[PATCH v3 08/10] Doc: usb: ci-hdrc-usb2: add tx(rx)-burst-config-dword for binding doc

Am Freitag, den 14.08.2015, 16:40 +0800 schrieb Peter Chen:

On Fri, Aug 14, 2015 at 10:37:30AM +0200, Lucas Stach wrote:

quoted

Am Freitag, den 07.08.2015, 15:15 +0800 schrieb Peter Chen:

quoted

It is used to override the default setting for burst size, changing
burst size takes effect only when the SBUSCFG.AHBBRST = 0.

Signed-off-by: Peter Chen <redacted>
---
 Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt b/Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt
index d52a747..d71ef07 100644
--- a/Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt
+++ b/Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

@@ -37,6 +37,14 @@ Optional properties:
   property is used to change AHB burst configuration, check the chipidea
   spec for meaning of each value. If this property is not existed, it
   will use the reset value.
+- tx-burst-size-dword: it is vendor dependent, the tx burst size in dword
+  (4 bytes), This register represents the maximum length of a the burst
+  in 32-bit words while moving data from system memory to the USB
+  bus, changing this value takes effect only the SBUSCFG.AHBBRST is 0.

The last bits about SBUSCFG.AHBBRST don't make any sense in the DT
binding. The DT writer doesn't know about the SBUSCFG.AHBBRST value and
should not care either.
If someone sets the burst size to something
non-standard in the DT, the driver should make sure to enable to
necessary bits to make this setting take effect.

The SBUSCFG.AHBBRST property description is just above this one,
and the user should read the spec for changing these system
configuration parameters.

This is not clear from the description. This should read something like
"The value of this property will only take effect if property
ahb-burst-config is set to 0."

The DT binding should be coherent and understandable without reading the
DW USB spec and/or the code. Please take some care about this.

This requirement is from the spec, and the code logic makes sure
the burst size changes only when SBUSCFG.AHBBRST is 0, since the
hardware will only change burst size if SBUSCFG.AHBBRST is 0.

quoted

Also both those descriptions are missing a description to what value the
burst sizes will be set if the DT properties are not found. If it's
implementation defined spell this out in the doc.

It is optional property, if without this property, it will use the
default value.

Then specify this in the binding. Again, the binding is a spec that one
should be able to understand without reading the code.

"If this property is missing the reset defaults of the hardware
implementation will be used."

quoted

Are there really cases where it makes sense to set RX and TX burst sizes
to different values?

Yes, the default burst size is the same for all SoCs, but the bus
burst size may be larger for some SoCs.

This question wasn't meant to dispute the RX/TX burst size
configuration, but I'm questioning if it really makes sense to set them
to different values. Do we really need 2 properties for this, or is 1
enough?

Do you see any use case where someone would like to do something like:
tx-burst-size-dword = <0x8>;
rx-burst-size-dword = <0x10>;

Regards,
Lucas
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