* Question about regmap_range_cfg and regmap_mmio
@ 2020-02-28 17:47 Lars Möllendorf
0 siblings, 0 replies; only message in thread
From: Lars Möllendorf @ 2020-02-28 17:47 UTC (permalink / raw)
To: linux-newbie
Hi,
this mail is copied from internal issue written in markdown - I hope
this is still readable as mail.
I am referring to kernel sources v4.9.87 but I think all my assumptions
do still apply to current kernel versions.
[regmap.h](https://elixir.bootlin.com/linux/v4.9.87/source/include/linux/regmap.h#L334)
contains:
```c
/**
* Configuration for indirectly accessed or paged registers.
* Registers, mapped to this virtual range, are accessed in two steps:
* 1. page selector register update;
* 2. access through data window registers.
*
* @name: Descriptive name for diagnostics
*
* @range_min: Address of the lowest register address in virtual range.
* @range_max: Address of the highest register in virtual range.
*
* @page_sel_reg: Register with selector field.
* @page_sel_mask: Bit shift for selector value.
* @page_sel_shift: Bit mask for selector value.
*
* @window_start: Address of first (lowest) register in data window.
* @window_len: Number of registers in data window.
*/
struct regmap_range_cfg {
const char *name;
/* Registers of virtual address range */
unsigned int range_min;
unsigned int range_max;
/* Page selector for indirect addressing */
unsigned int selector_reg;
unsigned int selector_mask;
int selector_shift;
/* Data window (per each page) */
unsigned int window_start;
unsigned int window_len;
};
```
Unfortunately this seems not to work for MMIO devices.
In
[`__regmap_init()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L711)
[`_regmap_bus_reg_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L2330)
is assigned to
[`regmap.reg_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/internal.h#L101)
if `!bus->read || !bus->write`, else
[`_regmap_bus_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L2338)
is assigned:
```c
if (!bus) {
map->reg_read = config->reg_read;
map->reg_write = config->reg_write;
map->defer_caching = false;
goto skip_format_initialization;
} else if (!bus->read || !bus->write) {
map->reg_read = _regmap_bus_reg_read;
map->reg_write = _regmap_bus_reg_write;
map->defer_caching = false;
goto skip_format_initialization;
} else {
map->reg_read = _regmap_bus_read;
map->reg_update_bits = bus->reg_update_bits;
}
```
[`_regmap_bus_reg_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L2330)
calls the `reg_read` function of the bus directly,
[`_regmap_bus_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L2338)
instead calls `_regmap_raw_read()`:
```c
static int _regmap_bus_reg_read(void *context, unsigned int reg,
unsigned int *val)
{
struct regmap *map = context;
return map->bus->reg_read(map->bus_context, reg, val);
}
static int _regmap_bus_read(void *context, unsigned int reg,
unsigned int *val)
{
int ret;
struct regmap *map = context;
if (!map->format.parse_val)
return -EINVAL;
ret = _regmap_raw_read(map, reg, map->work_buf, map->format.val_bytes);
if (ret == 0)
*val = map->format.parse_val(map->work_buf);
return ret;
}
```
[`_regmap_raw_read()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap.c#L2297)
in turn calls
[`_regmap_range_lookup()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap-mmio.c#L479)
and
[`_regmap_select_page()`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap-mmio.c#L1283)
which do the paging.
-
[`regmap_mmio`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap-mmio.c#L212)
does neither contain `.read` nor `.write`.
-
[`regmap_i2c`](https://elixir.bootlin.com/linux/v4.9.87/source/drivers/base/regmap/regmap-i2c.c#L204)
does contain both.
My assumption is that paging is not a common use case for Memory-mapped
I/O and thus has not been implemented for this case.
- Are my assumptions correct?
- If so, what would you recommend me to do:
- Continue using `regmap-mmio` and implement my custom paging
functions on top of that?
- Enhance the current `regmap-mmio` implementation so it does paging
and submit a patch?
- Write my own `better-regmap-mmio` implementation?
Thank you,
Lars
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