Block driver aligning BIO buffer address

beandigital · 11-08-2019, 11:34 AM

I am developing a block driver for an embedded device. The device has a DMA controller, but it expects the address to be aligned 8 byte boundary. I have using an example ram disk driver to develop my driver. But when I look at the address that is returned from the BIO, there is no alignment. Is there a way to align it? I have included the code I am using below.

Code:

#define BV_PAGE(bv) ((bv).bv_page)
#define BV_OFFSET(bv) ((bv).bv_offset)
#define BV_LEN(bv) ((bv).bv_len)

rq_for_each_segment(bv, req, iter)
{
  buffer = page_address(BV_PAGE(bv)) + BV_OFFSET(bv);

smallpond · 11-09-2019, 07:59 AM

There are two reasons that the kernel copies write data from user to kernel space before issuing the I/O request. One is to avoid problems if the user space buffer gets freed or changed during the I/O, and the other is to align the data. The kernel block cache is all aligned on page boundaries.

A ram disk probably doesn't care about boundaries so doesn't need to do this. It also doesn't need to do the extra copy since it can just copy directly to the target ram area.

If you look at the async I/O driver, it requires the user to provide aligned buffers and does DMA directly from/to user space. You can either do that or copy to aligned buffers allocated in the kernel.

beandigital · 11-09-2019, 09:33 AM

Thanks for your reply. I dont really understand what you said as I am relatively new to Linux programming. I am actually a hardware engineer, so just trying to work this out myself. Is there an example that you could point me to? Thanks

smallpond · 11-09-2019, 04:12 PM

The free book Linux Device Drivers contains working examples for all types of drivers, including block devices. There is a whole chapter on Memory management and DMA.

https://lwn.net/Kernel/LDD3/