[PATCH V3 13/14] ARM: dts: stm32: Adjust PLL4 settings on AV96

[Marek Vasut <marex@denx.de>]

On 4/1/20 12:24 PM, Patrick DELAUNAY wrote:
> Hi Gerald and Manivannan,

Hi,

>> From: Marek Vasut <marex@denx.de>
>> Sent: mardi 31 mars 2020 19:52
>>
>> The PLL4 is supplying SDMMC12, SDMMC3 and SPDIF with 120 MHz and
>> FDCAN with 96 MHz. This isn't good for the SDMMC interfaces, which can not
>> easily divide the clock down to e.g. 50 MHz for high speed SD and eMMC
>> devices, so those devices end up running at 30 MHz as that is 120 MHz / 4.
>> Adjust the PLL4 settings such that both PLL4P and PLL4R run at 100 MHz
>> instead, which is easy to divide to 50MHz for optimal operation of both SD and
>> eMMC, SPDIF clock are not that much slower and FDCAN is also unaffected.
>>
>> Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
>> Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
>> Signed-off-by: Marek Vasut <marex@denx.de>
>> Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
>> Cc: Patrick Delaunay <patrick.delaunay@st.com>
>> Cc: Patrice Chotard <patrice.chotard@st.com>
>> ---
>> V2: Move this patch before the split of AV96 into SoM and carrier
>> V3: No change
>> ---
> 
> This patch update the PLL4 frequency used on AV96 board,
> with different of reference clock tree used on ST board,
> this new setting allow to optimize the SDMMC frequency (50MHz vs 30Mz).
> 
> I don't know why the previous PLL4 frequency was chosen as a compromise
> on reference clock-tree  (PLL4 is used by mostly all the peripheral,
> with display and audio requirements).
> 
> Can you cross check the proposed clock tree and ack this patch
> if these ST requirements are not applicable on AV96 board.
> 
> Anyway the code is correct.

Likely because these PLL settings are being copied from reference
platform to other platforms etc.

But I did notice one odd thing, which is when running the SD1 in SDR104,
the read data transfers can be unstable, which I suspect is because the
bus runs at actual 100 MHz instead of some 60 MHz. I need to look at
that with a scope, so that's to be checked. For now I turned the SDR104 off.