From time to time I convert videos to H264. When I do this I want to get the best quality out of a give filesize. This means I create VBR videos. The question here is, how big the target video file shall be.
After searching around a little bit in the net I found a formula which is supposed to give a hint about the target filesize. Naturally this depends on the encoder (or even the encoder-version), the encoder settings and even the video.
The formula is at least a good hint for my use, so I wrote a script which calculates several filesize values for a given video (based upon the output of mediainfo, which the scripts expects in a file with the filename as an argument to the script). It calculates a CBR and a VBR value for a given video based upon the width, height and duration. It should work on all system with a POSIX compatible shell.
Example output for a video from my HD-ready cam, original filesize 1.8 GB:
Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 2
Per second: 6451200.000 bps / 6300 Kibps
Total CBR: 1148313600 bytes / 1121400 KiB / 1095 MiB
Total VBR: 861235200 bytes / 841050 KiB / 821 MiB
Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 3
Per second: 9676800.000 bps / 9450 Kibps
Total CBR: 1722470400 bytes / 1682100 KiB / 1642 MiB
Total VBR: 1291852800 bytes / 1261575 KiB / 1232 MiB
Width: 1280, Height: 720, FPS: 50.000, Time: 1424, Motion: 4
Per second: 12902400.000 bps / 12600 Kibps
Total CBR: 2296627200 bytes / 2242800 KiB / 2190 MiB
Total VBR: 1722470400 bytes / 1682100 KiB / 1642 MiB
There are 3 sections, the difference is the “motion” value. It is a kind of multiplicator depending on the amount of motion in the video. For the videos I made myself (family videos, and even some videos of volley ball games), the first section seems to be just fine. So I reduced the original MP4 file to about 50% (not visible here is the audio size, normally I copy the original audio unmodified).
For the curious ones, the formula is
width_in_pixels * height_in_pixels * fps * motion_value * 0.07
for the bps value. The CBR value is
bps * playtime_in_seconds / 8
and the VBR value is
3 / 4 * CBR_value.
GD Star Rating
GD Star Rating
The recent security incident triggered a discussion how to secure ssh/gpg keys.
One way I want to focus on here (because it is the way I want to use at home), is to store the keys on a crypto card. I did some research for suitable crypto cards and found one which is called Feitian PKI Smartcard, and one which is called OpenPGP card. The OpenPGP card also exists in a USB version (basically a small version of the card is already integrated into a small USB card reader).
The Feitian card is reported to be able to handle RSA keys upto 2048 bits. They do not seem to handle DSA (or ECDSA) keys. The smartcard quick starter guide they have (the Tuning smartcard file system part) tells how to change the parameters of the card to store upto 9 keys on it.
The spec of the OpenPGP card tells that it supports RSA keys upto 3072 bits, but there are reports that it is able to handle RSA keys upto 4096 bits (you need to have at least GPG 2.0.18 to handle that big keys on the crypto card). It looks to me like the card is not handle DSA (or ECDSA) cards. There are only slots for upto 3 keys on it.
If I go this way, I would also need a card reader. It seems a class 3 one (hardware PIN pad and display) would be the most “future-proof” way to go ahead. I found a Reiner SCT cyberJack secoder card reader, which is believed to be supported by OpenSC and seems to be a good balance between cost and features of the Reiner SCT card readers.
If anyone reading this can suggest a better crypto card (keys upto 4096 bits, more than 3 slots, and/or DSA/ECDSA support), or a better card reader, or has any practical experience with any of those components on FreeBSD, please add a comment.
GD Star Rating
GD Star Rating
In mid-April a woman from the marketing department of No Starch Press contacted me and asked if I am interested to do a public review of the FreeBSD Device Drivers book by Joseph Kong (no link to a book shop, go and have a look in your preferred one). Just this simple question, no strings attached.
I had my nose in some device drivers in the past, but I never wrote one, and never had a look at the big picture. I was interested to know how everything fits together, so this made me a good victim for a review (novice enough to learn something new and to have a look if enough is explained, and experienced enough to understand what is going on in the FreeBSD kernel).
Some minutes after I agreed to review it (but with a little notice that I do not know how long I need to review it), I had the PDF version of the book. That was faster than I expected (maybe I am too old-school and used to have paper versions of books in my hands).
Let the review begin… but bear with me, this is the first time I do a real public review of a book (instead of a technical review for an author). And as this is my very own personal opinion, I will not allow comments here. This page is all about my opinion while reading the book, questions I have while reading the book shall serve as a hint about the quality of the book and they should be answered in the book, not here.
In short, the book is not perfect, but it is a good book. There is room for improvement, but on a very high level. If you want to write a device driver for FreeBSD, this book is a must. I suggest to read it completely, even chapters which do not belong to the type of driver you want to write (specially the case studies of real drivers). The reason is that each chapter has some notes which may not only apply to the chapter in question, but to all kinds of device drivers. The long review follows now.
The first chapter is titled “Building and running modules”. The author begins with description of the usual device driver types (NIC driver, pseudo-device, …) and how they can be added to the kernel (statically linked in or as a module). The first code example is a small and easy kernel module, so that we do not have to reboot the system we use to develop a driver (except we make a fault during driver development which causes the machine to panic or hang). Every part of the example is well explained. This is followed by an overview about character devices (e.g. disks) and a simple character-device driver (so far a pseudo-device, as we do not have real hardware we access) which is not only as-well explained as the module-example, but there is also a note where the code was simplified and what should be done instead.
After reading this chapter you should be able to write your own kernel module in 5 minutes (well, after 5 minutes it will not be able to do a lot — just a “hello world” – but at least you can already load/unload/execute some code into/from/in the kernel).
I have not tried any example myself, but I compiled a lot of modules and drivers I modified in the past and remember to have seen the described parts.
The second chapter explains how to allocate and free memory in the kernel. There is the possibility to allocate maybe-contiguous memory (the normal case, when your hardware does not do DMA or does not have the requirement that the memory region it makes DMA from/too needs to be contiguous), and really contiguous. For the size argument of the freeing of the the contiguous memory there is the sentence “Generally, size should be equal the amount allocated.”. Immediately I wanted to know what happens if you specify a different size (as a non-native english speaker I understand this sentence in a way that I am allowed to specify a different size and as such are able to free only parts of the allocated memory). Unfortunately this is not answered. I had a look into the source, the kernel frees memory pages, so the size argument (and addr argument) will be rounded to include a full page. This means theoretically I am able to free parts of the allocated memory, but this is a source-maintenance nightmare (needs knowledge about the machine specific page boundaries and you need to make sure that you do the absolutely correct size calculations). To me this looks more like as long as nobody is pointing a gun at my head and tells me to use a different size, specifying the same size as made during the allocation of this memory region is the way to go.
After reading this chapter you should know how to kill the system by allocating all the RAM in the kernel.
Again, I did not try to compile the examples in this chapter, but the difference of the memory allocation in the kernel compared with memory allocation in the userland is not that big.
The third chapter explains the device communication and control interfaces (ioctl/sysctl) of a driver. The ioctl part teached me some parts I always wanted to know when I touched some ioctls, but never bothered to find out before. Unfortunately this makes me a little bit nervous about the way ioctls are handled in the FreeBSD linuxulator, but this is not urgent ATM (and can probably be handled by a commend in the right place). The sysctl part takes a little bit longer to follow through, but there is also more to learn about it. If you just modify an existing driver with an existing sysctl interface, it probably just comes down to copy&paste with little modifications, but if you need to make more complex changes or want to add a sysctl interface to a driver, this part of the book is a good way to understand what is possible and how everything fits together. Personally I would have wished for a more detailed guide when to pick the ioctl interface and when the sysctl interface than what was written in the conclusion of the chapter, but it is probably not that easy to come up with a good list which fits most drivers.
After reading this chapter you should be able to get data in and out of the kernel in 10 minutes.
As before, I did not compile the examples in this chapter. I already added ioctls and sysctls in various places in the FreeBSD kernel.
Chapter 4 is about thread synchronization – mutexes, shared/exclusive locks, reader/writer locks and condition variables. For me this chapter is not as good as the previous ones. While I got a good explanation of everything, I missed a nice overview table which compares the various methods of thread synchronization. Brendan Gregg did a nice table to give an overview of DTrace variable types and when to use them. Something like this would have been nice in this chapter too. Apart from this I got all the info I need (but hey, I already wrote a NFS client for an experimental computer with more than 200000 CPUs in 1998, so I’m familiar with such synchronization primitives).
Delayed execution is explained in chapter 5. Most of the information presented there was new to me. While there where not much examples presented (there will be some in a later chapter), I got a good overview about what exists. This time there was even an overview when to use which type of delayed execution infrastructure. I would have preferred to have this overview in the beginning of the chapter, but that is maybe some kind of personal preference.
In chapter 6 a complete device driver is dissected. It is the virtual null modem terminal driver. The chapter provides real-world examples of event-handlers, callouts and taskqueues which where not demonstrated in chapter five. At the same time the chapter serves as a description of the functions a TTY driver needs to have.
Automated device detection with Newbus and the corresponding resource allocation (I/O ports, device memory and interrupts) are explained in chapter 7. It is easy… if you have a real device to play with. Unfortunately the chapter missed a paragraph or two about the suspend and resume methods. If you think about it, it is not hard to come up with what they are supposed to do, but a little explicit description of what they shall do, in what state the hardware should be put and what to assume when being called would have been nice.
Chapter 8 is about interrupts. It is easy to add an interrupt handler (or to remove one), the hard part is to generate an interrupt. The example code uses the parallel port, and the chapter also contains a little explanation how to generate an interrupt… if you are not afraid to touch real hardware (the parallel port) with a resistor.
In chapter 9 the lpt(4) driver is explained, as most of the topics discussed so far are used inside. The explanation how everything is used is good, but what I miss sometimes is why they are used. The most prominent (and only) example here for me is why are callouts used to catch stray interrupts? That callouts are a good way of handling this is clear to me, the big question is why can there be stray interrupts. Can this happen only for the parallel port (respectively a limited amount of devices), or does every driver for real interrupt driven hardware need to come with something like this? I assume this is something specific to the device, but a little explanation regarding this would have been nice.
Accessing I/O ports and I/O memory for devices are explained in chapter 10 based upon a driver for a LED device (turn on and off 2 LEDs on an ISA bus). All the functions to read and write data are well explained, just the part about the memory barrier is a little bit short. It is not clear why the CPU reordering of memory accesses matter to what looks like function calls. Those function calls may be macros, but this is not explained in the text. Some little examples when to use the barriers instead of an abstract description would also have been nice at this point.
Chapter 11 is similar to chapter 10, just that a PCI bus driver is discussed instead of an ISA bus driver. The differences are not that big, but important.
In chapter 12 it is explained how to do DMA in a driver. This part is not easy to understand. I would have wanted to have more examples and explanations of the DMA tag and DMA map parts. I am also surprised to see different supported architectures for the flags BUS_DMA_COHERENT and BUS_DMA_NOCACHE for different functions. Either this means FreeBSD is not coherent in those parts, or it is a bug in the book, or it is supposed to be like this and the reasons are not explained in the book. As there is no explicit note about this, it probably leads to confusion of readers which pay enough attention here. It would also have been nice to have an explanation when to use those flags which are only implemented on a subset of the architectures FreeBSD supports. Anyway, the explanations give enough information to understand what is going on and to be able to have a look at other device drivers for real-live examples and to get a deeper understanding of this topic.
Disk drivers and block I/O (bio) requests are described in chapter 13. With this chapter I have a little problem. The author used the word “undefined” in several places where I as a non-native speaker would have used “not set” or “set to 0″. The word “undefined” implies for me that there may be garbage inside, whereas from a technical point of view I can not imagine that some random value in those places would have the desired result. In my opinion each such place is obvious, so I do not expect that an experienced programmer would lose time/hairs/sanity over it, but inexperienced programmers which try to assemble the corresponding structures on the (uninitialized) heap (for whatever reason), may struggle with this.
Chapter 14 is about the CAM layer. While the previous chapter showed how to write a driver for a disk device, chapter 14 gave an overview about how to an HBA to the CAM layer. It is just an overview, it looks like CAM needs a book on its own to be fully described. The simple (and most important) cases are described, with the hardware-specific parts being an exercise for the person writing the device driver. I have the impression it gives enough details to let someone with hardware (or protocol), and more importantly documentation for this device, start writing a driver.
It would have been nice if chapter 13 and 14 would have had a little schematic which describes at which level of the kernel-subsystems the corresponding driver sits. And while I am at it, a schematic with all the driver components discussed in this book at the beginning as an overview, or in the end as an annex, would be great too.
An overview of USB drivers is given in chapter 15 with the USB printer driver as an example for the explanation of the USB driver interfaces. If USB would not be as complex as it is, it would be a nice chapter to start driver-writing experiments (due to the availability of various USB devices). Well… bad luck for curious people. BTW, the author gives pointers to the official USB docs, so if you are really curious, feel free to go ahead.
Chapter 16 is the first part about network drivers. It deals with ifnet (e.g. stuff needed for ifconfig), ifmedia (simplified: which kind of cable and speed is supported), mbufs and MSI(-X). As in other chapters before, a little overview and a little picture in the beginning would have been nice.
Finally, in chapter 17, the packet reception and transmission of network drivers is described. Large example code is broken up into several pieces here, for more easy discussion of related information.
One thing I miss after reaching the end of the book is a discussion of sound drivers. And this is surely not the only type of drivers which is not discussed, I can come up with crypto, firewire, gpio, watchdog, smb and iic devices within a few seconds. While I think that it is much more easy to understand all those drivers now after reading the book, it would have been nice to have at least a little overview of other driver types and maybe even a short description of their driver methods.
Conclusion: As I wrote already in the beginning, the book is not perfect, but it is good. While I have not written a device driver for FreeBSD, the book provided enough insight to be able to write one and to understand existing drivers. I really hope there will be a second edition which addresses the minor issues I had while reading it to make it a perfect book.
GD Star Rating
GD Star Rating
Tags: book questions
, device driver
, device drivers
, freebsd kernel
, marketing department
, paper versions
, pdf version
, personal opinion
, preferred one
In several previous posts I wrote about my quest for the right source format to stream video to my Sony BRAVIA TV (build in 2009). The last week-end I finally found something which satisfies me.
What I found was serviio, a free UPnP-AV (DLNA) server. It is written in java and runs on Windows, Linux and FreeBSD (it is not listed on the website, but we have an not-so-up-to-date version in the ports tree). If necessary it transcodes the input to an appropriate format for the DLNA renderer (in my case the TV).
I tested it with my slow Netbook, so that I was able to see with which input format it will just remux the input container to a MPEG transport stream, and which input format would be really re-encoded to a format the TV understands.
The bottom line of the tests is, that I just need to use a supported container (like MKV or MP4 or AVI) with H.264-encoded video (e.g. encoded by x264) and AC3 audio.
The TV is able to chose between several audio streams, but I have not tested if serviio is able to serve files with multiple audio streams (my wife has a different mother language than me, so it is interesting for us to have multiple audio streams for a movie), and I do not know if DLNA supports something like this.
Now I just have to replace minidlna (which only works good with my TV for MP3s and Pictures) with serviio on my FreeBSD file server and we can forget about the disk-juggling.
GD Star Rating
GD Star Rating
Tags: audio streams
, dlna server
, input container
, input format
, mother language
, sony bravia tv
, source format
, transport stream