Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3774.200.91.1.1\))
Subject: Re: [PATCH RFC 01/10] dt-bindings: gpu: Add PowerVR Series5 SGX GPUs
From:   "H. Nikolaus Schaller" <hns@goldelico.com>
In-Reply-To: <20231206-wolverine-paprika-0674ca01e1f2@spud>
Date:   Wed, 6 Dec 2023 22:43:27 +0100
Cc:     Andrew Davis <afd@ti.com>, Frank Binns <frank.binns@imgtec.com>,
        Donald Robson <donald.robson@imgtec.com>,
        Matt Coster <matt.coster@imgtec.com>,
        Adam Ford <aford173@gmail.com>,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Chen-Yu Tsai <wens@csie.org>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Samuel Holland <samuel@sholland.org>,
        =?utf-8?Q?Beno=C3=AEt_Cousson?= <bcousson@baylibre.com>,
        Tony Lindgren <tony@atomide.com>, Nishanth Menon <nm@ti.com>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Tero Kristo <kristo@kernel.org>,
        Paul Cercueil <paul@crapouillou.net>,
        dri-devel@lists.freedesktop.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-sunxi@lists.linux.dev, linux-omap@vger.kernel.org,
        linux-mips@vger.kernel.org
Content-Transfer-Encoding: quoted-printable
Message-Id: <07CB504D-F4B7-425A-A24C-D0DC079ED48E@goldelico.com>
References: <20231204182245.33683-1-afd@ti.com>
 <20231204182245.33683-2-afd@ti.com>
 <CFF198DA-5C42-425E-86F4-759629489ECB@goldelico.com>
 <cb590a13-e0ff-49d9-8583-be613ad50dc5@ti.com>
 <FE0DBA5E-95A5-4C27-9F69-D1D8BDF56EC3@goldelico.com>
 <20231206-wolverine-paprika-0674ca01e1f2@spud>
To:     Conor Dooley <conor@kernel.org>
Precedence: bulk


> Am 06.12.2023 um 17:02 schrieb Conor Dooley <conor@kernel.org>:
>=20
> On Tue, Dec 05, 2023 at 07:04:05PM +0100, H. Nikolaus Schaller wrote:
>>> Am 05.12.2023 um 18:33 schrieb Andrew Davis <afd@ti.com>:
>>>=20
>>> On 12/5/23 2:17 AM, H. Nikolaus Schaller wrote:
>>>>> +          - enum:
>>>>> +              - ti,omap3430-gpu # Rev 121
>>>>> +              - ti,omap3630-gpu # Rev 125
>>>> Is the "Rev 121" and "Rev 125" a property of the SoC integration =
(clock/reset/power
>>>> hookup etc.) or of the integrated SGX core?
>>>=20
>>> The Rev is a property of the SGX core, not the SoC integration.
>>=20
>> Then, it should belong there and not be a comment of the ti,omap*-gpu =
record.
>> In this way it does not seem to be a proper hardware description.
>>=20
>> BTW: there are examples where the revision is part of the compatible =
string, even
>> if the (Linux) driver makes no use of it:
>>=20
>> drivers/net/ethernet/xilinx/xilinx_emaclite.c
>=20
> AFAICT these Xilinx devices that put the revisions in the compatible =
are
> a different case - they're "soft" IP intended for use in the fabric of
> an FPGA, and assigning a device specific compatible there does not =
make
> sense.
>=20
> In this case it appears that the revision is completely known once you
> see "ti,omap3630-gpu", so encoding the extra "121" into the compatible
> string is not required.

Well, I would not put my hand in the fire for this assumption.

And I am a friend of explicitly stating what is instead ot encoding =
indirectly.

>=20
>>=20
>>> But it seems that
>>> compatible string is being used to define both (as we see being =
debated in the other
>>> thread on this series).
>>>=20
>>>> In my understanding the Revs are different variants of the SGX core =
(errata
>>>> fixes, instruction set, pipeline size etc.). And therefore the =
current driver code
>>>> has to be configured by some macros to handle such cases.
>>>> So the Rev should IMHO be part of the next line:
>>>>> +          - const: img,powervr-sgx530
>>>> +          - enum:
>>>> +              - img,powervr-sgx530-121
>>>> +              - img,powervr-sgx530-125
>>>> We have a similar definition in the openpvrsgx code.
>>>> Example: compatible =3D "ti,omap3-sgx530-121", "img,sgx530-121", =
"img,sgx530";
>>>> (I don't mind about the powervr- prefix).
>>>> This would allow a generic and universal sgx driver (loaded through =
just matching
>>>> "img,sgx530") to handle the errata and revision specifics at =
runtime based on the
>>>> compatible entry ("img,sgx530-121") and know about SoC integration =
("ti,omap3-sgx530-121").
>=20
> The "raw" sgx530 compatible does not seem helpful if the sgx530-121 or
> sgx530-125 compatibles are also required to be present for the driver =
to
> actually function.

Indeed. This seems to be redundant (but may need some pattern =
processing).

> The revision specific compatibles I would not object
> to, but everything in here has different revisions anyway - does the
> same revision actually appear in multiple devices? If it doesn't then =
I
> don't see any value in the suffixed compatibles either.

Well, we don't know.

So far only a subset of SoC with the SGX GPU core variants has been =
considered
(mainly because lack of user space code and device owners).

Maybe someone with insider knowledge can give a hint if the SGX version =
numbers
were assigned uniquely for each SoC integration project.

>=20
>>>> And user-space can be made to load the right firmware variant based =
on "img,sgx530-121"
>>>> I don't know if there is some register which allows to discover the =
revision long
>>>> before the SGX subsystem is initialized and the firmware is up and =
running.
>>>> What I know is that it is possible to read out the revision after =
starting the firmware
>>>> but it may just echo the version number of the firmware binary =
provided from user-space.
>>>=20
>>> We should be able to read out the revision (register =
EUR_CR_CORE_REVISION), the problem is
>>> today the driver is built for a given revision at compile time.
>>=20
>> Yes, that is something we had planned to get rid of for a long time =
by using different compatible
>> strings and some variant specific struct like many others drivers are =
doing it.
>> But it was a to big task so nobody did start with it.
>>=20
>>> That is a software issue,
>>> not something that we need to encode in DT. While the core ID =
(SGX5xx) can be also detected
>>> (EUR_CR_CORE_ID), the location of that register changes, and so it =
does need encoded in
>>> DT compatible.
>>=20
>> Ok, I didn't know about such registers as there is not much public =
information available.
>> Fair enough, there are some error reports about in different forums.
>>=20
>> On the other hand we then must read out this register in more or less =
early initialization
>> stages. Even if we know this information to be static and it could be =
as simple as a list
>> of compatible strings in the driver.
>>=20
>>> The string "ti,omap3430-gpu" tells us the revision if we cannot =
detect it (as in the current
>>> driver), and the SoC integration is generic anyway (just a reg and =
interrupt).
>>=20
>> It of course tells, but may need a translation table that needs to be =
maintained in a
>> different format. Basically the same what the comments show in a =
non-machine readable
>> format.
>>=20
>> I just wonder why the specific version can or should not become =
simply part of the DTS
>> and needs this indirection.
>>=20
>> Basically it is a matter of openness for future (driver) development =
and why it needs
>> careful decisions.
>>=20
>> So in other words: I would prefer to see the comments about versions =
encoded in the device
>> tree binary to make it machine readable.
>=20
> It's already machine readable if it is invariant on an SoC given the
> patch had SoC-specific compatibles.

But needs a translation table to get to the revision number.

I have not yet brought into discussion that there are different =
firmwares for sgx530-121,
sgx530-125, sgx544-116 etc. And user-space code may also depend on to be =
able to chose
the right one if multiple firmware packages are installed. Currently =
this is not the case
but would be a major benfit for OS packages.

To automate this we need a mechanism to scan the device tree for a =
compatible string that
tells which firmware variant to load.

But why force this to depend on the SoC compatible if it only depends =
indirectly?

By the way, there is a tested and working driver using the scheme with =
the sub-versions:

=
https://github.com/openpvrsgx-devgroup/linux_openpvrsgx/blob/11cc7876ba39b=
6172d19ee0bf0a872c1d3d745e1/drivers/gpu/drm/pvrsgx/pvr-drv.c#L306

On the other hand As far as I see this will can of course be adapted to =
the newly
proposed scheme.

But it still seems a bit twisted to me. Maybe because we for example =
define LCD panel
compatibles not by the compatible of the device they are installed in.

BR,
Nikolaus=