[RFC v2 PATCH 0/3] Fix dma_alloc_coherent() and friends for NOMMU

Hi,

On 2016. Dec 13., at 14:45, Vladimir Murzin [off-list ref] wrote:

Hi,

It seem that addition of cache support for M-class cpus uncovered
latent bug in DMA usage. NOMMU memory model has been treated as being
always consistent; however, for R/M classes of cpu memory can be
covered by MPU which in turn might configure RAM as Normal
i.e. bufferable and cacheable. It breaks dma_alloc_coherent() and
friends, since data can stuck in caches now or be buffered.

This patch set is trying to address the issue by providing region of
memory suitable for consistent DMA operations. It is supposed that such
region is marked by MPU as non-cacheable. Since we have MPU support in
Linux for R-class only and M-class setting MPU in bootloader, proposed
interface to advertise such memory is via "memdma=size at start" command
line option, to avoid clashing with normal memory (which usually comes
from dts) it'd be safer to use it together with "mem=" command line
option. Meanwhile, I'm open to suggestions for the better way telling
Linux of such memory.

I have tested these patches on my ATMEL SAME70 armv7m board.

After setting the memory regions and attributes in the bootloader and providing the required 
command line parameters, the DMA issues fixed.

I have tested an usart, sdcard, and ethernet driver with DMA enabled.

Booting Linux on physical CPU 0x0
Linux version 4.9.0 (root at debian) (gcc version 4.8.3 20140320 (prerelease) (Sourcery CodeBench Lite 2014.05-29) ) #4 Mon Dec 12 20:27:21 CET 201
6
CPU: ARMv7-M [410fc271] revision 1 (ARMv7M), cr=00000000
CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
OF: fdt:Machine model: SAME70-sampione board
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 15748
Kernel command line: console=ttyS1,115200 root=/dev/mmcblk0p2 rw init=/linuxrc rootwait mem=62M memdma=2M at 0x73e00000

So you can add my Tested-by.

Thanks for the patches!

Regards,
Andras