[PATCH v5] fbtft: limit dirty rows based on damage range

[PATCH v5] fbtft: limit dirty rows based on damage range

[ChanSoo Shin]

Instead of marking the entire display as dirty, calculate the start
and end rows based on the damage offset and length and only mark the
affected rows dirty. This reduces unnecessary full framebuffer updates
for partial writes.

Signed-off-by: ChanSoo Shin <csshin9928@gmail.com>
---
 drivers/staging/fbtft/fbtft-core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c
index 8a5ccc8ae0a1..1d5cb45199d0 100644
--- a/drivers/staging/fbtft/fbtft-core.c
+++ b/drivers/staging/fbtft/fbtft-core.c
@@ -415,8 +415,12 @@ static void fbtft_ops_damage_range(struct fb_info *info, off_t off, size_t len)
 {
 	struct fbtft_par *par = info->par;
 
-	/* TODO: only mark changed area update all for now */
-	par->fbtftops.mkdirty(info, -1, 0);
+	__u32 width = info->var.xres;
+	__u32 start_row = off / width;
+	__u32 end_row = (off + len - 1) / width;
+	__u32 height = end_row - start_row + 1;
+
+	par->fbtftops.mkdirty(info, start_row, height);
 }
 
 static void fbtft_ops_damage_area(struct fb_info *info, u32 x, u32 y, u32 width, u32 height)
-- 
2.52.0

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Andy Shevchenko]

On Thu, Jan 29, 2026 at 05:39:38AM +0900, ChanSoo Shin wrote:
> Instead of marking the entire display as dirty, calculate the start
> and end rows based on the damage offset and length and only mark the
> affected rows dirty. This reduces unnecessary full framebuffer updates
> for partial writes.
> 
> Signed-off-by: ChanSoo Shin <csshin9928@gmail.com>
> ---

This is v5 and no changelog and no answers to the reviewers' questions.
Please, read all what people replied to your previous attempts.
Without that processed correctly NAK to this one.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Dan Carpenter]

On Thu, Jan 29, 2026 at 05:39:38AM +0900, ChanSoo Shin wrote:
> Instead of marking the entire display as dirty, calculate the start
> and end rows based on the damage offset and length and only mark the
> affected rows dirty. This reduces unnecessary full framebuffer updates
> for partial writes.
> 
> Signed-off-by: ChanSoo Shin <csshin9928@gmail.com>
> ---

TL/DR:  I suck as a reviewer so I would be nervous to apply this
without testing.  Andy is an expert here and we trust him so if he's
okay with it then great.  Or if some other expert could sign off, but
I don't know enough to sign off myself.


The problem for me is how do I review something like this?  Staging
is a grab bag of different modules and I'm not an expert in any of
the subsystems.  Normally, it's easy to review staging patches
because they are clean up work which does change how the code works
so I just look for unintentional side effects.

It's trickier to review a patch like this which changes runtime.  If
it were fixing a bug, then I could verify the bug is real and say
well, "Maybe the fix is wrong, but we were going to corrupt memory
anyway, so the worst case is that it is as bad as before.  It can't
make the problem worse."

This is your first kernel patch.  You don't work for a company that
makes the hardware.  You said earlier in a private email that this
hasn't been tested.

The patch looks reasonable to me, but it also looks simple.  If it
were that easy why didn't the original author do it?

regards,
dan carpenter

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Andy Shevchenko]

On Thu, Jan 29, 2026 at 09:29:26AM +0300, Dan Carpenter wrote:
> On Thu, Jan 29, 2026 at 05:39:38AM +0900, ChanSoo Shin wrote:
> > Instead of marking the entire display as dirty, calculate the start
> > and end rows based on the damage offset and length and only mark the
> > affected rows dirty. This reduces unnecessary full framebuffer updates
> > for partial writes.
> > 
> > Signed-off-by: ChanSoo Shin <csshin9928@gmail.com>
> > ---
> 
> TL/DR:  I suck as a reviewer so I would be nervous to apply this
> without testing.  Andy is an expert here and we trust him so if he's
> okay with it then great.  Or if some other expert could sign off, but
> I don't know enough to sign off myself.

The rule of thumb for _this_ driver (or set of drivers under FBTFT) is
that: we are in maintenance mode and we only accept bugfixes or treewide
changes. The rest can be accepted but unlikely. Either way, we really
want to see this (kind of changes) being tested on real HW. It's not as
simple as renaming variable 'i' to 'j'.

> The problem for me is how do I review something like this?  Staging
> is a grab bag of different modules and I'm not an expert in any of
> the subsystems.  Normally, it's easy to review staging patches
> because they are clean up work which does change how the code works
> so I just look for unintentional side effects.
> 
> It's trickier to review a patch like this which changes runtime.  If
> it were fixing a bug, then I could verify the bug is real and say
> well, "Maybe the fix is wrong, but we were going to corrupt memory
> anyway, so the worst case is that it is as bad as before.  It can't
> make the problem worse."
> 
> This is your first kernel patch.  You don't work for a company that
> makes the hardware.  You said earlier in a private email that this
> hasn't been tested.

Unfortunately it is not the best driver to go with this. At some point I might
be able to test this when I setup my fbtft minilab at home, I have a few I²C,
SPI, and parallel panels.

> The patch looks reasonable to me, but it also looks simple.  If it
> were that easy why didn't the original author do it?

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Nam Cao]

Hi ChanSoo,

ChanSoo Shin <csshin9928@gmail.com> writes:
> Instead of marking the entire display as dirty, calculate the start
> and end rows based on the damage offset and length and only mark the
> affected rows dirty. This reduces unnecessary full framebuffer updates
> for partial writes.
>
> Signed-off-by: ChanSoo Shin <csshin9928@gmail.com>

This looks useful and I prefer to see it applied. I understand lack of
testing is the main obstacle. Are you still trying to get it merged? If
not, I can take over.

Nam

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Andy Shevchenko]

On Fri, Feb 06, 2026 at 01:04:45AM +0100, Nam Cao wrote:
> ChanSoo Shin <csshin9928@gmail.com> writes:
> > Instead of marking the entire display as dirty, calculate the start
> > and end rows based on the damage offset and length and only mark the
> > affected rows dirty. This reduces unnecessary full framebuffer updates
> > for partial writes.
> 
> This looks useful and I prefer to see it applied. I understand lack of
> testing is the main obstacle.

Correct.

> Are you still trying to get it merged? If not, I can take over.

Do you have an actual HW to test? If not, won't be applied either.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Nam Cao]

Andy Shevchenko <andriy.shevchenko@intel.com> writes:
> Do you have an actual HW to test? If not, won't be applied either.

Yes, a pair with SPI interface.

Nam

Re: [PATCH v5] fbtft: limit dirty rows based on damage range

[Andy Shevchenko]

On Fri, Feb 06, 2026 at 05:51:45PM +0700, Nam Cao wrote:
> Andy Shevchenko <andriy.shevchenko@intel.com> writes:
> > Do you have an actual HW to test? If not, won't be applied either.
> 
> Yes, a pair with SPI interface.

Cool! Would be nice to have tested contribution, indeed.

-- 
With Best Regards,
Andy Shevchenko