[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@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

Re: [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@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

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

[Tomi Valkeinen]

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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@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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Re: [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@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

Re: [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@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