How to work with reserved CMA memory? - memory-management

Question

I would like to allocate piece of physically contiguous reserved memory (in predefined physical addresses) for my device with DMA support.
As I see CMA has three options:
1. To reserve memory via kernel config file. 2. To reserve memory via kernel cmdline. 3. To reserve memory via device-tree memory node.
In the first case: size and number of areas could be reserved.
CONFIG_DMA_CMA=y
CONFIG_CMA_AREAS=7
CONFIG_CMA_SIZE_MBYTES=8
So I could use:
start_cma_virt = dma_alloc_coherent(dev->cmadev, (size_t)size_cma, &start_cma_dma, GFP_KERNEL);
in my driver to allocate contiguous memory. I could use it max 7 times and it will be possible allocate up to 8M. But unfortunately
dma_contiguous_reserve(min(arm_dma_limit, arm_lowmem_limit));
from arch/arm/mm/init.c:
void __init arm_memblock_init(struct meminfo *mi,const struct machine_desc *mdesc)
it is impossible to set predefined physical addresses for contiguous allocation.
Of Course I could use kernel cmdline:
mem=cma=cmadevlabel=8M#32M cma_map=mydevname=cmadevlabel
//struct device *dev = cmadev->dev; /*dev->name is mydevname*/
After that dma_alloc_coherent() should alloc memory in physical memory area from 32M + 8M (0x2000000 + 0x800000) up to 0x27FFFFF.
But unfortunately I have problem with this solution. Maybe my cmdline has error?
Next one try was device tree implementation.
cmadev_region: mycma {
/*no-map;*/ /*DMA coherent memory*/
/*reusable;*/
reg = <0x02000000 0x00100000>;
};
And phandle in some node:
memory-region = <&cmadev_region>;
As I saw in kernel usual it should be used like:
of_find_node_by_name(); //find needed node
of_parse_phandle(); //resolve a phandle property to a device_node pointer
of_get_address(); //get DT __be32 physical addresses
of_translate_address(); //DT represent local (bus, device) addresses so translate it to CPU physical addresses
request_mem_region(); //reserve IOMAP memory (cat /proc/iomem)
ioremap(); //alloc entry in page table for reserved memory and return kernel logical addresses.
But I want use DMA via (as I know only one external API function dma_alloc_coherent) dma_alloc_coherent() instead IO-MAP ioremap(). But how call
start_cma_virt = dma_alloc_coherent(dev->cmadev, (size_t)size_cma, &start_cma_dma, GFP_KERNEL);
associate memory from device-tree (reg = <0x02000000 0x00100000>;) to dev->cmadev ? In case with cmdline it is clear it has device name and addresses region.
Does reserved memory after call of_parse_phandle() automatically should be booked for your special driver (which parse DT). And next call dma_alloc_coherent will allocate dma area inside memory from cmadev_region: mycma?

Answer 1

To use dma_alloc_coherent() on reserved memory node, you need to declare that area as dma_coherent. You can do some thing like:
In dt:
cmadev_region: mycma {
compatible = "compatible-name"
no-map;
reg = <0x02000000 0x00100000>;
};
In your driver:
struct device *cma_dev;
static int rmem_dma_device_init(struct reserved_mem *rmem, struct device *dev)
{
int ret;
if (!mem) {
ret = dma_declare_coherent_memory(cma_dev, rmem->base, rmem->base,
rmem->size, DMA_MEMORY_EXCLUSIVE);
if (ret) {
pr_err("Error");
return ret;
}
}
return 0;
}
static void rmem_dma_device_release(struct reserved_mem *rmem,
struct device *dev)
{
if (dev)
dev->dma_mem = NULL;
}
static const struct reserved_mem_ops rmem_dma_ops = {
.device_init = rmem_dma_device_init,
.device_release = rmem_dma_device_release,
};
int __init cma_setup(struct reserved_mem *rmem)
{
rmem->ops = &rmem_dma_ops;
return 0;
}
RESERVEDMEM_OF_DECLARE(some-name, "compatible-name", cma_setup);
Now on this cma_dev you can perform dma_alloc_coherent and get memory.