linux-kernel - Re: [PATCH] Tracing events with GPIOs

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Date:	Fri, 3 Jan 2014 17:39:34 +0100
From:	Jean-Jacques Hiblot <jjhiblot@...phandler.com>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Jean-Jacques Hiblot <jjhiblot@...phandler.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Tracing events with GPIOs

2014/1/3 Alexandre Courbot <gnurou@...il.com>:
> On Fri, Dec 20, 2013 at 5:40 PM, Jean-Jacques Hiblot
> <jjhiblot@...phandler.com> wrote:
>> 2013/12/20 Alexandre Courbot <gnurou@...il.com>:
>>> On Thu, Dec 19, 2013 at 8:38 PM, Jean-Jacques Hiblot
>>> <jjhiblot@...phandler.com> wrote:
>>>> 2013/12/19 Alexandre Courbot <gnurou@...il.com>:
>>>>> The problems I can see so far:
>>>>>
>>>>> - Using gpiod, GPIOs are not specified as integers, but are typically
>>>>> mapped to a given (device, function) pair (device can be NULL) using
>>>>> device tree/platform data/ACPI and obtained by the corresponding
>>>>> device driver through gpiod_get(). You would need to find a different
>>>>> way to specify GPIOs, maybe using the gpio_chip's label and the GPIO
>>>>> hardware number.
>>>>>
>>>>> - Even if you do so, there is currently no way to arbitrarily obtain a
>>>>> GPIO that has not been explicitly mapped to a (device, function), and
>>>>> IIUC you need to specify the tracing GPIO freely from user-space. This
>>>>> hints that we will need to add a function that is sensibly the same as
>>>>> gpio_request_one() to the gpiod API, but I wonder if that does not
>>>>> defeats the purpose somehow.
>>>>
>>>> This is something I was wondering about for another reason. In many
>>>> cases the GPIOs that are physically available for probing will be
>>>> limited to the GPIOs already assigned a function (backlight control
>>>> for example), others are usually not routed except in eval boards or
>>>> early prototypes. And consequently those GPIOs will be requested by a
>>>> driver long before a probe is set.
>>>> It would be nice not to have to remove the driver to be able to use
>>>> this GPIO  as a probe. Maybe a gpiod_steal() interface and a flag
>>>> indicating that the GPIO can be safely stolen?
>>>
>>> Mmm an explicit way to hijack a GPIO does not sound very safe. Do you
>>> have concrete cases where you need to do so? I guess most boards you
>>> may want to use this patch with would have at least some spare GPIOs
>>> with pins somewhere on the board for this kind of purpose.
>> It's not always true. There are quite a few platforms where GPIOs is a
>> scarce resource (ppc for example). For example, the board I'm working
>> on at the moment is built around a APM powerpc which has only 16
>> GPIOs. Of those 16 GPIOS, some are not routed and most of the others
>> are hidden by the shielding so that I can probe only those that go to
>> external connectors.
>> IMHO it's probably the case for most of the boards that go into a
>> final product where EMI and space constraints are tight.
>> But I agree that's not safe. I thought that maybe a flag indicating
>> when it is safe would help (on my board that would be : ok to use the
>> GPIO that turns on or off the backlight, not ok to use the GPIO that
>> controls the power supply)
>
> Rather than a flag per GPIO or board (my definition of "safe" is
> rather strict ; I'd consider being able to mess with the backlight as
> unsafe), I think I'd be more comfortable with having this as a kernel
> config option, defaulting to 'n', and clearly stating that this can
> potentially allow any GPIO to be hijacked (which is, apart for the
> last point, what you already did). That way people can make an
> informed decision about whether to enable it or not (disabled for
> distro kernels, enabled for the occasional hacker who wants to use
> this feature).
>
> But before doing this, I'd like to make sure we explore every
> possibility to make this safer by design.
I agree and to be honest I'm not comfortable with the hijacker's way.
Maybe I should just stick to the proper way of requesting a GPIO. When
this feature is used,  it'll not be in the field after production but
during the development when building a new kernel is not really an
issue.

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