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

[Neil Armstrong]

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.

> 
> 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.

> 
>> 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.

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".

> 
>> 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 ;-)

Neil

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