Re: [PATCH v2 1/5] dt-bindings: gpu: add bindings for the ARM Mali Midgard GPU

From: Guillaume Tucker
Date: Wed Apr 12 2017 - 05:36:32 EST

Hi Neil,

On 12/04/17 09:48, Neil Armstrong wrote:
Hi Guillaume,

On 04/12/2017 10:25 AM, Guillaume Tucker wrote:
Hi Heiko,

On 11/04/17 21:52, Heiko Stübner wrote:
Hi Guillaume,

Am Dienstag, 11. April 2017, 18:40:37 CEST schrieb Guillaume Tucker:
On 03/04/17 09:12, Neil Armstrong wrote:
On 04/02/2017 09:59 AM, Guillaume Tucker wrote:
+Optional:
+
+- clocks : Phandle to clock for the Mali Midgard device.
+- clock-names : Shall be "clk_mali".
+- mali-supply : Phandle to regulator for the Mali device. Refer to
+ Documentation/devicetree/bindings/regulator/regulator.txt for details.
+- operating-points : Refer to
Documentation/devicetree/bindings/power/opp.txt + for details.

Please add :
* Must be one of the following:
"arm,mali-t820"

* And, optionally, one of the vendor specific compatible:
"amlogic,meson-gxm-mali"

with my Ack for the amlogic platform.

It seems to me that as long as the GPU architecture hasn't been
modified (I don't think I've ever encountered such a case) then
it has to be a standard ARM Mali type regardless of the SoC
vendor. So unless a Mali-T820 in the Amlogic S912 SoC is not the
same as a T820 in a different SoC, please forgive me but I don't
understand why a vendor compatible string is needed. My main
concern is that it's going to be very hard to keep that list
up-to-date with all existing Midgard SoC variants. If do we need
to add vendor compatible strings to correctly describe the
hardware then I'm happy to add the amlogic one in my patch v3; I
would just like to understand why that's necessary.

SoC vendors in most cases hook ip blocks into their socs in different
and often strange ways. After all it's not some discrete ic you solder
onto a board, but instead a part of the soc itself.

Thanks for your explanation. I see, it's really about special
things that are not supported by the standard Midgard kernel
driver.

So in most cases you will have some hooks outside the actual gpu iospace
that can be used to tune different things about how the gpu interacts with
the system. Which is probably also the reason the midgard kernel driver
has this ugly "platform" subdirectory for compile-time platform selection.

I see the "platform" directory approach as an old and deprecated
way of supporting platforms, upstreaming the dt bindings goes in
the direction of using solely the device tree to describe the GPU
hardware (i.e. CONFIG_MALI_PLATFORM_DEVICETREE). If something
quirky is needed in the platform, it should be possible to
support it outside the GPU driver (platform devfreq etc...).

If this was so simple...

This is why the "vendor" compatible is optional.

And on another side, the binding were written by ARM, are may not be
compatible with how the mainline Linux handles these uses-cases.

ARM added some tweaks to handle some weird integration using DT properties,
but this should definitely go in platform specific code instead.

OK, sorry I was approaching the issue from a completely different
and somewhat more idealistic angle. My impression is that if the
driver was in mainline then it would be maintained in such a way
that vendor compatible strings would not be required, but this is
all hypothetical.

So in practice, I think I now better understand why vendor
compatible strings may still be needed. And they're optional, so
harmless to other platforms, so it's all fine with me :)

Back to the original intent of enabling distros to make Mali GPU
driver packages, when using the device tree you can have a single
kernel driver package for all Midgard platforms. When using the
third-party platform sources approach, you need to make an extra
package for each one of them.

Well, if the midgard driver was maintained in community-driven way, all vendor
could actually push their platform code, but it's not the case and somehow
never will.


So if there is value in supporting platforms that absolutely
require something special due to their hardware GPU integration,
then yes I see why vendor dt bindings might be useful. However
it seems to me that this should really be an exception and
avoided whenever possible.

I understand you would avoid changing the (horrible) way the midgard
driver handles the DT...

But ultimately, if we consider the eventual integration of the midgard
driver in mainline kernel (because why not ?), the driver would need
a complete re-write to conform the actual driver coding structure,
and so would also need to conform to the DT mainline rules.

Yes, it's hard to have a discussion without a mainline driver to
back things up. So I understand the pragmatic approach here.

On my rk3288 for example we have [0] in the chromeos tree, that handles
the oddities of the midgard on the rk3288 used in a lot of Chromebooks.
There are soc-specific oddities of frequencies, frequency-scaling and
whatnot. And there are also more gpu-specific setting in syscon areas
of the soc (pmu and grf) that can also influence the gpus performance
and might need tweaking at some point.

For the rk3288, this is purely a software implementation issue on
the chromeos-3.14 branch. With mainline kernel, you can use
devfreq and no platform files at all (that's how I tested these
dt bindings). So as far as I know, there's no need for a vendor
compatible string on rk3288.

That doesn't even take into account that there may even be differences
on how things are synthesized that we don't know about. See all the
variants of the dw_hdmi ip block (imx, rockchip, meson [more?]) .

I'm not too familiar with that driver, just had a quick look and
it seems to be a different issue as there's a kernel config for
each platform to build separate driver modules. And it looks
like they are actually needed to cope with variants of the
hardware inside the display processor block, unlike with the Mali
GPU which in principle should always be the same. I've run this
Midgard driver without any platform files and using devfreq at
least on rk3288 Firefly, Exynos 5422 ODROID-XU3 and Juno and they
all have a vanilla Mali GPU hw block. It's just wired
differently in each SoC.

Please, do not consider the "driver" in dt-bindings discussion, the
way it's implemented is irrelevant here, but the way the IP is
integrated is crucial.
And trust me, HW designers and integrators are very very creative
guys, and we *need* some platform code to handle integration corner
cases not handled by the "IP" driver.

Sure, I know this happens... I can't help but think that if
something quirky is done in the SoC around the GPU then it
shouldn't need to be fixed in the GPU driver. But that's talking
about the "driver" again, so not relevant.

In the case of "dw-hdmi", the "driver" acts like a "library" that
can be used differently among the various platforms.

I can talk about the "meson" Amlogic integration, in this particular
case Amlogic used their owm PHY and wrapped all the stuff in a way
we need to handle the PHY+HDMI+Wrapper in a single block and then
connect it to the Video pipeline.

For the midgard integration made by Amlogic, the vendor platform code
uses tweaks to start the IP and initialize the clocks.
And this code is part of the "amlogic,meson-gxm-mali" HW, not the
"arm,midgard".

Can't this be done in the clock driver or some other early init
stage of the SoC? But again, that code is not in mainline so
it's a non-issue.

So we really want to have the special compatibles in place, to be prepared
for the future per-soc oddities that always appear :-) .

How about aiming for the ideal case where vendor-specific things
are not needed and add them if and when they really become
inevitable and worth the cost?

Trust me, an amlogic compatible is needed ;-)

Fair enough, will add the optional vendor compatible string in my
patch v3 :) Glad this was discussed though, thank you.

Guillaume

[0] https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-3.14/drivers/gpu/arm/midgard/platform/rk/