linux-kernel - Re: [PATCH v2 net-next 1/3] net: dsa: lan9303: Refactor lan9303_xxx_packet

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Message-ID: <9f06fee2-563a-cbef-4913-fd3f24306a68@egil-hjelmeland.no>
Date:   Tue, 1 Aug 2017 16:43:06 +0200
From:   Egil Hjelmeland <privat@...l-hjelmeland.no>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     vivien.didelot@...oirfairelinux.com, f.fainelli@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...gutronix.de
Subject: Re: [PATCH v2 net-next 1/3] net: dsa: lan9303: Refactor
 lan9303_xxx_packet_processing()

On 01. aug. 2017 16:02, Andrew Lunn wrote:
> On Tue, Aug 01, 2017 at 03:50:14PM +0200, Egil Hjelmeland wrote:
>> On 01. aug. 2017 15:39, Andrew Lunn wrote:
>>>> @@ -704,7 +710,7 @@ static void lan9303_get_ethtool_stats(struct dsa_switch *ds, int port,
>>>>   	unsigned int u, poff;
>>>>   	int ret;
>>>> -	poff = port * 0x400;
>>>> +	poff = LAN9303_SWITCH_PORT_REG(port, 0);
>>>>   	for (u = 0; u < ARRAY_SIZE(lan9303_mib); u++) {
>>>>   		ret = lan9303_read_switch_reg(chip,
>>>
>>> So the actual code is:
>>>
>>> 	for (u = 0; u < ARRAY_SIZE(lan9303_mib); u++) {
>>>         		ret = lan9303_read_switch_reg(chip,
>>> 			      		      lan9303_mib[u].offset + poff,
>>> 					      &reg);
>>>
>>> Could this be written as
>>>
>>> 	for (u = 0; u < ARRAY_SIZE(lan9303_mib); u++) {
>>> 		ret = lan9303_read_switch_port(chip, port, lan9303_mib[u].offset, &reg);
>>>
>>> It is then clear you are reading the statistics from a port register.
>>>
>>>     Andrew
>>>
>>
>> Yes it can. Since it is (insignificantly) less efficient, I
>> chose not to touch it. But I can do it if you like.
> 
> I doubt it is less efficient. The compiler has seen
> lan9303_read_switch_port() and will probably inline it.  So what the
> optimiser gets to see is probably the same in both cases.
> 
> Try generating the assembler listing in both cases, and compare them
> 
> make drivers/net/dsa/lan9303-core.lst
> 
>       Andrew
> 

Thanks for the tips about generating assembler listing, can be useful
another time. But in this case I trust you :-)
And in this case it does not really matter, because its not in the
data path.

I did try to look at the listing. But I did not quite understand it.
Looks like it is doing both inlining and unrolling.

Anyway, you just decide how you like to have it in this series.

Egil