to hundreds of thousands, if not millions of units.
Can you separate code and data? Maybe the 256K contains tables, images, fonts, or something that is relatively fixed, and can be separated. It's a lot easier to add a little indirection to the access of such tables than to have more general dynamically linked libraries.
Can you patch the firmware, so that the user downloads a delta from one version to the next?
A straightforward solution is to have two sections for each library. One section contains addresses of subroutines in the other section. The main program and the libraries are linked to the section with the addresses. The linking can be difficult, but I blame those linkers, because this is conceptually simple.
Each pair of sections must be put in a fixed place. A pair of sections can be exchanged without affecting linking. The overhead is very small, an extra indirection.
In assembler it is a snap, especially with macro's. In c it looks bad. An instrumented c-compiler could handle it transparently.
The details are much dependant on the tools you have available.
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- being exponential -- ultimately falters.