[U-Boot] [PATCH] mx6: Fix the reading of CPU revision

[Eric Nelson]

Hi Fabio,

On 03/27/2013 06:37 AM, Fabio Estevam wrote:
> On Wed, Mar 27, 2013 at 5:57 AM, Dirk Behme <dirk.behme at de.bosch.com> wrote:
>
>> Some additional rethinking: I missed that we have a Linux kernel, too ;)
>>
>> c) It's the job of the Linux kernel to export the CPU revision to the VPU
>> library. In case the Linux kernel completely ignores what we are doing in
>> U-Boot and calculates the CPU revision itself (*), e.g. by something like
>>
>> http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/tree/arch/arm/mach-mx6/mm.c?h=imx_3.0.35_1.1.0&id=207f80453c77bc32e04b5fef863f6fe50a7fd1a8#n60
>>
>> we can do anything in U-Boot. Independent of the VPU library.
>
> Unfortunately VPU library relies on the bootloader to pass the correct
> silicon revision.
>

The VPU library relies on the output of /proc/cpuinfo (specifically
the line beginning with "Revision".

The snippet (from vpu_io.h) is:

	tmp = strstr(buf, "Revision");
	if (tmp != NULL) {
		rev = index(tmp, ':');
		if (rev != NULL) {
			rev++;
			system_rev = strtoul(rev, NULL, 16);
			ret = 0;
		}
	}

This code should really be changed, so we don't have to carry this
data all the way from boot loader to /proc/cpuinfo.

Similar (but different) code is present in mxc_ipu_hl_lib.c
for the IPU.

In the case of the VPU library, it seems more sane to have the
VPU driver expose the particular IP revision present on the
system.

> Eric's tested passing 0 as get_cpu_rev and showed that VPU simply
> cannot work on this case.
>
>> In this case I'd propose to just keep Troy's version of get_cpu_rev() as it
>> is [1].
>
> This is proven to not to work with mx6solo and VPU, so we need the fix
> I proposed.
>
> Here is what I am planning to do:
>
> 1. Send a v2 of this patch with the small correction pointed out by Eric
> 2. Include a weak function to pass get_cpu_rev in common mx6 code
>
> Then on top of that, one can send a patch that prints the mx6 silicon
> strings by differentiating between a mx6dual-lite and mx6solo, if it
> is worth.
>

It seems a reasonable interim solution to provide backward
compatibility until the kernel driver(s) and userspace can be
fixed.

Another way of doing this that prevents get_cpu_rev() from
hiding the precise CPU is to do this in the "weak" version
of get_board_rev().

Regards,


Eric