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@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
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@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 :-)
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@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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