* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
@ 2016-06-10 19:52 ` Doug Anderson
0 siblings, 0 replies; 23+ messages in thread
From: Doug Anderson @ 2016-06-10 19:52 UTC (permalink / raw)
To: Rob Clark
Cc: Thierry Reding, Mark Rutland, devicetree@vger.kernel.org,
dri-devel@lists.freedesktop.org, Linux Kernel Mailing List,
Rob Herring, Stéphane Marchesin
Rob,
On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>> using eDP interfaces.
>
> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
> we need panel drivers or DT bindings?
I was wondering about that too. As far as I can tell:
1. We need a panel driver because that appears to be what owns a
reference to the backlight / panel power regulator and that part is
not auto-probable.
2. As far as I could tell, there is no way to declare a generic
(unspecified) panel in the device tree. Everyone seems to include
"simple-panel" in their compatible string but as far as I can tell
nothing in the kernel looks at it.
3. In theory, all the info specified here should match the EDID
exactly and thus (as you said) be probable. However, it sounds like
(for power sequencing reasons) there might be reasons why you'd want
to know exactly what panel was present beforehand. You might need to
power the panel and backlight in very specific sequences, for
instance. I'm not sure it's always 100% possible in all embedded
designs to read the EDID before you know how the sequencing should
work (but, of course, I'm a NOOB).
4. Reading the EDID can be slow. If you happen to know all the info
on the panel beforehand you can significantly speed up boot speed,
notably how fast you can get something on the screen.
Anyway, maybe someone else who actually knows what they're talking
about will chime in. ;)
-Doug
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
2016-06-10 19:52 ` Doug Anderson
@ 2016-06-10 22:03 ` Rob Clark
-1 siblings, 0 replies; 23+ messages in thread
From: Rob Clark @ 2016-06-10 22:03 UTC (permalink / raw)
To: Doug Anderson
Cc: Mark Rutland, devicetree@vger.kernel.org,
Linux Kernel Mailing List, dri-devel@lists.freedesktop.org,
Rob Herring, Stéphane Marchesin
On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
> Rob,
>
> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>>> using eDP interfaces.
>>
>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
>> we need panel drivers or DT bindings?
>
> I was wondering about that too. As far as I can tell:
>
> 1. We need a panel driver because that appears to be what owns a
> reference to the backlight / panel power regulator and that part is
> not auto-probable.
oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
driver hell.. I guess being able to use panel-simple is at least an
improvement. But panel specific sequences is sounds like panel-simple
won't save us all the time :-(
> 2. As far as I could tell, there is no way to declare a generic
> (unspecified) panel in the device tree. Everyone seems to include
> "simple-panel" in their compatible string but as far as I can tell
> nothing in the kernel looks at it.
>
> 3. In theory, all the info specified here should match the EDID
> exactly and thus (as you said) be probable. However, it sounds like
> (for power sequencing reasons) there might be reasons why you'd want
> to know exactly what panel was present beforehand. You might need to
> power the panel and backlight in very specific sequences, for
> instance. I'm not sure it's always 100% possible in all embedded
> designs to read the EDID before you know how the sequencing should
> work (but, of course, I'm a NOOB).
>
> 4. Reading the EDID can be slow. If you happen to know all the info
> on the panel beforehand you can significantly speed up boot speed,
> notably how fast you can get something on the screen.
The theory is (although I think not true currently for most of the arm
drivers) that we should be reading back from hw the current config
from bootloader splash screen, to avoid a modeset (and conveniently
also the need to read edid) at boot.
BR,
-R
>
> Anyway, maybe someone else who actually knows what they're talking
> about will chime in. ;)
>
> -Doug
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
@ 2016-06-10 22:03 ` Rob Clark
0 siblings, 0 replies; 23+ messages in thread
From: Rob Clark @ 2016-06-10 22:03 UTC (permalink / raw)
To: Doug Anderson
Cc: Thierry Reding, Mark Rutland, devicetree@vger.kernel.org,
dri-devel@lists.freedesktop.org, Linux Kernel Mailing List,
Rob Herring, Stéphane Marchesin
On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
> Rob,
>
> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>>> using eDP interfaces.
>>
>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
>> we need panel drivers or DT bindings?
>
> I was wondering about that too. As far as I can tell:
>
> 1. We need a panel driver because that appears to be what owns a
> reference to the backlight / panel power regulator and that part is
> not auto-probable.
oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
driver hell.. I guess being able to use panel-simple is at least an
improvement. But panel specific sequences is sounds like panel-simple
won't save us all the time :-(
> 2. As far as I could tell, there is no way to declare a generic
> (unspecified) panel in the device tree. Everyone seems to include
> "simple-panel" in their compatible string but as far as I can tell
> nothing in the kernel looks at it.
>
> 3. In theory, all the info specified here should match the EDID
> exactly and thus (as you said) be probable. However, it sounds like
> (for power sequencing reasons) there might be reasons why you'd want
> to know exactly what panel was present beforehand. You might need to
> power the panel and backlight in very specific sequences, for
> instance. I'm not sure it's always 100% possible in all embedded
> designs to read the EDID before you know how the sequencing should
> work (but, of course, I'm a NOOB).
>
> 4. Reading the EDID can be slow. If you happen to know all the info
> on the panel beforehand you can significantly speed up boot speed,
> notably how fast you can get something on the screen.
The theory is (although I think not true currently for most of the arm
drivers) that we should be reading back from hw the current config
from bootloader splash screen, to avoid a modeset (and conveniently
also the need to read edid) at boot.
BR,
-R
>
> Anyway, maybe someone else who actually knows what they're talking
> about will chime in. ;)
>
> -Doug
^ permalink raw reply [flat|nested] 23+ messages in thread
[parent not found: <CAF6AEGsZxZuSVUCqVH=8FUOTMiWBVT52xQ+Qcaqt3yF=pRmf3w-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>]
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
2016-06-10 22:03 ` Rob Clark
@ 2016-06-10 22:08 ` Stéphane Marchesin
-1 siblings, 0 replies; 23+ messages in thread
From: Stéphane Marchesin @ 2016-06-10 22:08 UTC (permalink / raw)
To: Rob Clark
Cc: Doug Anderson, Thierry Reding, Mark Rutland,
devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org,
Linux Kernel Mailing List, Rob Herring
On Fri, Jun 10, 2016 at 3:03 PM, Rob Clark <robdclark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
>> Rob,
>>
>> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
>>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>>>> using eDP interfaces.
>>>
>>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
>>> we need panel drivers or DT bindings?
>>
>> I was wondering about that too. As far as I can tell:
>>
>> 1. We need a panel driver because that appears to be what owns a
>> reference to the backlight / panel power regulator and that part is
>> not auto-probable.
>
> oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
> driver hell.. I guess being able to use panel-simple is at least an
> improvement. But panel specific sequences is sounds like panel-simple
> won't save us all the time :-(
Yes, although you can probably make things mostly work with improper
sequencing and enough retries, a lot of ARM hw either requires
interesting sequencing, or has broken/unreliable DDC, which translates
into the use of panel simple on the sw side.
>
>> 2. As far as I could tell, there is no way to declare a generic
>> (unspecified) panel in the device tree. Everyone seems to include
>> "simple-panel" in their compatible string but as far as I can tell
>> nothing in the kernel looks at it.
>>
>> 3. In theory, all the info specified here should match the EDID
>> exactly and thus (as you said) be probable. However, it sounds like
>> (for power sequencing reasons) there might be reasons why you'd want
>> to know exactly what panel was present beforehand. You might need to
>> power the panel and backlight in very specific sequences, for
>> instance. I'm not sure it's always 100% possible in all embedded
>> designs to read the EDID before you know how the sequencing should
>> work (but, of course, I'm a NOOB).
>>
>> 4. Reading the EDID can be slow. If you happen to know all the info
>> on the panel beforehand you can significantly speed up boot speed,
>> notably how fast you can get something on the screen.
>
> The theory is (although I think not true currently for most of the arm
> drivers) that we should be reading back from hw the current config
> from bootloader splash screen, to avoid a modeset (and conveniently
> also the need to read edid) at boot.
On some devices the firmware doesn't set any video mode, so there
isn't anything we can read back. That is our case :)
Stéphane
>
> BR,
> -R
>
>>
>> Anyway, maybe someone else who actually knows what they're talking
>> about will chime in. ;)
>>
>> -Doug
--
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^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
@ 2016-06-10 22:08 ` Stéphane Marchesin
0 siblings, 0 replies; 23+ messages in thread
From: Stéphane Marchesin @ 2016-06-10 22:08 UTC (permalink / raw)
To: Rob Clark
Cc: Doug Anderson, Thierry Reding, Mark Rutland,
devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org,
Linux Kernel Mailing List, Rob Herring
On Fri, Jun 10, 2016 at 3:03 PM, Rob Clark <robdclark@gmail.com> wrote:
> On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
>> Rob,
>>
>> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
>>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
>>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>>>> using eDP interfaces.
>>>
>>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
>>> we need panel drivers or DT bindings?
>>
>> I was wondering about that too. As far as I can tell:
>>
>> 1. We need a panel driver because that appears to be what owns a
>> reference to the backlight / panel power regulator and that part is
>> not auto-probable.
>
> oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
> driver hell.. I guess being able to use panel-simple is at least an
> improvement. But panel specific sequences is sounds like panel-simple
> won't save us all the time :-(
Yes, although you can probably make things mostly work with improper
sequencing and enough retries, a lot of ARM hw either requires
interesting sequencing, or has broken/unreliable DDC, which translates
into the use of panel simple on the sw side.
>
>> 2. As far as I could tell, there is no way to declare a generic
>> (unspecified) panel in the device tree. Everyone seems to include
>> "simple-panel" in their compatible string but as far as I can tell
>> nothing in the kernel looks at it.
>>
>> 3. In theory, all the info specified here should match the EDID
>> exactly and thus (as you said) be probable. However, it sounds like
>> (for power sequencing reasons) there might be reasons why you'd want
>> to know exactly what panel was present beforehand. You might need to
>> power the panel and backlight in very specific sequences, for
>> instance. I'm not sure it's always 100% possible in all embedded
>> designs to read the EDID before you know how the sequencing should
>> work (but, of course, I'm a NOOB).
>>
>> 4. Reading the EDID can be slow. If you happen to know all the info
>> on the panel beforehand you can significantly speed up boot speed,
>> notably how fast you can get something on the screen.
>
> The theory is (although I think not true currently for most of the arm
> drivers) that we should be reading back from hw the current config
> from bootloader splash screen, to avoid a modeset (and conveniently
> also the need to read edid) at boot.
On some devices the firmware doesn't set any video mode, so there
isn't anything we can read back. That is our case :)
Stéphane
>
> BR,
> -R
>
>>
>> Anyway, maybe someone else who actually knows what they're talking
>> about will chime in. ;)
>>
>> -Doug
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
2016-06-10 22:08 ` Stéphane Marchesin
@ 2016-06-13 11:39 ` Thierry Reding
-1 siblings, 0 replies; 23+ messages in thread
From: Thierry Reding @ 2016-06-13 11:39 UTC (permalink / raw)
To: Stéphane Marchesin
Cc: Mark Rutland, devicetree@vger.kernel.org, Doug Anderson,
dri-devel@lists.freedesktop.org, Linux Kernel Mailing List,
Rob Herring
[-- Attachment #1.1: Type: text/plain, Size: 4170 bytes --]
On Fri, Jun 10, 2016 at 03:08:50PM -0700, Stéphane Marchesin wrote:
> On Fri, Jun 10, 2016 at 3:03 PM, Rob Clark <robdclark@gmail.com> wrote:
> > On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
> >> Rob,
> >>
> >> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
> >>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
> >>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
> >>>> using eDP interfaces.
> >>>
> >>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
> >>> we need panel drivers or DT bindings?
> >>
> >> I was wondering about that too. As far as I can tell:
> >>
> >> 1. We need a panel driver because that appears to be what owns a
> >> reference to the backlight / panel power regulator and that part is
> >> not auto-probable.
> >
> > oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
> > driver hell.. I guess being able to use panel-simple is at least an
> > improvement. But panel specific sequences is sounds like panel-simple
> > won't save us all the time :-(
>
> Yes, although you can probably make things mostly work with improper
> sequencing and enough retries, a lot of ARM hw either requires
> interesting sequencing, or has broken/unreliable DDC, which translates
> into the use of panel simple on the sw side.
panel-simple has support for very simple sequencing. You can specify
delays after the prepare and enable stages. This is useful because most
panels have specific requirements when it comes to the amount of time it
takes them to receive video data (after being powered up) and the amount
of time it takes them to show the first valid frame after it has been
received.
The former is used to keep drivers from sending video data to make sure
it can be properly received by the panel, and the latter is used to keep
the backlight off until the first valid frame is visible on the display.
This is used to avoid glitches and seems to work well enough for simple
panels. More complex panels have more involved sequences, so separate
drivers are required.
Also note that the simple-panel driver will try to use EDID if available
and only fall back to the hard-coded mode or timings if there is no DDC
to probe or no modes could be parsed from EDID.
> >> 2. As far as I could tell, there is no way to declare a generic
> >> (unspecified) panel in the device tree. Everyone seems to include
> >> "simple-panel" in their compatible string but as far as I can tell
> >> nothing in the kernel looks at it.
> >>
> >> 3. In theory, all the info specified here should match the EDID
> >> exactly and thus (as you said) be probable. However, it sounds like
> >> (for power sequencing reasons) there might be reasons why you'd want
> >> to know exactly what panel was present beforehand. You might need to
> >> power the panel and backlight in very specific sequences, for
> >> instance. I'm not sure it's always 100% possible in all embedded
> >> designs to read the EDID before you know how the sequencing should
> >> work (but, of course, I'm a NOOB).
> >>
> >> 4. Reading the EDID can be slow. If you happen to know all the info
> >> on the panel beforehand you can significantly speed up boot speed,
> >> notably how fast you can get something on the screen.
> >
> > The theory is (although I think not true currently for most of the arm
> > drivers) that we should be reading back from hw the current config
> > from bootloader splash screen, to avoid a modeset (and conveniently
> > also the need to read edid) at boot.
>
> On some devices the firmware doesn't set any video mode, so there
> isn't anything we can read back. That is our case :)
Reading out hardware state also doesn't give you all the information
that you need. I've never seen hardware that is programmed with the
physical size of the panel, so there's no way to read that back and
you'd still have to either parse EDID or use the value hard-coded in
the simple-panel driver if you want to compute the pixel density.
Thierry
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_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
@ 2016-06-13 11:39 ` Thierry Reding
0 siblings, 0 replies; 23+ messages in thread
From: Thierry Reding @ 2016-06-13 11:39 UTC (permalink / raw)
To: Stéphane Marchesin
Cc: Rob Clark, Doug Anderson, Mark Rutland,
devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org,
Linux Kernel Mailing List, Rob Herring
[-- Attachment #1: Type: text/plain, Size: 4170 bytes --]
On Fri, Jun 10, 2016 at 03:08:50PM -0700, Stéphane Marchesin wrote:
> On Fri, Jun 10, 2016 at 3:03 PM, Rob Clark <robdclark@gmail.com> wrote:
> > On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
> >> Rob,
> >>
> >> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
> >>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
> >>>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
> >>>> using eDP interfaces.
> >>>
> >>> so drive-by comment... but shouldn't eDP be probe-able? Not sure why
> >>> we need panel drivers or DT bindings?
> >>
> >> I was wondering about that too. As far as I can tell:
> >>
> >> 1. We need a panel driver because that appears to be what owns a
> >> reference to the backlight / panel power regulator and that part is
> >> not auto-probable.
> >
> > oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
> > driver hell.. I guess being able to use panel-simple is at least an
> > improvement. But panel specific sequences is sounds like panel-simple
> > won't save us all the time :-(
>
> Yes, although you can probably make things mostly work with improper
> sequencing and enough retries, a lot of ARM hw either requires
> interesting sequencing, or has broken/unreliable DDC, which translates
> into the use of panel simple on the sw side.
panel-simple has support for very simple sequencing. You can specify
delays after the prepare and enable stages. This is useful because most
panels have specific requirements when it comes to the amount of time it
takes them to receive video data (after being powered up) and the amount
of time it takes them to show the first valid frame after it has been
received.
The former is used to keep drivers from sending video data to make sure
it can be properly received by the panel, and the latter is used to keep
the backlight off until the first valid frame is visible on the display.
This is used to avoid glitches and seems to work well enough for simple
panels. More complex panels have more involved sequences, so separate
drivers are required.
Also note that the simple-panel driver will try to use EDID if available
and only fall back to the hard-coded mode or timings if there is no DDC
to probe or no modes could be parsed from EDID.
> >> 2. As far as I could tell, there is no way to declare a generic
> >> (unspecified) panel in the device tree. Everyone seems to include
> >> "simple-panel" in their compatible string but as far as I can tell
> >> nothing in the kernel looks at it.
> >>
> >> 3. In theory, all the info specified here should match the EDID
> >> exactly and thus (as you said) be probable. However, it sounds like
> >> (for power sequencing reasons) there might be reasons why you'd want
> >> to know exactly what panel was present beforehand. You might need to
> >> power the panel and backlight in very specific sequences, for
> >> instance. I'm not sure it's always 100% possible in all embedded
> >> designs to read the EDID before you know how the sequencing should
> >> work (but, of course, I'm a NOOB).
> >>
> >> 4. Reading the EDID can be slow. If you happen to know all the info
> >> on the panel beforehand you can significantly speed up boot speed,
> >> notably how fast you can get something on the screen.
> >
> > The theory is (although I think not true currently for most of the arm
> > drivers) that we should be reading back from hw the current config
> > from bootloader splash screen, to avoid a modeset (and conveniently
> > also the need to read edid) at boot.
>
> On some devices the firmware doesn't set any video mode, so there
> isn't anything we can read back. That is our case :)
Reading out hardware state also doesn't give you all the information
that you need. I've never seen hardware that is programmed with the
physical size of the panel, so there's no way to read that back and
you'd still have to either parse EDID or use the value hard-coded in
the simple-panel driver if you want to compute the pixel density.
Thierry
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^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
2016-06-10 19:52 ` Doug Anderson
@ 2016-06-13 11:28 ` Thierry Reding
-1 siblings, 0 replies; 23+ messages in thread
From: Thierry Reding @ 2016-06-13 11:28 UTC (permalink / raw)
To: Doug Anderson
Cc: Mark Rutland, devicetree@vger.kernel.org,
Linux Kernel Mailing List, dri-devel@lists.freedesktop.org,
Rob Herring, Stéphane Marchesin
[-- Attachment #1.1: Type: text/plain, Size: 3339 bytes --]
On Fri, Jun 10, 2016 at 12:52:41PM -0700, Doug Anderson wrote:
> Rob,
>
> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
> > On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
> >> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
> >> using eDP interfaces.
> >
> > so drive-by comment... but shouldn't eDP be probe-able? Not sure why
> > we need panel drivers or DT bindings?
>
> I was wondering about that too. As far as I can tell:
>
> 1. We need a panel driver because that appears to be what owns a
> reference to the backlight / panel power regulator and that part is
> not auto-probable.
Yes, that's one of the reasons why we still need DT nodes for panels,
even on a probe-able bus.
eDP has a mechanism to allow brightness control via DPCD, but I've never
actually seen it implemented. Even if we had that, we'd likely need to
represent supplies and GPIOs for the backlight, and we'd be back to
square one.
DSI also provides a means to control backlight brightness, but in the
same way that eDP doesn't go all the way, we'd require external
resources to be hooked up via DT again.
> 2. As far as I could tell, there is no way to declare a generic
> (unspecified) panel in the device tree. Everyone seems to include
> "simple-panel" in their compatible string but as far as I can tell
> nothing in the kernel looks at it.
The ones that do are wrong and should eventually be updated. This was
originally done, and the driver used to match on simple-panel as well,
but in retrospect that didn't make any sense at all so it was removed
from the driver again.
One of the reasons why it doesn't make any sense is because even if you
have an EDID that's reachable, the EDID itself doesn't fully specify the
panel. Power sequences are one example of data that's not represented in
EDID.
> 3. In theory, all the info specified here should match the EDID
> exactly and thus (as you said) be probable. However, it sounds like
> (for power sequencing reasons) there might be reasons why you'd want
> to know exactly what panel was present beforehand. You might need to
> power the panel and backlight in very specific sequences, for
> instance. I'm not sure it's always 100% possible in all embedded
> designs to read the EDID before you know how the sequencing should
> work (but, of course, I'm a NOOB).
It doesn't really matter whether you can access EDID or not, because
EDID contains absolutely nothing on the power sequences for the panel.
Even if it did, there'd be no way to relate that information to the
device tree binding phandles.
> 4. Reading the EDID can be slow. If you happen to know all the info
> on the panel beforehand you can significantly speed up boot speed,
> notably how fast you can get something on the screen.
The motivation for potentially duplicating this information was not that
it's slow to read EDID, but that occasionally you might not be able to
access EDID. There could be a number of reasons why this might happen: a
device might end up with an erased or invalid EDID or the DDC used to
get at the EDID might be broken (I've been told that it's fairly common
for OEMs to not wire through the DDC wires in cables to reduce costs).
Thierry
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^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
@ 2016-06-13 11:28 ` Thierry Reding
0 siblings, 0 replies; 23+ messages in thread
From: Thierry Reding @ 2016-06-13 11:28 UTC (permalink / raw)
To: Doug Anderson
Cc: Rob Clark, Mark Rutland, devicetree@vger.kernel.org,
dri-devel@lists.freedesktop.org, Linux Kernel Mailing List,
Rob Herring, Stéphane Marchesin
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On Fri, Jun 10, 2016 at 12:52:41PM -0700, Doug Anderson wrote:
> Rob,
>
> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
> > On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.org> wrote:
> >> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
> >> using eDP interfaces.
> >
> > so drive-by comment... but shouldn't eDP be probe-able? Not sure why
> > we need panel drivers or DT bindings?
>
> I was wondering about that too. As far as I can tell:
>
> 1. We need a panel driver because that appears to be what owns a
> reference to the backlight / panel power regulator and that part is
> not auto-probable.
Yes, that's one of the reasons why we still need DT nodes for panels,
even on a probe-able bus.
eDP has a mechanism to allow brightness control via DPCD, but I've never
actually seen it implemented. Even if we had that, we'd likely need to
represent supplies and GPIOs for the backlight, and we'd be back to
square one.
DSI also provides a means to control backlight brightness, but in the
same way that eDP doesn't go all the way, we'd require external
resources to be hooked up via DT again.
> 2. As far as I could tell, there is no way to declare a generic
> (unspecified) panel in the device tree. Everyone seems to include
> "simple-panel" in their compatible string but as far as I can tell
> nothing in the kernel looks at it.
The ones that do are wrong and should eventually be updated. This was
originally done, and the driver used to match on simple-panel as well,
but in retrospect that didn't make any sense at all so it was removed
from the driver again.
One of the reasons why it doesn't make any sense is because even if you
have an EDID that's reachable, the EDID itself doesn't fully specify the
panel. Power sequences are one example of data that's not represented in
EDID.
> 3. In theory, all the info specified here should match the EDID
> exactly and thus (as you said) be probable. However, it sounds like
> (for power sequencing reasons) there might be reasons why you'd want
> to know exactly what panel was present beforehand. You might need to
> power the panel and backlight in very specific sequences, for
> instance. I'm not sure it's always 100% possible in all embedded
> designs to read the EDID before you know how the sequencing should
> work (but, of course, I'm a NOOB).
It doesn't really matter whether you can access EDID or not, because
EDID contains absolutely nothing on the power sequences for the panel.
Even if it did, there'd be no way to relate that information to the
device tree binding phandles.
> 4. Reading the EDID can be slow. If you happen to know all the info
> on the panel beforehand you can significantly speed up boot speed,
> notably how fast you can get something on the screen.
The motivation for potentially duplicating this information was not that
it's slow to read EDID, but that occasionally you might not be able to
access EDID. There could be a number of reasons why this might happen: a
device might end up with an erased or invalid EDID or the DDC used to
get at the EDID might be broken (I've been told that it's fairly common
for OEMs to not wire through the DDC wires in cables to reduce costs).
Thierry
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^ permalink raw reply [flat|nested] 23+ messages in thread