Re: [RFC 1/4] pwm: sifive: Add DT documentation for SiFive PWM Controller.

From: Atish Patra
Date: Mon Oct 15 2018 - 18:57:38 EST


On 10/10/18 6:49 AM, Thierry Reding wrote:
On Tue, Oct 09, 2018 at 11:51:22AM -0700, Atish Patra wrote:
From: "Wesley W. Terpstra" <wesley@xxxxxxxxxx>

DT documentation for PWM controller added with updated compatible
string.

Signed-off-by: Wesley W. Terpstra <wesley@xxxxxxxxxx>
[Atish: Compatible string update]
Signed-off-by: Atish Patra <atish.patra@xxxxxxx>
---
.../devicetree/bindings/pwm/pwm-sifive.txt | 32 ++++++++++++++++++++++
1 file changed, 32 insertions(+)
create mode 100644 Documentation/devicetree/bindings/pwm/pwm-sifive.txt

diff --git a/Documentation/devicetree/bindings/pwm/pwm-sifive.txt b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt
new file mode 100644
index 00000000..532b10fc
--- /dev/null
+++ b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt
@@ -0,0 +1,32 @@
+SiFive PWM controller
+
+Unlike most other PWM controllers, the SiFive PWM controller currently only
+supports one period for all channels in the PWM. This is set globally in DTS.
+The period also has significant restrictions on the values it can achieve,
+which the driver rounds to the nearest achievable frequency.

What restrictions are these? If "nearest achievable" is too far off the
target period it might be preferable to error out.


@Wes: Any comments?

+Required properties:
+- compatible: should be one of
+ "sifive,fu540-c000-pwm0","sifive,pwm0".

What's the '0' in here? A version number?


I think yes. Since fu540 is the first Linux capable RISC-V core, SiFive Guys decided mark it as version 0.

@Wesly: Please correct me if I am wrong.

+ PWM controller is HiFive Unleashed specific chip which warrants a
+ specific compatible string. The second string is kept for backward
+ compatibility until a firmware update with latest compatible string.
+- reg: physical base address and length of the controller's registers
+- clocks: The frequency the controller runs at
+- #pwm-cells: Should be 2.
+ The first cell is the PWM channel number
+ The second cell is the PWM polarity
+- sifive,approx-period: the driver will get as close to this period as it can

Given the above comment, maybe "sifive,period"?


ok.
In Unleashed board, the DT is loaded by FSBL (first stage boot loader). Thus, changing device tree entries requires a FSBL update. If we update this string, we need to update the driver to parse both properties so that existing devices with older firmware continue to work.

This is probably ok as we anyways do that for compatible strings. Just wanted to update that here for the record.

Regards,
Atish
Thierry