linux-kernel - Re: [RFC PATCH 2/3] mxc: move serial driver init()/exit() to platform

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Date:	Wed, 25 Feb 2009 11:19:45 +0200
From:	Paulius Zaleckas <paulius.zaleckas@...tonika.lt>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	greg@...ah.com, s.hauer@...gutronix.de,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] mxc: move serial driver init()/exit() to platform_device

Russell King - ARM Linux wrote:
> On Tue, Feb 24, 2009 at 05:57:37PM +0200, Paulius Zaleckas wrote:
>> Signed-off-by: Paulius Zaleckas <paulius.zaleckas@...tonika.lt>
>> ---
>>
>>  arch/arm/mach-mx1/devices.c |   43 ++++++++++++++
>>  arch/arm/mach-mx1/mx1ads.c  |   45 ---------------
>>  arch/arm/mach-mx2/mx27ads.c |  131 -------------------------------------------
>>  arch/arm/mach-mx2/pcm038.c  |   64 ---------------------
>>  arch/arm/mach-mx2/serial.c  |  127 ++++++++++++++++++++++++++++++++++++++++++
>>  5 files changed, 170 insertions(+), 240 deletions(-)
>>
>>
>> diff --git a/arch/arm/mach-mx1/devices.c b/arch/arm/mach-mx1/devices.c
>> index a956441..5fd4ee3 100644
>> --- a/arch/arm/mach-mx1/devices.c
>> +++ b/arch/arm/mach-mx1/devices.c
>> @@ -26,6 +26,7 @@
>>  
>>  #include <mach/irqs.h>
>>  #include <mach/hardware.h>
>> +#include <mach/iomux-mx1-mx2.h>
>>  
>>  static struct resource imx_csi_resources[] = {
>>  	[0] = {
>> @@ -96,11 +97,32 @@ static struct resource imx_uart1_resources[] = {
>>  	},
>>  };
>>  
>> +static int mxc_uart1_pins[] = {
>> +	PC9_PF_UART1_CTS,
>> +	PC10_PF_UART1_RTS,
>> +	PC11_PF_UART1_TXD,
>> +	PC12_PF_UART1_RXD,
>> +};
>> +
>> +static int uart1_mxc_init(struct platform_device *pdev)
>> +{
>> +	return mxc_gpio_setup_multiple_pins(mxc_uart1_pins,
>> +			ARRAY_SIZE(mxc_uart1_pins), "UART1");
>> +}
>> +
>> +static void uart1_mxc_exit(struct platform_device *pdev)
>> +{
>> +	mxc_gpio_release_multiple_pins(mxc_uart1_pins,
>> +			ARRAY_SIZE(mxc_uart1_pins));
>> +}
>> +
>>  struct platform_device imx_uart1_device = {
>>  	.name		= "imx-uart",
>>  	.id		= 0,
>>  	.num_resources	= ARRAY_SIZE(imx_uart1_resources),
>>  	.resource	= imx_uart1_resources,
>> +	.init		= uart1_mxc_init,
>> +	.exit		= uart1_mxc_exit,
> 
> I really don't like this approach to controlling multiplex pins, which
> is to setup the SoC pin configuration when the driver is being bound and
> to remove it when the driver is unbound.
> 
> Let's take the issue of a serial driver.
> 
> The outputs of a serial port have defined inactive levels - for TXD, RTS
> and DTR, that's logic one.  If a driver is not loaded and you have a
> peripheral connected to this port, you probably don't want them to see
> a break level on TXD, or active RTS or DTR signal.
> 
> However, by hooking the SoC pin configuration into the binding and
> unbinding of the driver, the state of the pins are indeterminent until
> the driver is initialised.
> 
> I can think of other cases in hardware I've dealt with which required
> careful handling of SSP signals to ensure that a flip-flop in a FPGA is
> correctly set to ensure that left/right channels aren't swapped.
> 
> Basically, my point is that for 99.9% of the time, SoC pin configuration
> is determined by the platform board layout, and the right place to set
> this configuration up is in the board support file, just like we do on
> PXA platforms.

I see your point and have to agree with you.
After all that is why it was RFC!
It was quick idea by looking at MXC drivers and amount of platform_data
with init()/exit()...
Now it seems for me, like this was just bad approach.

Thanks for comments!

> For the 0.1% of cases where a board needs to manipulate the SoC pin
> configuration depending on which drivers are loaded, doing so at driver
> probe time _may_ make sense, but it feels all together cumbersome,
> especially as unloading drivers has historically had question marks
> over it.
> 
> Surely, for this 0.1% of cases, the right solution would be to have an
> interface which allows a platform device to be unregistered, the SoC pin
> mux settings changed by platform code, and the new device registered?
> 
> Finally, note that the approach of putting init/exit into struct
> platform_device doesn't cater for all cases - we're still going to need
> to have init/exit pointers in platform data for some platform devices,
> such as MMC drivers, which have to pass parameters to those hooks.
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