ZwQueryVirtualMemory reports on virtual memory in the address space of a process. I would like to do the same thing, but for paged memory in system space. Is there an equivalent function that deals with system space instead of process space?
You could use ZwQuerySystemInformation with SystemModuleInformation to get all running drivers. Then you can find the entry you want and get the base and size of the driver. You could if you want to do it properly only get the base of a driver with using the same method of above or using the PsLoadedModuleList to get the base of the targeted driver and then just walk the sections manually with the headers.
Also a tip, if you are going to copy it to dump it, use MmCopyMemory.
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I write a kernel driver which exposes to the user space my I/O device.
Using mmap the application gets virtual address to write into the device.
Since i want the application write uses a big PCIe transaction, the driver maps this memory to be write combining.
According to the memory type (write-combining or non-cached) the application applies an optimal method to work with the device.
But, some architectures do not support write-combining or may support but just for part of the memory space.
Hence, it is important that the kernel driver tell to application if it succeeded to map the memory to be write-combining or not.
I need a generic way to check in the kernel driver if the memory it mapped (or going to map) is write-combining or not.
How can i do it?
here is part of my code:
vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
io_remap_pfn_range(vma, vma->vm_start, pfn, PAGE_SIZE, vma->vm_page_prot);
First, you can find out if an architecture supports write-combining at compile time, with the macro ARCH_HAS_IOREMAP_WC. See, for instance, here.
At run-time, you can check the return values from ioremap_wc, or set_memory_wc and friends for success or not.
I am just writing my very first linux device driver, and I have ran into a problem. I want to prevent one memory region from being cached, so I have been trying to use flush_cache_range() and flush_tlb_range() to flush the cache for this memory region. Everything compiles well, but when I try to load the kernel module I get the following errors:
Unknown symbol flush_cache_range (err 0)
Unknown symbol flush_tlb_range (err 0)
I find this very strange. Shouldn't they be defined in kernel?
I know that alternatively I could also use dma_alloc_coherent() to allocate a non-cached memory region. But I don't have a device structure and passing NULL for this parameter didn't cause any errors, but I also couldn't see any of the data that was supposed to be there.
Some information about my system: I'm trying to get this running on a ARM microcontroller with an integrated FPGA (the Xilinx Zynq). The FPGA copies some data to a memory location specified by the CPU. Now I want to access this memory without getting old data from the caches.
Any help is very appreciated.
You cannot use functions such as flush_cache_range() because they are not intended to be used by modules.
To allocate memory that can be accessed by a DMA device, you must use dma_alloc_coherent().
This requires a valid device structure so that it can do proper mapping between memory addresses and bus addresses.
If your device is not on a bus that is handled by an existing framework (such as PCI), you have to create a platform device.
A few notes:
1- flush_cache_range doesn't "prevent one memory region from being cached" .. It just simply flush (clean + invalidate) the caches. Any future writes/reads to this memory region through the same virtual range will go through the cache again.
2- If the FPGA is writing to memory and then the CPU are going to read from this memory, probably flushing the cache isn't the correct thing to do any way. Usually what you need to do is to invalidate the memory region and then tell the FPGA to write.
3- Please take a look at "${kernel-src}/Documentation/DMA-API.txt" in the kernel sources. It has plenty of information about how you can safely ( cache maintenance + phys_to_dma translation ) use a specific region of memory for DMA.
I am writting a Kernel Module that is going to trigger and external PCIe device to read a block of data from my internel memory. To do this I need to send the PCIe device a pointer to the physical memory address of the data that I would like to send. Ultimately this data is going to be written from Userspace to the kernel with the write() function (userspace) and copy_from_user() (kernel space). As I understand it, the address that my kernel module will see is still a virtual memory address. I need a way to get the physical address of it so that the PCIe device can find it.
1) Can I just use mmap() from userspace and place my data in a known location in DDR memory, instead of using copy_from_user()? I do not want to accidently overwrite another processes data in memory though.
2) My kernel module reserves PCIe data space at initialization using ioremap_nocache(), can I do the same from my kernel module or is it a bad idea to treat this memory as io memory? If I can, what would happen if the memory that I try to reserve is already in use? I do not want to hard code a static memory location and then find out that it is in use.
Thanks in advance for you help.
You don't choose a memory location and put your data there. Instead, you ask the kernel to tell you the location of your data in physical memory, and tell the board to read that location. Each page of memory (4KB) will be at a different physical location, so if you are sending more data than that, your device likely supports "scatter gather" DMA, so it can read a sequence of pages at different locations in memory.
The API is this: dma_map_page() to return a value of type dma_addr_t, which you can give to the board. Then dma_unmap_page() when the transfer is finished. If you're doing scatter-gather, you'll put that value instead in the list of descriptors that you feed to the board. Again if scatter-gather is supported, dma_map_sg() and friends will help with this mapping of a large buffer into a set of pages. It's still your responsibility to set up the page descriptors in the format expected by your device.
This is all very well written up in Linux Device Drivers (Chapter 15), which is required reading. http://lwn.net/images/pdf/LDD3/ch15.pdf. Some of the APIs have changed from when the book was written, but the concepts remain the same.
Finally, mmap(): Sure, you can allocate a kernel buffer, mmap() it out to user space and fill it there, then dma_map that buffer for transmission to the device. This is in fact probably the cleanest way to avoid copy_from_user().
I am developing a real-time application for Linux 3.5.7. The application needs to manage a PCI-E device.
In order to access the PCI-E card spaces, I have been using mmap in combination with /dev/mem. However (please correct me if I am wrong) each time I read or write the mapped memory, a system call is required for the /dev/mem pseudo-driver to handle the memory access.
To avoid the overhead of this system call, I think it should be possible to write a kernel module so that, within e.g. a ioctl call I can modify the process page table, in order to map the physical device pages to userspace pages and avoid the system call.
Can you give me some orientation on this?
Thanks and regards
However (please correct me if I am wrong) each time I read or write the mapped memory, a system call is required
You are wrong.
it should be possible to write a kernel module so that, within e.g. a ioctl call I can modify the process page table
This is precisely what mmap() does.
From my understanding, after a PC/embedded system booted up, the OS will occupy the entire RAM region, the RAM will look like this:
Which means, while I'm running a program I write, all the variables, dynamic memory allocated in the stacks, heaps and etc, will remain inside the region. If I run firefox, paint, gedit, etc, they will also be running in this region. (Is this understanding correct?)
However, I would like to shrink the OS region. Below is an illustration of how I want to divide the RAM:
The reason that I want to do this is because, I want to store some data receive externally through the driver into the Custom Region at fixed physical location, then I will be able to access it directly from the user space without using copy_to_user().
I think it is possible to do that by configuring u-boot, but I have no experience in u-boot, can anyone give me some directions where to begin with, such as: do I need to modify the source of u-boot, or changing the environment variables of u-boot will be sufficient?
Or is there any alternative method of doing this?
Any help is much appreciated. Thanks!
p/s: I'm using TI ARM processor, and booting up from an SD card, I'm not sure if it matters.
The platform is ARM. min_addr and max_addr will not work on these platform since these are for Intel-only implementations.
For the ARM platform try to look at "mem=size#start" kernel parameter. Read up on Documentation/kernel-parameters.txt and arch/arm/kernel/setup.c. This option is available on most new Linux code base (ie. 2.6.XX).
You need to set the following parameters:
max_addr=some_max_physical
min_addr=some_min_physical
to be passed to the kernel through uboot in the 'bootargs' u-boot environment variable.
I found myself trying to do the opposite recently - in other words get Linux to use the additional memory in my system - although I'm using Barebox rather than u-boot on a OMAP4 platform.
I found (a bit to my surprise) that once the Barebox MLO first stage boot-loader was aware of the extra RAM, the kernel then detected and used it as well without any bootargs. Since the memory size is not passed anywhere on the boot-line, I can only assume the kernel inspects the memory mappings set up by the boot-loader to determine RAM size. This suggests that modifying your u-boot to not map all of the RAM is the way to go.
On the subject of boot-args, there was a time when you it was recommended that you mapped out a chunk of RAM (used by the frame buffer?) on OMAP4 systems, using the boot-line. It's still unclear whether this is still necessary.