[U-Boot] [PATCH RFC] imx: add multi-architecture README

[Eric Nelson <eric.nelson@boundarydevices.com>]

Hi Tapani,

On 11/12/2013 04:05 AM, Tapani wrote:
>
> Thank you Eric,
>
< snip >

>>
>> and paste it into the spot where it's used:
>> 	MX6_PAD_DEF(SD2_DAT1__USDHC2_DAT1)
>>
>> (As opposed to having to hand-edit to remove the MX6Q_PAD_ prefix
>> from one of the declarations)
>>
>>> Technical point: Could you clarify how this approach scales? There are still
>>> new imx6 models to be released (imx6-next).
>>>
>>
>> It's always tough to tell until we hash through the reference manuals.
>>
>
> Maybe you misunderstood me. We both know there are many more iMX6 CPUs
> coming next year (and they are under NDA; but the public Freescale roadmap
> confirms a imx6-next line, so we can safely say that stuff is coming).
>
> My point was that the approach we take to make multi-variant code now
> should be scalable to add additional variants in the future. Assuming the
> new variants are compatible enough, but maybe requiring their own padconfs.
>
> So we have families of cpus like (just an example): { iMX6Q, iMX6D },
> { iMX6S, iMX6DL }, { iMX6SL }, { iMX6x1, iMX6x2 }, { iMX6x3, iMX6x4 }, ...
> We should be able to support multi-variant over as many families of cpus
> as possible, without having another macro-fu nightmare when we need to
> support another new CPU group.
>
> My worry was that your approach could grow in complexity when a hypothetical
> iMX6X cpu is to be supported.
>

I think this scales linearly with the number of CPU variants.

I'm also not sure that any of the new processors will be pin-compatible,
which is really the only time this effort helps.

< snip >

>>>
>>> We have suggested an alternative solution, but somehow nobody seem
>>> to notice. We avoid almost all the preprocessor messing, and have the
>>> definitions only once. And it would scale for even more cpu types.t
>>> Admittedly, the drawback is duplicate padconf macro definitions
>>> (or having to convert the existing boards padconfigs).
>>>
>> Somehow I missed it. What I recall involved duplicating code and data.
>>
>> Can you explain?
>>
>
> Ok, for the third time :-) But this time combined with some of your suggestions.
>
> In mx6_pins.h we can do the 20 lines of macro-fu you suggested, to create
> the enums for MX6Q_PAD_x + MX6DL_PAD_x or MX6_PAD_x, depending on CPUs to
> support. Or use separate includes with duplicate padconf definitions.
>
> Anyway assume MX6Q_PAD_x and MX6DL_PAD_x definitions are in scope.
>
>
> Now somewhere (board file? mx6_pins.h?) we need one helper macro:
>
> #define IMX6_SET_PAD(p) \
> 	if ( is_mx6q ) \
> 		mxc_iomux_v3_setup_pad(MX6Q_##p); \
> 	else \
> 		mxc_iomux_v3_setup_pad(MX6DL_##p)
>
>
> Using this macro a pad can be set in code, and no need for tables!
> (The compiler will do a good job on that, don't worry about the resulting code)
>
> For instance, a board file can now initialize UART1 with:
>
> static __init void edm_cf_imx6_init_uart(void) {
>          IMX6_SET_PAD( PAD_CSI0_DAT10__UART1_TXD );
>          IMX6_SET_PAD( PAD_CSI0_DAT11__UART1_RXD );
>          IMX6_SET_PAD( PAD_EIM_D19__UART1_CTS );
>          IMX6_SET_PAD( PAD_EIM_D20__UART1_RTS );
>
> 	imx6_add_uart(0, NULL);
> }
>

I didn't catch that you were using a global (local) variable
for is_mx6q so the compiler can optimize this, but I would still
prefer an array and a single call to imx_iomux_v3_setup_multiple_pads().

I would also suggest re-visiting the pad setting in each block
(uart, SD, i2c, etc) and consider a single block of pad setup,
regardless of whether it's done with a table or individual calls.

> An example of this architecture is in our kernel board file:
> https://github.com/TechNexion/linux/blob/imx-3.0.35-4.1.0/arch/arm/mach-mx6/board-edm_cf_imx6.c
>
> Using this method the boardfile part contains less macro hacks, is clearer,
> no tables, and results in shorter C code than your suggestion. (Ok, the
> clearer part could be an opinion.)
>
> Actually, I am not sure this is mutually exclusive with your suggestion.
> (Since we suspect you like your board file as much as we like ours, maybe
> it is better to not mandate how board files should do their pinmuxing?).
>

I actually don't like our board file so much, but don't want to thrash
it without a clear(er) direction.

>
> Anyway, let's work together on this, so we can avoid the maintenance mess the
> iMX6 products otherwise could become.
>

Sounds good.

I'm hoping to get some feedback from at least Fabio and Stefano on this.

They've been notably silent, but I'm sure it's because they're busy...

Regards,


Eric