linux-kernel - Re: [RFC PATCH] net: dsa: mv88e6xxx: support single chip sw

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Date:   Tue, 5 Mar 2019 14:45:49 +0000
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Rasmus Villemoes <Rasmus.Villemoes@...vas.se>,
        Per Noergaard Christensen <pnc@...f.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] net: dsa: mv88e6xxx: support single chip sw_addr
 offset

On 04/03/2019 14.45, Andrew Lunn wrote:
> On Mon, Mar 04, 2019 at 12:59:42PM +0000, Rasmus Villemoes wrote:
>> From: Per Noergaard Christensen <pnc@...f.com>
>>
>> The 88e6250 does not support multi-chip addressing. However, one can
>> still have two of them on the same mdio bus, since the device only
>> uses 16 of the 32 possible addresses, either addresses 0x00-0x0F or
>> 0x10-0x1F depending on the ADDR4 pin at reset [since ADDR4 is
>> internally pulled high, the latter is the default].
>>
>> In order to prepare for supporting the 88e6250, change
>> mv88e6xxx_smi_init and the single_chip_{read,write} functions to allow
>> and honour a non-zero sw_addr in single chip mode. Since this only
>> changes the behaviour in the sw_addr!=0 && !chip->info->multi_chip
>> case from returning -EINVAL, it should not break existing setups.
> 
> Hi Rasmus
> 
> We have the nice abstraction of mv88e6xxx_smi_multi_chip_ops and
> mv88e6xxx_smi_single_chip_ops. I think we should extend this
> abstraction and implement mv88e6xxx_smi_dual_chip_ops. Also, add a
> chip->info->dual_chip flag, so we know when it can be used.
> 

Hi Andrew

I should probably give a little more context, and mention that I don't
actually have a board with two 88e6250 sitting on the same bus, so I
can't test that such a setup will actually work (but I assume that's the
whole point of the reset behaviour). But I do have a single 88e6250, at
addresses 0x10-0x1f, and for that to get recognized I need to get
mv88e6xxx_detect to look at smi address 0x10+0x8 (i.e., I also need to
add another family to mv88e6xxx_of_match with port_base_addr = 0x08, and
then somehow have mv88e6xxx_smi_read add a 0x10 offset).

I can certainly do as you suggest instead, but I'm not sure what that
buys us? Especially given that I can't test an actual dual chip setup, I
think it's more natural to use a "single chip ops with an addr offset".

Thanks,
Rasmus