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

[Brian Starkey]

(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
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.

> 
> 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 :-)

> +	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:

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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