Lars Friedrich wrote: > Hello, > > thanks for your comments. > > Am Mittwoch, den 25.10.2006, 11:28 +0900 schrieb David Cournapeau: > >> Andrew Straw wrote: >> >>> David Cournapeau wrote: >>> >>> >>>> I don't know anything about your device, but a driver directly accessing >>>> a memory buffer from a userland program sounds like a bug to me. >>>> >>>> >>> David, DMA memory (yes, I know thats an example of RAS Syndrome, >>> apologies) allows hardware to fill a chunk of RAM and then hand it over >>> to a userspace program. In my experience, RAM used for this purpose must >>> be pre-allocated, usually in a ring-buffer type arrangement. So this is >>> normal operating procedure for something like a frame grabber and not a >>> bug at all. >>> >>> >> What I understood from former emails was that the user is allocating a >> memory buffer, and that it gives this memory buffer to the hardware. In >> this sense, I don't see how it is possible to avoid kernel panic or >> equivalents. If on the contrary, the driver gives you the memory buffer, >> then, ok, by eg a mmap-like call, you can access directly the device >> memory, but within a range fixed by the driver, which is valid if the >> driver is not buggy. >> > > The API of the camera provides a function > pl_exp_start_cont(hCam, buffer, size) > > The usage is to pass a Camera-Identifier "hCam", a pointer "buffer" and > a size "size" in bytes. The camera will then start to acquire frames and > put them to the buffer, which has to be size-bytes long. When the > function reaches the end of the buffer, it will start at the beginning > again (circular buffer mode). There is also a function available to ask > for the last buffer position that is written to. The buffer has to be > allocated by the caller of this API-function, so I guess it will be in > *userland*. > In this case, I think the data are copied from the device to your buffer. > To have enough time to read a frame, the buffer has to be sufficiently > big. I don't know what is sufficient, but I made it 50 frames big. With > an acquisition time of lets say 10ms I have about 0.5seconds to get a > frame, that was recently written to the buffer. > You have no way to check if the ringbuffer is full or not ? In audio programming under linux, what happens normally is that you have a ring buffer locked in memory (that is cannot be paged out), and you need to check if your read/write pointers overlap to detect a buffer overrun. > I know, these things are not numpy and even not Python-questions, but > C-questions, and I think I should go and read something about it. Does > anyone have a recommendation where to find information on this > memory-locking things? How is this kind of programming called? > Driver-Programming? Hardware-Programming? System-Programming? > > Real time programming, maybe ? Without a knowledge of the API of your device, it is hard to know what the problem is for me.
By the way, I found the information about locking pages into memory for windows: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/memory/base/virtuallock.asp It may not work as expected, though, as it happens quite often with windows :) David ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ Numpy-discussion mailing list Numpy-discussion@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/numpy-discussion
