Increasing Linux DMA_ZONE memory on ARM i.MX287

Question

I am working in an Embedded Linux system which has the 2.6.35.3 kernel.

Within the device we require a 4MB+192kB contiguous DMA capable buffer for one of our data capture drivers. The driver uses SPI transfers to copy data into this buffer.

The user space application issues a mmap system call to map the buffer into user space and after that, it directly reads the available data.

The buffer is allocated using "alloc_bootmem_low_pages" call, because it is not possible to allocate more than 4 MB buffer using other methods, such as kmalloc.

However, due to a recent upgrade, we need to increase the buffer space to 22MB+192kB. As I've read, the Linux kernel has only 16MB of DMA capable memory. Therefore, theoretically this is not possible unless there is a way to tweak this setting.

If there is anyone who knows how to perform this, please let me know?

Is this a good idea or will this make the system unstable?

Answer 1

The ZONE_DMA 16MB limit is imposed by a hardware limitation of certain devices. Specifically, on the PC architecture in the olden days, ISA cards performing DMA needed buffers allocated in the first 16MB of the physical address space because the ISA interface had 24 physical address lines which were only capable of addressing the first 2^24=16MB of physical memory. Therefore, device drivers for these cards would allocate DMA buffers in the ZONE_DMA area to accommodate this hardware limitation.

Depending on your embedded system and device hardware, your device either is or isn't subject to this limitation. If it is subject to this limitation, there is no software fix you can apply to allow your device to address a 22MB block of memory, and if you modify the kernel to extend the DMA address space beyond 16MB, then of course the system will become unstable.

On the other hand, if your device is not subject to this limitation (which is the only way it could possibly write to a 22MB buffer), then there is no reason to allocate memory in ZONE_DMA. In this case, I think if you simply replace your alloc_bootmem_low_pages call with an alloc_bootmem_pages call, it should work fine to allocate your 22MB buffer. If the system becomes unstable, then it's probably because your device is subject to a hardware limitation, and you cannot use a 22MB buffer.

Answer 2

It looks like my first attempt at an answer was a little too generic. I think that for the specific i.MX287 architecture you mention in the comments, the DMA zone size is configurable through the CONFIG_DMA_ZONE_SIZE parameter which can be made as large as 32Megs. The relevant configuration option should be under "System Type -> Freescale i.MXS implementations -> DMA memory zone size".

On this architecture, it's seems safe to modify it, as it looks like it's not addressing a hardware limitation (the way it was on x86 architectures) but just determining how to lay out memory.

If you try setting it to 32Meg and testing both alloc_bootmem_pages and alloc_bootmem_low_pages in your own driver, perhaps one of those will work.

Otherwise, I think I'm out of ideas.