Re: [PATCH RFC PoC 0/2] platform: different approach to early platform drivers

[Bartosz Golaszewski <bgolaszewski@baylibre.com>]

2018-04-27 14:40 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
> On Fri, Apr 27, 2018 at 1:53 PM, Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>> 2018-04-27 12:18 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>> On Fri, Apr 27, 2018 at 10:57 AM, Bartosz Golaszewski
>>> <bgolaszewski@baylibre.com> wrote:
>>>> 2018-04-27 9:52 GMT+02:00 Arnd Bergmann <arnd@arndb.de>:
>>>>> On Fri, Apr 27, 2018 at 4:28 AM, David Lechner <david@lechnology.com> wrote:
>>>> My patch tries to address exactly the use cases we're facing - for
>>>> example by providing means to probe devices twice (early and late) and
>>>> to check the state we're at in order for users to be able to just do
>>>> the critical initialization early on and then continue with regular
>>>> stuff later.
>>>
>>> Maybe the problem is reusing the name and some of the code from
>>> an existing functionality that we've been trying to get rid of.
>>>
>>
>> I'm not reusing the name - in fact I set the prefix to earlydev_
>> exactly in order to not confuse anyone. I'm also not reusing any code
>> in the second series.
>
> Ok.
>
>>> If what you want to do is completely different from the existing
>>> early_platform implementation, how about starting by moving that
>>> out of drivers/base/platform.c into something under arch/sh/
>>> and renaming it to something with an sh_ prefix.
>>>
>>
>> Yes, this is a good idea, but what about the sh-specific drivers that
>> rely on it? Is including headers from arch/ in driver code still an
>> accepted practice?
>
> I think it's fine here, since we're just move it out of the way and
> there are only very few drivers using it:
>
> drivers/clocksource/sh_cmt.c:early_platform_init("earlytimer",
> &sh_cmt_device_driver);
> drivers/clocksource/sh_mtu2.c:early_platform_init("earlytimer",
> &sh_mtu2_device_driver);
> drivers/clocksource/sh_tmu.c:early_platform_init("earlytimer",
> &sh_tmu_device_driver);
> drivers/clocksource/timer-ti-dm.c:early_platform_init("earlytimer",
> &omap_dm_timer_driver);
> drivers/tty/serial/sh-sci.c:early_platform_init_buffer("earlyprintk",
> &sci_driver,
>
> For timer-ti-dm, it seems like a leftover from old times that can
> be removed. The other four are shared between arch/sh and
> arch/arm/mach-shmobile and already have some #ifdef
> to handle those two cases.
>

I'm also seeing that we also call early_platform_cleanup() from
platform_bus_init(). Any ideas for this one?

>>> Let's just leave the non-DT part out of it by making it sh specific.
>>> Then we can come up with improvements to the current
>>> platform_device handling for DT based platforms that you can
>>> use on DT-based davinci to replace what currently happens on
>>> board-file based davinci systems, without mixing up those
>>> two code paths too much in the base driver support.
>>
>> I don't see why we wouldn't want to unify these two cases. The best
>> solution to me seems having only one entry point into the driver code
>> - the probe() function stored in platform_driver - whether we're
>> probing it from DT, platform data, ACPI or early boot code.
>
> I'd rather keep those separate and would prefer not to have
> that many different ways of getting there instead. DT and
> board files can already share most of the code through the
> use of platform_device, especially when you start using
> device properties instead of platform_data, and the other
> two are rare corner cases and ideally left that way.
>
> The early boot code is always special and instead of making
> it easier to use, we should focus on using it as little as
> possible: every line of code that we call before even
> initializing timers and consoles means it gets harder to
> debug when something goes wrong.
>

I'm afraid I don't quite understand your reasoning. I fully agree that
devices that need to be initialized that early are a rare corner case.
We should limit any such uses to the absolute minimum. But when we do
need to go this way, we should do it right. Having a unified mechanism
for early devices will allow maintainers to enforce good practices
(using resources for register mapping, devres, reusing driver code for
reading/writing to registers). Having initialization code in machine
code will make everybody use different APIs and duplicate solutions. I
normally assume that code consolidation is always good.

If we add a way for DT-based platform devices to be probed early - it
would be based on platform device/driver structures anyway. Why would
we even want to not convert the board code into a simple call to
early_platform_device_register() if we'll already offer this API for
device tree?

Best regards,
Bartosz Golaszewski