linux-kernel - Re: [PATCH] drm/msm/a6xx: fix for kernels without CONFIG

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Date:   Mon, 22 Feb 2021 21:15:39 +0530
From:   Akhil P Oommen <akhilpo@...eaurora.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Jonathan Marek <jonathan@...ek.ca>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Eric Anholt <eric@...olt.net>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/a6xx: fix for kernels without CONFIG_NVMEM

On 2/19/2021 9:30 PM, Rob Clark wrote:
> On Fri, Feb 19, 2021 at 2:44 AM Akhil P Oommen <akhilpo@...eaurora.org> wrote:
>>
>> On 2/18/2021 9:41 PM, Rob Clark wrote:
>>> On Thu, Feb 18, 2021 at 4:28 AM Akhil P Oommen <akhilpo@...eaurora.org> wrote:
>>>>
>>>> On 2/18/2021 2:05 AM, Jonathan Marek wrote:
>>>>> On 2/17/21 3:18 PM, Rob Clark wrote:
>>>>>> On Wed, Feb 17, 2021 at 11:08 AM Jordan Crouse
>>>>>> <jcrouse@...eaurora.org> wrote:
>>>>>>>
>>>>>>> On Wed, Feb 17, 2021 at 07:14:16PM +0530, Akhil P Oommen wrote:
>>>>>>>> On 2/17/2021 8:36 AM, Rob Clark wrote:
>>>>>>>>> On Tue, Feb 16, 2021 at 12:10 PM Jonathan Marek <jonathan@...ek.ca>
>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>> Ignore nvmem_cell_get() EOPNOTSUPP error in the same way as a
>>>>>>>>>> ENOENT error,
>>>>>>>>>> to fix the case where the kernel was compiled without CONFIG_NVMEM.
>>>>>>>>>>
>>>>>>>>>> Fixes: fe7952c629da ("drm/msm: Add speed-bin support to a618 gpu")
>>>>>>>>>> Signed-off-by: Jonathan Marek <jonathan@...ek.ca>
>>>>>>>>>> ---
>>>>>>>>>>     drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 6 +++---
>>>>>>>>>>     1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>>>>>>>>> b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>>>>>>>>> index ba8e9d3cf0fe..7fe5d97606aa 100644
>>>>>>>>>> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>>>>>>>>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
>>>>>>>>>> @@ -1356,10 +1356,10 @@ static int a6xx_set_supported_hw(struct
>>>>>>>>>> device *dev, struct a6xx_gpu *a6xx_gpu,
>>>>>>>>>>
>>>>>>>>>>            cell = nvmem_cell_get(dev, "speed_bin");
>>>>>>>>>>            /*
>>>>>>>>>> -        * -ENOENT means that the platform doesn't support
>>>>>>>>>> speedbin which is
>>>>>>>>>> -        * fine
>>>>>>>>>> +        * -ENOENT means no speed bin in device tree,
>>>>>>>>>> +        * -EOPNOTSUPP means kernel was built without CONFIG_NVMEM
>>>>>>>>>
>>>>>>>>> very minor nit, it would be nice to at least preserve the gist of the
>>>>>>>>> "which is fine" (ie. some variation of "this is an optional thing and
>>>>>>>>> things won't catch fire without it" ;-))
>>>>>>>>>
>>>>>>>>> (which is, I believe, is true, hopefully Akhil could confirm.. if not
>>>>>>>>> we should have a harder dependency on CONFIG_NVMEM..)
>>>>>>>> IIRC, if the gpu opp table in the DT uses the 'opp-supported-hw'
>>>>>>>> property,
>>>>>>>> we will see some error during boot up if we don't call
>>>>>>>> dev_pm_opp_set_supported_hw(). So calling "nvmem_cell_get(dev,
>>>>>>>> "speed_bin")"
>>>>>>>> is a way to test this.
>>>>>>>>
>>>>>>>> If there is no other harm, we can put a hard dependency on
>>>>>>>> CONFIG_NVMEM.
>>>>>>>
>>>>>>> I'm not sure if we want to go this far given the squishiness about
>>>>>>> module
>>>>>>> dependencies. As far as I know we are the only driver that uses this
>>>>>>> seriously
>>>>>>> on QCOM SoCs and this is only needed for certain targets. I don't
>>>>>>> know if we
>>>>>>> want to force every target to build NVMEM and QFPROM on our behalf.
>>>>>>> But maybe
>>>>>>> I'm just saying that because Kconfig dependencies tend to break my
>>>>>>> brain (and
>>>>>>> then Arnd has to send a patch to fix it).
>>>>>>>
>>>>>>
>>>>>> Hmm, good point.. looks like CONFIG_NVMEM itself doesn't have any
>>>>>> other dependencies, so I suppose it wouldn't be the end of the world
>>>>>> to select that.. but I guess we don't want to require QFPROM
>>>>>>
>>>>>> I guess at the end of the day, what is the failure mode if you have a
>>>>>> speed-bin device, but your kernel config misses QFPROM (and possibly
>>>>>> NVMEM)?  If the result is just not having the highest clk rate(s)
>>>>
>>>> Atleast on sc7180's gpu, using an unsupported FMAX breaks gmu. It won't
>>>> be very obvious what went wrong when this happens!
>>>
>>> Ugg, ok..
>>>
>>> I suppose we could select NVMEM, but not QFPROM, and then the case
>>> where QFPROM is not enabled on platforms that have the speed-bin field
>>> in DT will fail gracefully and all other platforms would continue on
>>> happily?
>>>
>>> BR,
>>> -R
>>
>> Sounds good to me.
>>
> 
> You probably should do a quick test with NVMEM enabled but QFPROM
> disabled to confirm my theory, but I *think* that should work
> 
> BR,
> -R
> 

I tried it on an sc7180 device. The suggested combo (CONFIG_NVMEM + no 
CONFIG_QCOM_QFPROM) makes the gpu probe fail with error "failed to read 
speed-bin. Some OPPs may not be supported by hardware". This is good 
enough clue for the developer that he should fix the broken speedbin 
detection.

-Akhil.