On Wed, Jul 06, 2011 at 11:19:00AM -0500, Christoph Lameter wrote: > What I described is the basic memory architecture of Linux. I am not that > familiar with ARM and the issue discussed here. Only got involved because > ZONE_DMA was mentioned. The nature of ZONE_DMA is often misunderstood. > > The allocation of the memory banks for the Samsung devices has to fit > somehow into one of these zones. Its probably best to put the memory banks > into ZONE_NORMAL and not have any dependency on ZONE_DMA at all. Let me teach you about the ARM memory management on Linux. Firstly, lets go over the structure of zones in Linux. There are three zones - ZONE_DMA, ZONE_NORMAL and ZONE_HIGHMEM. These zones are filled in that order. So, ZONE_DMA starts at zero. Following on from ZONE_DMA is ZONE_NORMAL memory, and lastly ZONE_HIGHMEM. At boot, we pass all memory over to the kernel as follows: 1. If there is no DMA zone, then we pass all low memory over as ZONE_NORMAL. 2. If there is a DMA zone, by default we pass all low memory as ZONE_DMA. This is required so drivers which use GFP_DMA can work. Platforms with restricted DMA requirements can modify that layout to move memory from ZONE_DMA into ZONE_NORMAL, thereby restricting the upper address which the kernel allocators will give for GFP_DMA allocations. 3. In either case, any high memory as ZONE_HIGHMEM if configured (or memory is truncated if not.) So, when we have (eg) a platform where only the _even_ MBs of memory are DMA-able, we have a 1MB DMA zone at the beginning of system memory, and everything else in ZONE_NORMAL. This means GFP_DMA will return either memory from the first 1MB or fail if it can't. This is the behaviour we desire. Normal allocations will come from ZONE_NORMAL _first_ and then try ZONE_DMA if there's no other alternative. This is the same desired behaviour as x86. So, ARM is no different from x86, with the exception that the 16MB DMA zone due to ISA ends up being different sizes on ARM depending on our restrictions. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
