Re: [PATCH 5/5 v3] drm/bridge: adv7511: Reuse __adv7511_power_on/off() when probing EDID

[Laurent Pinchart]

Hi John,

On Monday 16 Jan 2017 12:14:48 John Stultz wrote:
> On Mon, Jan 16, 2017 at 8:03 AM, Laurent Pinchart wrote:
> > 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."

You're right that most registers will be out of sync.

> 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()?

It would be if we had to keep the device powered up, but we're powering it 
down right after reading the EDID. I don't think there's a need to reconfigure 
it completely, only setting the registers needed to read the EDID should be 
enough.

> 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?

I'm not too picky either :-) If we can't find a reliable way to read the EDID 
by just configuring the registers we need, we could go for a full 
reconfiguration. However, restoring the value of all cached registers will 
result in lots of I2C writes, which are time-consuming operations. EDID read 
would be sped up if we could avoid that.

-- 
Regards,

Laurent Pinchart

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Re: [PATCH 5/5 v3] drm/bridge: adv7511: Reuse __adv7511_power_on/off() when probing EDID

[John Stultz]

On Mon, Jan 16, 2017 at 8:03 AM, Laurent Pinchart
 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
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Re: [PATCH 5/5 v3] drm/bridge: adv7511: Reuse __adv7511_power_on/off() when probing EDID

[Laurent Pinchart]

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 ?

> Cc: David Airlie 
> Cc: Archit Taneja 
> Cc: Wolfram Sang 
> Cc: Lars-Peter Clausen 
> Cc: Laurent Pinchart 
> Cc: dri-devel@lists.freedesktop.org
> Signed-off-by: John Stultz 
> ---
>  drivers/gpu/drm/bridge/adv7511/adv7511_drv.c | 17 +++--
>  1 file changed, 3 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
> b/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c index dbdb71c..24573e0
> 100644
> --- a/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
> +++ b/drivers/gpu/drm/bridge/adv7511/adv7511_drv.c
> @@ -572,24 +572,13 @@ static int adv7511_get_modes(struct adv7511 *adv7511,
>   unsigned int count;
> 
>   /* Reading the EDID only works if the device is powered */
> - if (!adv7511->powered) {
> - regmap_update_bits(adv7511->regmap, ADV7511_REG_POWER,
> -ADV7511_POWER_POWER_DOWN, 0);
> - if (adv7511->i2c_main->irq) {
> - regmap_write(adv7511->regmap, 
ADV7511_REG_INT_ENABLE(0),
> -  ADV7511_INT0_EDID_READY);
> - regmap_write(adv7511->regmap, 
ADV7511_REG_INT_ENABLE(1),
> -  ADV7511_INT1_DDC_ERROR);
> - }
> - adv7511->current_edid_segment = -1;
> - }
> + if (!adv7511->powered)
> + __adv7511_power_on(adv7511);
> 
>   edid = drm_do_get_edid(connector, adv7511_get_edid_block, adv7511);
> 
>   if (!adv7511->powered)
> - regmap_update_bits(adv7511->regmap, ADV7511_REG_POWER,
> -ADV7511_POWER_POWER_DOWN,
> -ADV7511_POWER_POWER_DOWN);
> + __adv7511_power_off(adv7511);
> 
>   kfree(adv7511->edid);
>   adv7511->edid = edid;

-- 
Regards,

Laurent Pinchart

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