how to understand devm_regmap_init_mmio function in kernel?

DeanAnderson · 11-13-2019, 09:14 AM

Hi
I am newbie in kernel linux programming and I am investigating probe function in one of kernel driver.
In device tree we have node:

Code:

reg = <0xF0001000 0x3C0>;  //start addr + size
reg-names = "some_name";

In kernel module at the beginning of file:

Code:

struct Data {
         ....
	void __iomem *pReg;
	struct regmap *pRegMap;
          ...
}
stuct Data* pData;

static const struct regmap_config regCfg= {
	.reg_bits	= 32,
	.reg_stride	= 4,
	.val_bits	= 32,
	.fast_io	= true,
};

In probe in kernel module:

Code:

pData= devm_kzalloc(...  //allocate memory for pData
struct resource* res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "some_name"); //go through pdev->resource and find "some_name"
pData->pReg = devm_ioremap(dev, res->start, resource_size(res)); //my understanding is that this one maps physical register address into virtual kernel address space, so beginning from now You can access by pData->pReg[3]  so it is similar to mechanism used in user space with mmap
pData->pRegmap = devm_regmap_init_mmio(dev, pData->pReg, &regCfg); //this unfortunately I ma not able to understand

Does anyone know what does function:
"devm_regmap_init_mmio"
do? I am not able to understand even looking at google. I don't see a need of existence such function, as I think after ioremap we can access to pData->pReg directly by indexing or initialize memory with some memset(&pData->pReg,...) or equivalent function in kernel, is it true?

It seems that devm_regmap_init_mmio is not needed at all if you plan to use iowrite32, ioread32 api on __iomem pointer. Above instead the idea was to use regmap_write, regmap_read api. For this API you have to firstly do some kind of initialization and that is why there is devm_regmap_init_mmio. Also it is not recommended practise to iterate through index __iomem pointer like in mmap case.

smallpond · 11-17-2019, 04:19 PM

The function sets up memory-mapped I/O. It is documented in the header file https://elixir.bootlin.com/linux/lat.../regmap.h#L943 If you prefer to use io instructions, you are free to do so.

DeanAnderson · 11-18-2019, 09:53 AM

and what is the advantage of using memory mapped io than iowrite/read32 API? I saw only one that write/reads are secured with spinlock. Apart from there is different API to write and read and I don't see any other advantage.