Subject: Re: [PATCH 05/11] drm/bridge: analogix-anx78xx: correct value of TX_P0

From: Brian Masney <[email-masked]>
Date: Mon, 16 Sep 2019 06:36:14 -0400

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 <[email-masked]>
> > ---
> >  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?

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.

Brian

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