[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[Bartosz Golaszewski]

It has been determined that the maximum resolution supported correctly
by tilcdc rev1 on da850 SoCs is 800x600 at 60. Due to memory throughput
constraints we must filter out higher modes.

Specify the max-bandwidth property for the display node for
da850-based boards.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
---
 arch/arm/boot/dts/da850.dtsi | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/boot/dts/da850.dtsi b/arch/arm/boot/dts/da850.dtsi
index 8e30d9b..9b7c444 100644
--- a/arch/arm/boot/dts/da850.dtsi
+++ b/arch/arm/boot/dts/da850.dtsi
@@ -452,6 +452,7 @@
 			compatible = "ti,da850-tilcdc";
 			reg = <0x213000 0x1000>;
 			interrupts = <52>;
+			max-bandwidth = <28800000>;
 			status = "disabled";
 
 			ports {
-- 
2.9.3

[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[Sekhar Nori]

On Friday 25 November 2016 09:07 PM, Bartosz Golaszewski wrote:
> It has been determined that the maximum resolution supported correctly
> by tilcdc rev1 on da850 SoCs is 800x600 at 60. Due to memory throughput
> constraints we must filter out higher modes.
> 
> Specify the max-bandwidth property for the display node for
> da850-based boards.
> 
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> ---
>  arch/arm/boot/dts/da850.dtsi | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/boot/dts/da850.dtsi b/arch/arm/boot/dts/da850.dtsi
> index 8e30d9b..9b7c444 100644
> --- a/arch/arm/boot/dts/da850.dtsi
> +++ b/arch/arm/boot/dts/da850.dtsi
> @@ -452,6 +452,7 @@
>  			compatible = "ti,da850-tilcdc";
>  			reg = <0x213000 0x1000>;
>  			interrupts = <52>;
> +			max-bandwidth = <28800000>;

If this is effectively the max pixel clock that the device supports,
then why not use the datasheet specified value of 37.5 MHz (Tc = 26.66 ns).

Thanks,
Sekhar

[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[Tomi Valkeinen]

On 28/11/16 07:24, Sekhar Nori wrote:
> On Friday 25 November 2016 09:07 PM, Bartosz Golaszewski wrote:
>> It has been determined that the maximum resolution supported correctly
>> by tilcdc rev1 on da850 SoCs is 800x600 at 60. Due to memory throughput
>> constraints we must filter out higher modes.
>>
>> Specify the max-bandwidth property for the display node for
>> da850-based boards.
>>
>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>> ---
>>  arch/arm/boot/dts/da850.dtsi | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm/boot/dts/da850.dtsi b/arch/arm/boot/dts/da850.dtsi
>> index 8e30d9b..9b7c444 100644
>> --- a/arch/arm/boot/dts/da850.dtsi
>> +++ b/arch/arm/boot/dts/da850.dtsi
>> @@ -452,6 +452,7 @@
>>  			compatible = "ti,da850-tilcdc";
>>  			reg = <0x213000 0x1000>;
>>  			interrupts = <52>;
>> +			max-bandwidth = <28800000>;
> 
> If this is effectively the max pixel clock that the device supports,
> then why not use the datasheet specified value of 37.5 MHz (Tc = 26.66 ns).

There's a separate property for max-pixelclock. This one is maximum
pixels per second (which does sound almost the same), but the doc says
it's about the particular memory interface + LCDC combination.

But this 'max-bandwidth' does sound quite odd, as the it really should
be bytes, not pixels... Bad bindings again, which we just have to use.

 Tomi

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[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[Sekhar Nori]

On Monday 28 November 2016 01:12 PM, Tomi Valkeinen wrote:
> On 28/11/16 07:24, Sekhar Nori wrote:
>> On Friday 25 November 2016 09:07 PM, Bartosz Golaszewski wrote:
>>> It has been determined that the maximum resolution supported correctly
>>> by tilcdc rev1 on da850 SoCs is 800x600 at 60. Due to memory throughput
>>> constraints we must filter out higher modes.
>>>
>>> Specify the max-bandwidth property for the display node for
>>> da850-based boards.
>>>
>>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>>> ---
>>>  arch/arm/boot/dts/da850.dtsi | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/arch/arm/boot/dts/da850.dtsi b/arch/arm/boot/dts/da850.dtsi
>>> index 8e30d9b..9b7c444 100644
>>> --- a/arch/arm/boot/dts/da850.dtsi
>>> +++ b/arch/arm/boot/dts/da850.dtsi
>>> @@ -452,6 +452,7 @@
>>>  			compatible = "ti,da850-tilcdc";
>>>  			reg = <0x213000 0x1000>;
>>>  			interrupts = <52>;
>>> +			max-bandwidth = <28800000>;
>>
>> If this is effectively the max pixel clock that the device supports,
>> then why not use the datasheet specified value of 37.5 MHz (Tc = 26.66 ns).
> 
> There's a separate property for max-pixelclock. This one is maximum
> pixels per second (which does sound almost the same), but the doc says
> it's about the particular memory interface + LCDC combination.

DA850 supports both mDDR and DDR2, at slightly different speeds. So
memory bandwidth limitation is also board specific. This should probably
move to board file.

But I would like to know why using max-pixelclock is not good enough.
Have experiments shown that LCDC on DA850 LCDK underflows even if pixel
clock is below the datasheet recommendation?

Thanks,
Sekhar

[PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

[Bartosz Golaszewski]

2016-11-28 8:58 GMT+01:00 Sekhar Nori <nsekhar@ti.com>:
> On Monday 28 November 2016 01:12 PM, Tomi Valkeinen wrote:
>> On 28/11/16 07:24, Sekhar Nori wrote:
>>> On Friday 25 November 2016 09:07 PM, Bartosz Golaszewski wrote:
>>>> It has been determined that the maximum resolution supported correctly
>>>> by tilcdc rev1 on da850 SoCs is 800x600 at 60. Due to memory throughput
>>>> constraints we must filter out higher modes.
>>>>
>>>> Specify the max-bandwidth property for the display node for
>>>> da850-based boards.
>>>>
>>>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>>>> ---
>>>>  arch/arm/boot/dts/da850.dtsi | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/arch/arm/boot/dts/da850.dtsi b/arch/arm/boot/dts/da850.dtsi
>>>> index 8e30d9b..9b7c444 100644
>>>> --- a/arch/arm/boot/dts/da850.dtsi
>>>> +++ b/arch/arm/boot/dts/da850.dtsi
>>>> @@ -452,6 +452,7 @@
>>>>                     compatible = "ti,da850-tilcdc";
>>>>                     reg = <0x213000 0x1000>;
>>>>                     interrupts = <52>;
>>>> +                   max-bandwidth = <28800000>;
>>>
>>> If this is effectively the max pixel clock that the device supports,
>>> then why not use the datasheet specified value of 37.5 MHz (Tc = 26.66 ns).
>>
>> There's a separate property for max-pixelclock. This one is maximum
>> pixels per second (which does sound almost the same), but the doc says
>> it's about the particular memory interface + LCDC combination.
>
> DA850 supports both mDDR and DDR2, at slightly different speeds. So
> memory bandwidth limitation is also board specific. This should probably
> move to board file.
>
> But I would like to know why using max-pixelclock is not good enough.
> Have experiments shown that LCDC on DA850 LCDK underflows even if pixel
> clock is below the datasheet recommendation?
>

Hi Sekhar,

I've just tested 1024x768 at 37000 KHz - indeed seems like the
underflows are gone as soon as we go below 37500 KHz. I'll submit a
new patch using the max-pixelclock property.

Thanks,
Bartosz Golaszewski