(Adding back James again - did you use get_maintainer.pl?)
On Thu, Mar 11, 2021 at 12:08:46PM +0000, carsten.haitzler@xxxxxxxxxxxx wrote:
> From: Carsten Haitzler <carsten.haitzler@xxxxxxx>
>
> 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@xxxxxxx>
> ---
> 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@xxxxxxx>
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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