[PATCH] drm/komeda: Fix off-by-1 when with readback conn due to rounding

[Carsten Haitzler]

On 3/12/21 10:55 AM, Brian Starkey wrote:
> (Adding back James again - did you use get_maintainer.pl?)
> 
> On Thu, Mar 11, 2021 at 12:08:46PM +0000, carsten.haitzler at foss.arm.com wrote:
>> From: Carsten Haitzler <carsten.haitzler at arm.com>
>>
>> When setting up a readback connector that writes data back to memory
>> rather than to an actual output device (HDMI etc.), rounding was set
>> to round. As the DPU uses a higher internal number of bits when generating
>> a color value, this round-down back to 8bit ended up with everything
>> being off-by one. e.g. #fefefe became #ffffff. This sets
> 
> Perhaps overly pedantic, but now we've tracked down what was actually
> happening I think we can be more precise here. Not _everything_ is
> off-by-one, it's just rounding in the standard sense - if the most

Well a very large number of pixels were off-by-1 ... I guess it's an 
exaggeration but a "vast number of pixels were off by 1". I guess I was 
just using common terms like "everything is expensive here" doesn't 
actually mean absolutely everything but a very vast number of things. 
You know what I mean. :) The comment as a whole describing rounding 
policies should provide more details. I just write the log as a "when 
spelunking through history, this log will give me some broader insight 
into what this change is without being war and peace and If I want to 
see more and this commit is interesting to my spelunking efforts, I'll 
git log -U to read that".

> significant bit-to-be-discarded is set, the value is rounded up to
> minimise the absolute error introduced by bit-depth reduction.
> 
>> rounding to "round-down" so things end up correct by turning on the LW_TRC
>> round down flag.
> 
> Can we call it "truncate" rather than round down? I think it makes
> "TRC" a bit more understandable.

That's the official name from the docs though (TRC)... makes it easier 
to match to them... So I think you can argue this both ways. The comment 
where it's used though does make it clear...

>>
>> Signed-off-by: Carsten Haitzler <carsten.haitzler at arm.com>
>> ---
>>   drivers/gpu/drm/arm/display/komeda/d71/d71_component.c | 7 ++++++-
>>   drivers/gpu/drm/arm/display/komeda/d71/d71_regs.h      | 1 +
>>   2 files changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/arm/display/komeda/d71/d71_component.c b/drivers/gpu/drm/arm/display/komeda/d71/d71_component.c
>> index 8a02ade369db..e97acc5519d1 100644
>> --- a/drivers/gpu/drm/arm/display/komeda/d71/d71_component.c
>> +++ b/drivers/gpu/drm/arm/display/komeda/d71/d71_component.c
>> @@ -468,7 +468,12 @@ static void d71_wb_layer_update(struct komeda_component *c,
>>   	struct komeda_layer_state *st = to_layer_st(state);
>>   	struct drm_connector_state *conn_st = state->wb_conn->state;
>>   	struct komeda_fb *kfb = to_kfb(conn_st->writeback_job->fb);
>> -	u32 ctrl = L_EN | LW_OFM, mask = L_EN | LW_OFM | LW_TBU_EN;
>> +	/* LW_TRC sets rounding to truncate not round which is needed for
>> +	 * the output of writeback to match the input in the most common
>> +	 * use cases like RGB888 -> RGB888, so set this bit by default
>> +	 */
> 
> Hm, not sure why this file uses "net/" style comments, but as you
> said, this is in-keeping with the rest of the file, so meh :-)

Yup. Just stick to "follow the style there" unless there is seemingly a 
good reason that what is there is horribly "broken" and needs fixing up.

>> +	u32 ctrl = LW_TRC | L_EN | LW_OFM;
>> +	u32 mask = LW_TRC | L_EN | LW_OFM | LW_TBU_EN;
> 
> If you were aiming for matching register order, this should be:
> 
>      L_EN | LW_TRC | LW_OFM | LW_TBU_EN
> 
> 
> I think it'd be nice to have the exact behaviour in the commit
> message, but either way this seems OK as a pragmatic fix so:

git log -U ? :)

> Reviewed-by: Brian Starkey <brian.starkey at arm.com>
> 
> Thanks,
> -Brian
> 
>>   	u32 __iomem *reg = c->reg;
>>   
>>   	d71_layer_update_fb(c, kfb, st->addr);
>> diff --git a/drivers/gpu/drm/arm/display/komeda/d71/d71_regs.h b/drivers/gpu/drm/arm/display/komeda/d71/d71_regs.h
>> index e80172a0b320..a8036689d721 100644
>> --- a/drivers/gpu/drm/arm/display/komeda/d71/d71_regs.h
>> +++ b/drivers/gpu/drm/arm/display/komeda/d71/d71_regs.h
>> @@ -321,6 +321,7 @@
>>   #define LAYER_WR_FORMAT		0x0D8
>>   
>>   /* Layer_WR control bits */
>> +#define LW_TRC			BIT(1)
>>   #define LW_OFM			BIT(4)
>>   #define LW_LALPHA(x)		(((x) & 0xFF) << 8)
>>   #define LW_A_WCACHE(x)		(((x) & 0xF) << 28)
>> -- 
>> 2.30.0
>>
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