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

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Date:   Mon, 28 Nov 2016 09:42:30 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Sekhar Nori <nsekhar@...com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Russell King <linux@...linux.org.uk>
CC:     LKML <linux-kernel@...r.kernel.org>,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        linux-drm <dri-devel@...ts.freedesktop.org>,
        linux-devicetree <devicetree@...r.kernel.org>,
        Jyri Sarha <jsarha@...com>, David Airlie <airlied@...ux.ie>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] ARM: dts: da850: specify the maximum bandwidth for tilcdc

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@...libre.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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