linux-kernel - Re: [PATCH 0/1] Request to review progress decoupling vchiq platform code

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Message-ID: <2212368e-b597-b717-0d21-70b24322ca09@i2se.com>
Date:   Tue, 15 Jun 2021 00:06:14 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Ojaswin Mujoo <ojaswin98@...il.com>, nsaenz@...nel.org
Cc:     gregkh@...uxfoundation.org, dan.carpenter@...cle.com,
        phil@...pberrypi.com, bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Request to review progress decoupling vchiq platform
 code

Hi,

Am 14.06.21 um 21:32 schrieb Ojaswin Mujoo:
> Greetings,
>
> I'm working on addressing item 10 of the following TODO list:
>
>     drivers/staging/vc04-services/interface/TODO
>
> For reference, the task is:
>
>     10) Reorganize file structure: Move char driver to it's own file and join
>     both platform files
>
>     The cdev is defined alongside with the platform code in vchiq_arm.c. It
>     would be nice to completely decouple it from the actual core code. For
>     instance to be able to use bcm2835-audio without having /dev/vchiq created.
>     One could argue it's better for security reasons or general cleanliness. It
>     could even be interesting to create two different kernel modules, something
>     the likes of vchiq-core.ko and vchiq-dev.ko. This would also ease the
>     upstreaming process.
>
>
> This patch is the first step towards decoupling the platform and the cdev code.
> It moves all the cdev related code from vchiq_arm.c to vchiq_dev.c. However, for
> now, I have aimed to keep the functionality untouched, hence the platform code
> still calls the cdev initialisation function, and isn't truly decoupled yet.
>
> The summary of the changes is as follows:
>
>
>  *  Definition of functions and variables shared by cdev and platform
>     code are moved to vchiq_arm.h while declaration stays in vchiq_arm.c
>
>  *  Declaration and definition of functions and variables only used by
>     cdev code are moved to vchiq_dev.c file.
>
>  *  Defined vchiq_deregister_chrdev() and vchiq_register_chrdev(..) in
>     vchiq_dev.c which handle cdev creation and deletion. They are called by the
>     platfrom code during probe().
looks like this should be 3 separate patches. So you have the pure move
at the end.
>
>
> I mainly wanted to put this patch out to see if I have the right idea of the
> task at hand and to ensure I'm heading into the right direction. I would love to
> hear your thoughts and suggestions on this. Once I have some feedback on this, I
> can accordingly work towards a newer version to completely decouple the code. 
>
> Lastly, I had some questions related to the the task: 
>
> 1. So regarding the following line in the TODO:
>
>     "For instance to be able to use bcm2835-audio without having /dev/vchiq
>     created." 
>
>   I was wondering about the possible ways to achieve this. Specifically, I was
>   thinking of the following 2 ways:
>
>   1.1  Making a KConfig entry for Cdev creation, like CONFIG_VCHIQ_CDEV, and
>        then do something like:
>
>          vchiq_probe(..) 
>          {
>            /* platform init code */
>
>            #if defined(CONFIG_VCHIQ_CDEV)
>
>            /* Call cdev register function */
>
>            #endif 
>          }
A common pattern is to keep the calls, but have "empty" definitions of
the those functions in the header file in case CONFIG_VCHIQ_CDEV is not
defined.
>
>   1.2  The second approach is creating an entirely separate module for the cdev,
>        as suggested in the TODO. 
>
>   So I'm just wondering what the right approach should be?
>
> 2. Second, I currently tested by installing my patches to a pi3 B+ and running
>    `cat /dev/vchiq` to compare the output with the original kernel.  Also, to
>    see if the platform code works without the cdev code, I commented out the
>    call to vchiq_register_cdev() and made sure the platform device (and
>    children) was registered but the char device was not present. However, I'm
>    not sure if these tests are comprehensive enough. What would be the right way
>    to test my changes?

Sounds okay, but a functional test is still necessary (tool is provided
by Raspberry Pi OS):

vchiq_test -f 10
vchiq_test -p 10

Regards
Stefan