Re: [PATCH] serial: 8250: Add support for using platform_device resources

From: Esben Haabendal
Date: Mon May 06 2019 - 13:40:57 EST

> On Mon, May 06, 2019 at 05:46:56PM +0200, Esben Haabendal wrote:
>> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> writes:
>> >> > On Wed, May 01, 2019 at 09:17:37AM +0200, Esben Haabendal wrote:
>> >> >> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> writes:
>> As an example, the sm501.c driver, the only driver in drivers/mfd/ which
>> uses serial8250 driver, does not use any code from mfd-core.
>> Incidentally, it is 1 year older than mfd-core.c, and as never been
>> refactored to use mfd-core functionality.
> So, sm501.c should not request resources for its children. This as simple as
> that.

Funny thing. Even though sm501.c does not use mfd-core at all, it does
request resources for all its child devices, except for the uart

sm501_register_usbhost(), sm501_register_display() and
sm501_register_gpio() all creates/requests resources. But
sm501_register_uart() does not.

How many concrete examples are needed to convince you that what I am
trying to do is how it is done everywhere else (than
serial8250_core.c/serial8250_port.c, even in other 8250_*.c drivers)
(obviously not 100% true, there are ofcourse other pieces of code not
working well with resource management) ?

> What you are trying to do here is a hack workaround on the current
> behaviour in the Linux device model (resource management) as I told
> you already.

No. If it was, then all (most) mfd drivers added after 2008 are hacky
workarounds, because the use mfd_add_devices().

There are currently 53 drivers in drivers/mfd/ that calls
mfd_add_devices() with one or more cells with resources attached.

Are they all hacky workarounds?

I am not trying to do anything that they are not already doing.

>> > Why not? Again, *slicing* resources is OK and that's what MFD for,
>> > *requesting*
>> > them in the parent is not.
>> Why we cannot use request_mem_region() for those memory resources again?
> Because it's how it was designed. "One device per one resource". If you would
> like to fix this, it should be done obviously not in 8250 driver or any other
> driver, but driver core.
> Nevertheless there is one particular exception here,

I am not trying to fix the problem of having multiple drivers owning the
same resource. I am just trying to make serial8250 driver behave so it
can use the resources that it is handed by mfd-core.

This really is how it (mfd and also device resource management) is
designed. I am not inventing anything, or making a workaround.

Actually, you should take a look at the following specialized 8250

They all use platform_get_resource(), and will work nicely with mfd.
And of-course, none of them use request_mem_region().

So, if you want to insist that I create a clone of the current standard
serial8250 driver (serial8250_isa_driver in 8520_core.c), even though I
want absolutely nothing specialized, just need it to play nicely with
platform_get_resource(), what should I call the driver? 8520_plat.c ?

>> It fails because the resources are now already owned the mfd driver, on
>> behalf of the child.
> Yes. Behaves in order how it's implementer. No issues here.

If that was the case, then mfd-core would be implemented in order to not
work with existing platform drivers. There definitely is an issue
here. And it is in 8250_core.c and 8250_port.c.

>> > Nope, *requesting* resources as you mentioned lock them to the certain user.
>> I still think there is some confusion in relation to your use of the
>> word "requesting". There is no explicit request/lock action in
>> kernel/resource.c.
> You have to check IORESOURCE_BUSY. It seems that what you missed in your
> picture.

Point taken. I haven't put much focus on that. But I don't see how
that is going to help making use of serial8250_isa_driver in combination
with mfd_add_devices(). I am not creating/requesting the resources.
That is done by mfd_add_device(), which I fail to see why I would need
to change.

> I didn't comment the rest until we will figure out the IO resource management
> in general.

I believe all my comments were related to this same resource management