On 4/25/22 1:48 AM, Simon Glass wrote:
Hi Sean,
On Mon, 18 Apr 2022 at 13:37, Sean Anderson sean.anderson@seco.com wrote:
This adds support for "nvmem cells" as seen in Linux. The nvmem device class in Linux is used for various assorted ROMs and EEPROMs. In this sense, it is similar to UCLASS_MISC, but also includes UCLASS_I2C_EEPROM, UCLASS_RTC, and UCLASS_MTD. New drivers corresponding to a Linux-style nvmem device should be implemented as one of the previously-mentioned uclasses. The nvmem API acts as a compatibility layer to adapt the (slightly different) APIs of these uclasses. It also handles the lookup of nvmem cells.
While nvmem devices can be accessed directly, they are most often used by reading/writing contiguous values called "cells". Cells typically hold information like calibration, versions, or configuration (such as mac addresses).
nvmem devices can specify "cells" in their device tree:
qfprom: eeprom@700000 { #address-cells = <1>; #size-cells = <1>; reg = <0x00700000 0x100000>; /* ... */ tsens_calibration: calib@404 { reg = <0x404 0x10>; }; };which can then be referenced like:
tsens { /* ... */ nvmem-cells = <&tsens_calibration>; nvmem-cell-names = "calibration"; };The tsens driver could then read the calibration value like:
struct nvmem_cell cal_cell; u8 cal[16]; nvmem_cell_get_by_name(dev, "calibration", &cal_cell); nvmem_cell_read(&cal_cell, cal, sizeof(cal));Because nvmem devices are not all of the same uclass, supported uclasses must register a nvmem_interface struct. This allows CONFIG_NVMEM to be enabled without depending on specific uclasses. At the moment, nvmem_interface is very bare-bones, and assumes that no initialization is necessary. However, this could be amended in the future.
Although I2C_EEPROM and MISC are quite similar (and could likely be unified), they present different read/write function signatures. To abstract over this, NVMEM uses the same read/write signature as Linux. In particular, short read/writes are not allowed, which is allowed by MISC.
The functionality implemented by nvmem cells is very similar to that provided by i2c_eeprom_partition. "fixed-partition"s for eeproms does not seem to have made its way into Linux or into any device tree other than sandbox. It is possible that with the introduction of this API it would be possible to remove it.
I still think this would be better as a separate uclass, with child devices created at bind time in each of the respective uclasses, like mmc_bind() does. Then you will see the nvmem devices in the DM tree. Wouldn't we want to add a command to access the nvmem devices?
We already do. E.g. the misc/rtc/eeprom commands. The problem is that for software to access them, they would have to use misc_read/dm_rtc_read/ i2c_eeprom_read.
This patch feels like a shortcut to me and I'm not sure of the benefit of that shortcut.
Well, I suppose it's because "nvmem" devices are strict subsets of existing devices. There is no new functionality here (except adapting between semantics like for misc). We should always be able to use the existing API to implement support for a new underlying uclass. There should never be device-specific read/write methods, because we can use the existing read/write uclass methods.
What I'm trying to get at is that we sort of already have an nvmem uclass with nvmem devices, they're just not accessible in a uniform way. This series is trying to address the uniformity aspect. But I don't think we need new devices for each nvmem interface, because all they would do would take up ram/rom.
--Sean
PS. In an ideal world we'd have something like
struct nvmem_ops { read(); write(); };
struct dm_rtc_ops { nvmem_ops nvmem; /* the other ops minus read/write */ };
int nvmem_read (...) { struct nvmem_ops *ops = cell->nvmem->ops; /* ... */
return ops->read(...); }
but unfortunately, we already have fragmented implementations.