linux-kernel - Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

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Message-ID: <04306a5e-f9be-35a4-1aa1-5795d780e289@ti.com>
Date:   Mon, 30 Sep 2019 17:20:22 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Adam Ford <aford173@...il.com>,
        "H. Nikolaus Schaller" <hns@...delico.com>
CC:     Tero Kristo <t-kristo@...com>, Tony Lindgren <tony@...mide.com>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        Adam Ford <adam.ford@...icpd.com>,
        Benoît Cousson <bcousson@...libre.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to
 dts

On 30/09/2019 17:12, Adam Ford wrote:

>> I don't know the implications, so if the people from TI say stick with
>> 16, I'm fine with that, but at least there is some evidence that it
>> can be higher than 16, but lower than 32.
>>
> 
> Sorry for all the spam, but I moved both of them to 31 from 32, and it
> also seems to work successfully at 31.
> 
> [   26.923004] DSS: set fck to 36000000
> [   26.923034] DISPC: lck = 36000000 (1)
> [   26.923034] DISPC: pck = 9000000 (4)
> [   26.925048] DISPC: channel 0 xres 480 yres 272
> [   26.925048] DISPC: pck 9000000
> [   26.925048] DISPC: hsync_len 42 hfp 3 hbp 2 vsw 11 vfp 2 vbp 3
> [   26.925079] DISPC: vsync_level 1 hsync_level 1 data_pclk_edge 1
> de_level 1 sync_pclk_edge -1
> [   26.925079] DISPC: hsync 17077Hz, vsync 59Hz
> [   27.384613] DISPC: dispc_runtime_put
> 
> Is it possible to use 31?

Let's see what Tero says, but yeah, something is odd here. I expected 
the max divider to be 16 with Tero's patch, but I don't see it having 
that effect. I can get the div to 31.

You can see this from the clock register 0x48004e40 (CM_CLKSEL_DSS). The 
lowest bits are the divider, 5 to 0. The TRM says max div is 32.

Tero said for him the dividers > 16 didn't "stick" to the register. I'm 
now wondering if he has an old beagleboard with OMAP34xx, which has max 
div 16.

  Tomi

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