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

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Message-ID: <CAMpxmJXwq-w4WXwh+oF0_NACfcu4nE4n6MZPA19oMpt8j1Hdog@mail.gmail.com>
Date:   Mon, 28 Nov 2016 11:43:59 +0100
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Sekhar Nori <nsekhar@...com>
Cc:     Tomi Valkeinen <tomi.valkeinen@...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>,
        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

2016-11-28 8:58 GMT+01:00 Sekhar Nori <nsekhar@...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@...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.
>
> 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