Writing a simple usb driver linux journal

So this post is an attempt to write down my opinions on the matter.

Writing a simple usb driver linux journal

Mount time linearly depends on the flash size True, the dependency is linear, because JFFS2 has to scan whole flash media when mounting. UBIFS mount time does not depend on the flash size.

Memory consumption linearly depends on the flash size True, the dependency is linear. But it is not difficult to implement the LPT shrinker and get rid of the dependency. It is not implemented only because the memory consumption is too small to make the coding work worth it.

Related Work

UBI memory consumption linearly depends on flash size. Mount time linearly depends on the file system contents True, the more data are stored on the file system, the longer it takes to mount it, because JFFS2 has to do more scanning work.

False, mount time does not depend on the file system contents. At the worst case if there was an unclean rebootUBIFS has to scan and replay the journal which has fixed and configurable size. Full file system checking is required after each mount True.

For example, this may be observed by running top just after JFFS2 has been mounted. This slows down overall system boot-up time.

Release Notes | SUSE Linux Enterprise Server 12 SP2

Fundamentally, this is needed because JFFS2 does not store space accounting information i. Memory consumption linearly depends on file system contents True. UBIFS memory consumption does not depend on how much data are stored on the flash media. File access time linearly depends on its size True.

The fragment tree is an in-memory RB-tree which is indexed by file offset and refers on-flash nodes corresponding to this offset. The fragment tree is not stored on the flash media. Instead, it is built on-the-flight when the file is opened for the first time.

writing a simple usb driver linux journal

To build the fragment tree, JFFS2 has to read each data node corresponding to this inode from the flash. This means, the larger is the file, the longer it takes to open it for the first time. And the larger is the file the more memory it takes when it is opened.

Depending on the system, JFFS2 becomes nearly unusable starting from certain file size. Whenever a piece of data has to be read from the file system, the B-tree is looked-up and the corresponding flash address to read is found.

There is a TNC cache which caches the B-tree nodes when the B-tree is looked-up, and the cache is shrinkable, which means it can be shrunk when the kernel needs more memory. Since JFFS2 is fully synchronous, it writes data to the flash media as soon as the data arrives.

If one changes few bytes in the middle of a file, JFFS2 writes a data node which contains those bytes to the flash. If there are many random small writes all over the place, the file system becomes fragmented. But this "defragmentation" is happening during garbage collection and at random time, because JFFS2 wear-leveling algorithm is based on random eraseblock selection.

So if there were a lot of small writes, JFFS2 becomes slower some time later - the performance just goes down out of the blue which makes the system less predictable. This does not hurt the performance much because of the write-back support:If you are going to write a Linux USB driver, please become familiar with the USB protocol specification.

It can be found, along with many other useful documents, at the USB home page (see Resources). iridis-photo-restoration.com: News analysis, commentary, and research for business technology professionals.

This article explains the creation process of a Linux kernel device driver for an undocumented USB device. After having reverse-engineered the USB communication protocol, I present the architecture of the USB device driver.

In addition to the kernel driver I introduce a simple user-space tool that can be used to control the device.

writing a simple usb driver linux journal

Dear Twitpic Community - thank you for all the wonderful photos you have taken over the years. We have now placed Twitpic in an archived state. Sep 01,  · Research Resources. A Subject Tracer™ Information Blog developed and created by Internet expert, author, keynote . The Linux file system is a hierarchically structured tree where every location has its distinct meaning.

The file system structure is standardized through the file system hierarchy standard of which you'll find this chapter to be a description off.

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