[U-Boot] [PATCH 38/60] ARM: tegra: remove tegra_get_chip()

[Simon Glass]

Hi Stephen,

On 25 April 2016 at 13:25, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 04/23/2016 11:14 AM, Simon Glass wrote:
>>
>> Hi Stephen,
>>
>> On 19 April 2016 at 14:59, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>
>>> From: Stephen Warren <swarren at nvidia.com>
>>>
>>> U-Boot is compiled for a single board, which in turn uses a specific SoC.
>>> There's no need to make runtime decisions based on SoC ID. While there's
>>> certainly an argument for making the code support different SoCs at
>>> run-time, the Tegra code is so far from that possible ideal that the
>>> existing runtime code is an anomaly. If this changes in the future, all
>>> runtime decisions should likely be based on DT anyway.
>>>
>>> Signed-off-by: Stephen Warren <swarren at nvidia.com>
>>> ---
>>>   arch/arm/mach-tegra/ap.c               | 106
>>> ++++++++++-----------------------
>>>   arch/arm/mach-tegra/cache.c            |  20 +++----
>>>   arch/arm/mach-tegra/cpu.c              |  16 ++---
>>>   arch/arm/mach-tegra/cpu.h              |   6 --
>>>   arch/arm/mach-tegra/tegra20/warmboot.c |  20 ++-----
>>>   5 files changed, 51 insertions(+), 117 deletions(-)
>>
>>
>> What exactly is missing to prevent multi-arch support?
>
>
> In a word: everything:-)
>
> Pretty much all decisions in core architecture code, core Tegra code,
> drivers, and even board files are currently made at compile time. For
> example, consider drivers where the register layouts are different between
> different SoCs; not just new fields added, but existing fields moved to
> different offsets. Right now, we handle this by changing the register struct
> definition at compile time. To support multiple chips, we'd have to either
> (a) link in n copies of the driver, one per register layout, or (b) rework
> the driver to use #defines and runtime calculations for register offsets,
> like the Linux kernel drivers do. Tegra USB is one example. The pinmux and
> clock drivers have a significantly different sets of pins/clocks/resets/...
> per SoC, and enums/tables describing those sets are currently configured at
> compile time. Some PMIC constants (e.g. vdd_cpu voltage) are configured at
> compile-time, and even differ per board.

I wonder how far we would get by converting clock, pinctrl, reset to
driver model drivers?

>
>> Shouldn't we head towards that rather than making it harder?
>
>
> I don't see any need for that, no.
>
> U-Boot is built for a specific board (or in some cases a set of extremely
> closely related set of boards, such as the RPI A/B/A+/B+). There's no need
> to determine almost anything at run-time since almost all information is
> known at compile time, with exceptions such as standardized enumerable buses
> such as USB, PCIe. If we support multiple HW in a single binary, it gets
> bloated with code that simply isn't going to be used, since all the extra
> code is either for a platform that the build won't be installed on (e.g.
> clock/pinmux tables), or is overhead to add runtime detection of which block
> of code to use, which simply isn't needed in the current model.

It's not so much that. Presumably a build for a particular board would
not include support for and SoC it doesn't have. But it is still
useful to build the code. For example it would be nice to have an
overall Tegra build that enables all SoCs to avoid building every
board.

So it is a serious question. I suspect the main impediment may be
moving the clock and other core stuff to driver model.

>
> In my opinion, firmware/bootloaders run on a single specific board, whereas
> full-featured operating systems support multiple systems.

Except when the boards are pretty similar. Also, doesn't barebox have
only one build for Tegra?

>
> As an aside, I've wondered whether U-Boot should be split into multiple
> parts; one HW-specific binary providing various drivers (e.g. via DM-related
> APIs?) and the other containing just high-level user-interface code such as
> the shell, high-level USB/... protocols, which would only call into those
> APIs. Still, I don't think we're anywhere close to that, and I'm not aware
> that it's a goal of the project at the moment.

Well it gets built as one binary, but there's a pretty clear
separation in the code, at least with driver model. What's the purpose
of this?

Regards,
Simon