linux-kernel - Re: [PATCH 5/5 v3] drm/bridge: adv7511: Reuse __adv7511_power

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Message-ID: <CALAqxLVN=-9eUFaYWtu_MxTixfHZt_gRvWO7TDDFeDaDQ-uFZg@mail.gmail.com>
Date:   Mon, 16 Jan 2017 12:14:48 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: [PATCH 5/5 v3] drm/bridge: adv7511: Reuse __adv7511_power_on/off()
 when probing EDID

On Mon, Jan 16, 2017 at 8:03 AM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
> Hi John,
>
> Thank you for the patch.
>
> On Tuesday 03 Jan 2017 11:41:42 John Stultz wrote:
>> I've found that by just turning the chip on and off via the
>> POWER_DOWN register, I end up getting i2c_transfer errors
>> on HiKey.
>>
>> Investigating further, it seems some of the register state
>> in the regmap cache is somehow getting lost. Using the logic
>> in __adv7511_power_on/off() which syncs and dirtys the cache
>> avoids this issue.
>>
>> Thus this patch changes the EDID probing logic so that we
>> re-use the __adv7511_power_on/off() calls.
>
> regcache_sync() is quite costly as it will write a bunch of registers.
> Wouldn't it be more efficient to only write the registers that are needed for
> EDID access ?

So yes, you've mentioned this concern before, and I did spend some
time to narrow which lost-register state (0x43
 - ADV7511_REG_EDID_I2C_ADDR) was causing the trouble with i2c
trasnfer errors I was seeing:
  https://lkml.org/lkml/2016/11/22/677

However, I didn't get much feedback on that, and it seems (to me at
least) concerning that we are losing the underlying state of a
register in the cache, so just syncing that one register back to the
hardware might solve the issue I was seeing, but I worry what other
registers might also be out of sync.

The comment above the regmap_sync in adv7511_power_on after all states:
   "Most of the registers are reset during power down or when HPD is low."

So it seems like if we're setting the power down (and setting HPD in
cases where Archit had a patch to add HPD pulsing to the
adv7511_get_modes path), it seems reasonable to do the same
regmap_sync()?

But, I'm not really picky here, and I'm very open to other approaches
(including something like the patch in the link above) if you have
suggestions/preferences. I just want it to work reliably on my
hardware. :)

And just so I can better understand it, can you explain some about the
impact of your efficiency concerns?

thanks
-john