RE: RFC Need advice on reworking gpio-ep93xx.c to DT support

[Hartley Sweeten <HartleyS@xxxxxxxxxxxxxxxxxxx>]

On Monday, March 22, 2021 9:00 AM, Alexander Sverdlin wrote:
> On Mon, 2021-03-22 at 15:43 +0000, Hartley Sweeten wrote:
>>> Showing only ports A, F and no interrupt capable C:
>> 
>> Are you just going to drop the other GPIO ports?
>> 
>> The EP93xx has eight 8-bit ports total (Ports A-H). Only 3 port support
>> interrupts: A B and F. Ports A and B share a single interrupt and port F
>> has an interrupt for each pin.
>> 
>> Depending on the chip type  (01, 02, 07, 12, or 15) not all the GPIOs are
>> pinned out due to the chip pin count. But the registers exist so the current
>> GPIO support always registers all the ports.
>> 
>> Note that the GPIO banks are registered a bit goofy, Ports C and F are 
>> not in order. They have been that way since the original Cirrus "crater"
>> code base. If I remember correctly this was somewhere back in the 2.6.x
>> kernel. Please make sure the GPIO numbers stay the same so that any
>> userspace code does not break.

> I'm sceptical about this DT convertion.

I'm in the same boat. One of the reasons I have not tried to convert it...

> Not only will it make the kernel bigger (back then is was a concern), but
> they also do not guarantee any order of GPIOs with DT:
>
> https://patchwork.kernel.org/comment/24009887/

I bigger kernel can be an issue for some ep93xx boards. Depending on the SDRAM
chip size and how they are connected, some boards can have as small as a 2MiB
segment size. That limits the size of kernel that can be loaded. I had a lot of issues
with a TS-7200 board booting until I figured that out.

Hartley