linux-kernel - Re: [PATCH 05/11] drm/bridge: analogix-anx78xx: correct value of TX

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Date:   Mon, 16 Sep 2019 13:32:58 +0200
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Brian Masney <masneyb@...tation.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>, bjorn.andersson@...aro.org,
        robh+dt@...nel.org, agross@...nel.org, narmstrong@...libre.com,
        robdclark@...il.com, sean@...rly.run, airlied@...ux.ie,
        daniel@...ll.ch, mark.rutland@....com, jonas@...boo.se,
        jernej.skrabec@...l.net, linus.walleij@...aro.org,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH 05/11] drm/bridge: analogix-anx78xx: correct value of
 TX_P0

Hi,

On 16/9/19 12:49, Laurent Pinchart wrote:
> Hi Brian,
> 
> On Mon, Sep 16, 2019 at 06:36:14AM -0400, Brian Masney wrote:
>> On Mon, Sep 16, 2019 at 12:02:09PM +0200, Andrzej Hajda wrote:
>>> On 15.08.2019 02:48, Brian Masney wrote:
>>>> When attempting to configure this driver on a Nexus 5 phone (msm8974),
>>>> setting up the dummy i2c bus for TX_P0 would fail due to an -EBUSY
>>>> error. The downstream MSM kernel sources [1] shows that the proper value
>>>> for TX_P0 is 0x78, not 0x70, so correct the value to allow device
>>>> probing to succeed.
>>>>
>>>> [1] https://github.com/AICP/kernel_lge_hammerhead/blob/n7.1/drivers/video/slimport/slimport_tx_reg.h
>>>>
>>>> Signed-off-by: Brian Masney <masneyb@...tation.org>
>>>> ---
>>>>  drivers/gpu/drm/bridge/analogix-anx78xx.h | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/bridge/analogix-anx78xx.h b/drivers/gpu/drm/bridge/analogix-anx78xx.h
>>>> index 25e063bcecbc..bc511fc605c9 100644
>>>> --- a/drivers/gpu/drm/bridge/analogix-anx78xx.h
>>>> +++ b/drivers/gpu/drm/bridge/analogix-anx78xx.h
>>>> @@ -6,7 +6,7 @@
>>>>  #ifndef __ANX78xx_H
>>>>  #define __ANX78xx_H
>>>>  
>>>> -#define TX_P0				0x70
>>>> +#define TX_P0				0x78
>>>
>>>
>>> This bothers me little. There are no upstream users, grepping android
>>> sources suggests that both values can be used [1][2]  (grep for "#define
>>> TX_P0"), moreover there is code suggesting both values can be valid [3].
>>>
>>> Could you verify datasheet which i2c slave addresses are valid for this
>>> chip, if both I guess this patch should be reworked.
>>>
>>>
>>> [1]:
>>> https://android.googlesource.com/kernel/msm/+/android-msm-flo-3.4-jb-mr2/drivers/misc/slimport_anx7808/slimport_tx_reg.h
>>>
>>> [2]:
>>> https://github.com/AndroidGX/SimpleGX-MM-6.0_H815_20d/blob/master/drivers/video/slimport/anx7812/slimport7812_tx_reg.h
>>>
>>> [3]:
>>> https://github.com/commaai/android_kernel_leeco_msm8996/blob/master/drivers/video/msm/mdss/dp/slimport_custom_declare.h#L73
>>
>> This address is 0x78 on my Nexus 5. Given [3] above it looks like we
>> need to support both addresses. What do you think about moving these
>> addresses into device tree?
> 
> Assuming that the device supports different addresses (I can't validate
> that as I don't have access to the datasheet), and different addresses
> need to be used on different systems, then the address to be used needs
> to be provided by the firmware (DT in this case). Two options are
> possible, either specifying the address explicitly in the device's DT
> node, or specifying free addresses (in the form of a white list or black
> list) and allocating an address from that pool. The latter has been
> discussed in a BoF at the Linux Plumbers Conference last week,
> https://linuxplumbersconf.org/event/4/contributions/542/.
> 
>> The downstream and upstream kernel sources divide these addresses by two
>> to get the i2c address. Here's the code in upstream:
>>
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/bridge/analogix-anx78xx.c#L1353
>> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/bridge/analogix-anx78xx.c#L41
>>
>> I'm not sure why the actual i2c address isn't used in this code.
> 

The ANX7802/12/14/16 has a slave I2C bus that provides the interface to access
or control the chip from the AP. The I2C slave addresses used to control the
ANX7802/12/14/16 are 70h, 72h, 7Ah, 7Eh and 80h. Every address allows you to
access to different registers of the chip and AFAICS is not configurable.

I don't think these addresses should be configured via DT but for the driver itself.

My wild guess is that the ANX7808 has different addresses, but I don't have the
datasheet of this version.

Best regards,
 Enric