Re: How to pass software configuration to driver built as module in boot up phase

[Tingwei Zhang <quic_tingweiz@quicinc.com>]

On 10/31/2023 2:58 PM, Greg Kroah-Hartman wrote:
> On Tue, Oct 31, 2023 at 10:56:23AM +0800, Tingwei Zhang wrote:
>> On 10/30/2023 7:30 PM, Greg Kroah-Hartman wrote:
>>> On Mon, Oct 30, 2023 at 07:21:48PM +0800, Tingwei Zhang wrote:
>>>> Hi Masami, Greg and Rafael,
>>>>
>>>> I have one driver that needs a bunch to software configuration like 1000
>>>> settings. These settings can be different according to the use case.
>>>
>>> That seems very odd, what type of driver is this?
>> The example use case is below DCC driver which is a DMA engine. It can be
>> configured to read/write registers while whose registers need to be
>> configured by software. It's mainly used to debug device crash issue.
>> https://lore.kernel.org/lkml/cover.1691496290.git.quic_schowdhu@quicinc.com/T/
> 
> Given that no one listened to my last review cycle of that driver, why
> are you asking us again?
> 

There's some change on developer owner of DCC driver. We preparing and 
reviewing the driver internally.  It will be submitted once ready.

>>>> They are required in boot up phase so we can't reply on user space to make
>>>> the configuration.
>>>>
>>>> Boot config is not preferred since we'd like to build that driver as dynamic
>>>> load Kernel module.
>>>>
>>>> Could you let me know if there's any mechanism in Kernel to do that?
>>>
>>> What is wrong with the existing ways of doing this that all other
>>> drivers use?
>> I'm not aware of all the existing ways. Below ways are what I know and they
>> may not be suitable for some reason.
>> 1. Let user space to configure with sysfs/debugfs/ioctl interface. Since we
>> need this configuration to be done in early stage before user space is up.
> 
> I really doubt that you need this before userspace starts, as that's not
> what "device crash" stuff should be dealing with.  That happens after
> init starts, because you have a working kernel (i.e. this is not for
> hardware bringup.)
> 
>> This doesn't work.
> 
> Why do you need this before init?  Specific reasons please.
> 
There are some case that some driver broke Kernel in development phase 
and those bad driver could cause device crash in very early boot up phase.
>> 2. Use device tree to pass the configuration to driver. Since the
>> configuration is software configuration instead of hardware descirption. It
>> doesn't fit in device tree.
>> 3. Boot config or commandline parameter. If the driver is built in, it can
>> work. But it doesn't work for dynamically loadable Kernel module.
> 
> If you have a module, then you can do this after init starts, so you
> are reporting conflicting "requirements" here, which make this
> impossible for us to understand :(

You are correct.  I missed that part. We should already have user space 
up when install the module.

> 
> Please work with the kernel developers at your company to come up with a
> solution for this, you have the experience, do not require the community
> to do your work for you.
> 
> thanks,
> 
> greg k-h

-- 
Thanks,
Tingwei