linux-kernel - Re: Questions about NVMEM

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Date:   Mon, 11 Sep 2017 10:38:25 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Keiji Hayashibara <hayashibara.keiji@...ionext.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        Carlo Caione <carlo@...lessm.com>,
        Michael Grzeschik <m.grzeschik@...gutronix.de>
Subject: Re: Questions about NVMEM



On 11/09/17 05:44, Masahiro Yamada wrote:
> Hi Srinivas,
> 
> 
> I have 3 questions about the nvmem sybsystem.
> 
> Please correct me if something is missing from my thought.
> 
> 
> 
> 
> (Q1) How to allocate struct nvmem_config?
> 
> I see 3 ways in allocating struct nvmem_config.
> What is a good / bad practice?
> 
> 
>     (A)  Allocate statically in .data section
> 
>              bcm-ocotp.c
>              imx-ocotp.c
>              lpc18xx_eeprom.c
>              lpc18xx_otp.c
>              mxs-octop.c
>              qfprom.c
>              rockchip-efuse.c
>              sunxi_sid
>              vf610-ocotp.c
>              meson-efuse.c
> 
>     (B) devm_kzalloc()
> 
>             imx-iim.c
>             mtk-efuse.c
>             drivers/misc/eeprom/at24.c
> 
>     (C) Stack
> 
>             drivers/thunderbolt/switch.c
> 
> 
> 
> I think (A) is safe only when we know the system has
> just one instance of the device.
> (A) should not be used if two or more instances exist.
> Is this correct?

That is correct.

> 
> 
> I think (B) is wasting memory because nvmem_register()
> copies all members of nvmem_config to nvmem_device.
> nvmem_config is never dereferenced after nvmem_register() finished.
> I do not see much sense to keep it until the driver is detached.
> 
I agree.

> 
> 
> (C) looks reasonable because nvmem_config is pretty small.
> (sizeof(struct nvmem_config) = 104 byte on 64bit systems)
>
Yep, thats much better indeed!


> Several subsystems receive configuration data from stack,
> for example,
> 
>     "struct clk_init_data" in clk drivers,
>     "struct uart_8250_port" in 8250 serial drivers.
> 
> sizeof(struct uart_8250_port) = 528 byte,
> but it is still working in stack.
> 
> 
> 
> 
> 
> (Q2) Is nvmem_config::read_only necessary?
> 
> If .reg_write() callback is set, it is probably writable.
> If .reg_write() is missing, it must be read-only.
> 
> I have no idea when nvmem_config::read_only is useful...

You can mark particular instance of provider as read-only which could be 
specific to board.

reg_write callbacks can be implemented by provider driver, but read-only 
flag would give the flexibility at board level.

> 
> 
> 
> 
> 
> (Q3) The style of  drivers/nvmem/Makefile
> 
> This Makefile looks ugly to me.
> All nvmem drivers are just single file modules.
> Why are they renamed when modules are created?
> 
> For the name-space reason for modules,
> prefix "nvmem-" makes sense to me.
> 
> It is true that adding "nvmem-" prefix is redundant while
> they are located in drivers/nvmem/ directory,
> but renaming in the Makefile is even more annoying to me.
> Having said that, we may not want to churn this.
This is mainly done for consistent module naming.
I prefer to have nvmem- prefix for nvmem modules.

thanks,
srini
> 
> 
> 
> 
>