[PATCH v1] drm/tegra: dpaux: Fix always-failing probing of the driver

Dmitry Osipenko digetx at gmail.com
Mon Sep 24 13:56:28 UTC 2018

On 9/24/18 4:36 PM, Thierry Reding wrote:
> On Mon, Sep 24, 2018 at 03:40:35PM +0300, Dmitry Osipenko wrote:
>> On 9/24/18 3:32 PM, Jon Hunter wrote:
>>> On 24/09/18 12:59, Thierry Reding wrote:
>>>> On Fri, Sep 21, 2018 at 02:42:41PM +0300, Dmitry Osipenko wrote:
>>>>> Some of definitions in the code changed the meaning, unfortunately one
>>>>> place missed the change.
>>>>> Fixes: 0751bb5c44fe ("drm/tegra: dpaux: Add pinctrl support")
>>>>> Cc: <stable at vger.kernel.org> # v4.8+
>>>>> Signed-off-by: Dmitry Osipenko <digetx at gmail.com>
>>>>> ---
>>>>> I don't have HW to test DPAUX driver, apparently it has been broken for
>>>>> 2+ years now. There is also a known issue on with the DPAUX driver that
>>>>> prevents it from probing, that was discussed on the #tegra IRC. Thierry,
>>>>> please take a closer look at this driver and test it thoroughly, it has
>>>>> some obvious problems.
>>>>>    drivers/gpu/drm/tegra/dpaux.c | 2 +-
>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>> It's odd that you claim that the driver is always failing probe and at
>>>> the same time you say that you don't have hardware to test the driver.
>>>> =)
>>>> I know for a fact that this driver does not usually fail because it is
>>>> required on all recent chips (Tegra210 and later) to drive HDMI, which
>>>> we support on all boards, so it is indeed thoroughly tested.
>>>>> diff --git a/drivers/gpu/drm/tegra/dpaux.c b/drivers/gpu/drm/tegra/dpaux.c
>>>>> index d84e81ff36ad..ba5681fab73b 100644
>>>>> --- a/drivers/gpu/drm/tegra/dpaux.c
>>>>> +++ b/drivers/gpu/drm/tegra/dpaux.c
>>>>> @@ -521,7 +521,7 @@ static int tegra_dpaux_probe(struct platform_device *pdev)
>>>>>    	 * is no possibility to perform the I2C mode configuration in the
>>>>>    	 * HDMI path.
>>>>>    	 */
>>>>> -	err = tegra_dpaux_pad_config(dpaux, DPAUX_HYBRID_PADCTL_MODE_I2C);
>>>>> +	err = tegra_dpaux_pad_config(dpaux, DPAUX_PADCTL_FUNC_I2C);
>>>>>    	if (err < 0)
>>>>>    		return err;
>>>> If you look at the definitions of both DPAUX_HYBRID_PADCTL_MODE_I2C and
>>>> DPAUX_PADCTL_FUNC_I2C, you'll see that both are actually the same, which
>>>> is a good explanation for why the driver performs flawlessly.
>>>> That said, your change is obviously correct. I've applied it, but since
>>>> it doesn't actually fix anything, and doesn't change anything from a
>>>> binary point of view, I've removed the Fixes: and Cc: stable tags.
>>> Did you change the subject for the patch because as you mentioned it
>>> does not seem related to the change?
>>> Otherwise for the fix you can have my ...
>>> Acked-by: Jon Hunter <jonathanh at nvidia.com>
>>> The dpaux driver has been working fine for me on Tegra210 Smaug when
>>> using the pins for I2C and I have not seen any probing problems.
>> Guy with nickname "vlado" said on the IRC that the panel stopped to work on
>> T124 chromebook since 4.16 kernel, reverting commit [0] helps.
>> [0] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/gpu/drm/drm_dp_helper.c?id=17ab7806de0c10d27cdbda8ef57ca680bdd24315
> I'm pretty sure that's something that we fixed, but I'll have to check.
> Is this patch reported to help with that issue, or where's the
> connection?

Reverting of the patch was reported to help with the problem. Check the IRC 
history, you can also just ask the original reporter for more details on #tegra. 
IIUC, the DPAUX driver probing is getting deferred because regulator isn't ready 
during of the first invocation and then for some reason driver is never getting 

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