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Subject: Re: [PATCH 05/11] drm/bridge: analogix-anx78xx: correct value of
 TX_P0
To:     Brian Masney <masneyb@onstation.org>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>
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From:   Andrzej Hajda <a.hajda@samsung.com>
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Date:   Mon, 16 Sep 2019 14:25:52 +0200
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On 16.09.2019 14:02, Brian Masney wrote:
> On Mon, Sep 16, 2019 at 01:32:58PM +0200, Enric Balletbo i Serra wrote:
>> 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@onstation.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.
> I'm able to communicate with the 7808 on my Nexus 5 using the 0x78
> address. Given that the addresses appear to be fixed per model, maybe it
> makes sense to drop the address #defines and add the addresses to the
> data pointer in the driver's of_match_table like so:
>
> static const struct of_device_id anx78xx_match_table[] = {
>         { .compatible = "analogix,anx7808", .data = PTR_TO_7808_ADDRS },
>         { .compatible = "analogix,anx7812", .data = PTR_TO_781X_ADDRS },
>         { .compatible = "analogix,anx7814", .data = PTR_TO_781X_ADDRS },
>         { .compatible = "analogix,anx7818", .data = PTR_TO_781X_ADDRS },
>         { /* sentinel */ },
> };
>
> Brian
>
>

I have spotted following comment on chromium's ML[1]:

> The locations are hard coded in the register spec.  Furthermore, each
> one can be changed independently--for example the Android driver puts
> 0x38 at 0x3c but leaves the rest alone.

It is not entirely clear, but IMO it suggests these addresses are
hardware configurable.


[1]:
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/44601/2/drivers/auxdisplay/slimport.c#331


Regards

Andrzej