Re: [PATCH v2 1/3] Ethernet packet sniffer: Device tree binding and vendor prefix

From: Stathis Voukelatos
Date: Tue Feb 17 2015 - 11:22:25 EST

Hi Mark,

On 17/02/15 14:51, Mark Rutland wrote:

+Matched packet bytes and timestamp values are returned through a
+FIFO. Timestamps are provided to the module through an externally
+generated Gray-encoded counter.

Does this counter unit need to be enabled (or have any input clocks

Yes it does, that is the purpose of the 'tstamp' entry in the
'clock-names' property below.

+Required properties:
+- compatible : must be "linn,eth-sniffer"

Is there not a more precise name for this IP block?

It is generally called 'ethernet packet sniffer', maybe
linn,eth-packet-sniffer would be a more descriptive name?

+- reg : physical addresses and sizes of registers. Must contain 3 entries:
+ - registers memory space
+ - TX command string memory
+ - RX command string memory
+- reg-names : must contain the following 3 entries:
+ "regs", "tx-ram", "rx-ram"

It would be nicer to format this as:

- reg: A list of physical address and size pairs corresponding to each
entry in reg-names

- reg-names: must contain:
* "regs" for the control registers
* "tx-ram" for the TX command string memory
* "rx-ram" for the RX command string memory

Which avoids redundancy.

Will change formatting as suggested

The phrase "command string" sounds a bit odd. What are these used for

In these two memory areas we program a sequence of bytes in the
format: [cmd][value][cmd][value] to configure the data patterns that
the sniffer should match for Ethernet TX and RX packets respectively.
Maybe 'command memory' would be clearer?

+- interrupts : sniffer interrupt specifier
+- clocks : specify the system clock for the peripheral and
+ the enable clock for the timestamp counter
+- clock-names : must contain the "sys" and "tstamp" entries

Similarly here clocks should just be defined in terms of clock-names.

Will reformat similar to the 'regs' field

+- fifo-block-words : number of words in one data FIFO entry

I didn't see a data FIFO described. Is that dynamically allocated and
handed to the sniffer, or does that correspond to one of the memory
regions above?

It is a H/W FIFO internal to the module and accessed through
a register. It is divided in blocks and 'fifo-block-words'
specify the number of words in each block. It is needed by
the driver to make sure it reads an entire block, in order
to clear the 'data available' interrupt.

+- tstamp-hz : frequency of the timestamp counter
+- tstamp-shift : shift value for the timestamp cyclecounter struct

What exactly is this used for?

Are there any docs?

See: include/linux/clocksource.h
The driver uses a cyclecounter and timecounter to convert raw timestamps
to nanoseconds. 'tstamp-shift' refers to the 'shift' field of the
cyclecounter structure, that can be used to improve the precision of
the conversion

+- tstamp-bits : width in bits of the timestamp counter
+sniffer@1814a000 {
+ compatible = "linn,eth-sniffer";
+ reg = <0x1814a000 0x100>, <0x1814a400 0x400>,
+ <0x1814a800 0x400>;
+ reg-names = "regs", "tx-ram", "rx-ram";
+ interrupts = <GIC_SHARED 58 IRQ_TYPE_LEVEL_HIGH>;
+ clocks = <&clk_core CLK_AUDIO>,
+ <&cr_periph SYS_CLK_EVENT_TIMER>;
+ clock-names = "sys", "tstamp";
+ fifo-block-words = <4>;
+ tstamp-hz = <52000000>;
+ tstamp-shift = <27>;
+ tstamp-bits = <30>;

This property wasn't documented.

As mentioned previously, I think the relation between this unit and the
MAC and/or PHY needs to be explicitly described in the DT.

Do you suggest a field along the lines of:
mac = <&eth_controller_0>;
The driver could check that it exists and is valid but does
not need to make use of it.

diff --git a/Documentation/devicetree/bindings/vendor-prefixes.txt b/Documentation/devicetree/bindings/vendor-prefixes.txt
index d443279..891c224 100644
--- a/Documentation/devicetree/bindings/vendor-prefixes.txt
+++ b/Documentation/devicetree/bindings/vendor-prefixes.txt
@@ -90,6 +90,7 @@ lacie LaCie
lantiq Lantiq Semiconductor
lenovo Lenovo Group Ltd.
lg LG Corporation
+linn Linn Products Ltd.

This addition looks fine to me. For some reason it seems to be padded
with spaces instead of tabs though; is my mail server corrupting things
or is that the case in the original patch?

Sorry, it was spaces. Will be fixed


Thank you,

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