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Date:   Mon, 16 Sep 2019 13:49:07 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Brian Masney <masneyb@onstation.org>
Cc:     Andrzej Hajda <a.hajda@samsung.com>, bjorn.andersson@linaro.org,
        robh+dt@kernel.org, agross@kernel.org, narmstrong@baylibre.com,
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        enric.balletbo@collabora.com, dri-devel@lists.freedesktop.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        freedreno@lists.freedesktop.org
Subject: Re: [PATCH 05/11] drm/bridge: analogix-anx78xx: correct value of
 TX_P0
Message-ID: <20190916104907.GB4734@pendragon.ideasonboard.com>
References: <20190815004854.19860-1-masneyb@onstation.org>
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 <20190815004854.19860-6-masneyb@onstation.org>
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 <20190916103614.GA1644@onstation.org>
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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.

-- 
Regards,

Laurent Pinchart