Re: [RFC 1/2] dt-bindings: clk: versaclock5: Add load capacitance properties

Hi Adam,

On 09/01/21 03:48, Adam Ford wrote:

On Fri, Jan 8, 2021 at 4:49 PM Luca Ceresoli [off-list ref] wrote:

quoted

Hi Adam,

On 06/01/21 18:38, Adam Ford wrote:

quoted

There are two registers which can set the load capacitance for
XTAL1 and XTAL2. These are optional registers when using an
external crystal.  Update the bindings to support them.

Signed-off-by: Adam Ford <redacted>
---
 .../devicetree/bindings/clock/idt,versaclock5.yaml   | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml b/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml
index 2ac1131fd922..e5e55ffb266e 100644
--- a/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml
+++ b/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml

@@ -59,6 +59,18 @@ properties:
     minItems: 1
     maxItems: 2

+  idt,xtal1-load-femtofarads:

I wonder whether we should have a common, vendor independent property.

That would be nice.

quoted

In mainline we have xtal-load-pf (ti,cdce925.txt bindings) which has no
vendor prefix. However I don't know how much common it is to need

rtc-pcf85063.c uses  quartz-load-femtofarads, so there is already some
discrepancy.

Since the unit of measure here is femtofarads, using pF in the name seems wrong.
We need to read the data as a u32, so femtofarads works better than
pF, which would require a decimal point.

quoted

different loads for x1 and x2. Any hardware engineer around?

I talked to a hardware engineer where I work, and he said it makes
sense to keep them the same.  I only separated them because there are
two registers, and I assumed there might be a reason to have X1 and X2
be different, but I'm ok with reading one value and writing it to two
different registers.

If both your HW engineer and the Renesas docs say setting different
values is not useful in real life, and other drivers don't set different
values as well, it looks like that is the reasonable way. I think it
also increases likelihood of establishing a unique property name to be
used for all future chips.

-- 
Luca