Date:	Tue, 09 Sep 2014 20:33:17 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
CC:	NetDev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	John Linville <linville@...driver.com>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH net-next v5 11/13] net: dsa: add Broadcom SF2 switch driver

On 09/09/14 19:17, Alexander Duyck wrote:

[snip]

>>>
>>>> +static char *bcm_sf2_sw_probe(struct mii_bus *bus, int sw_addr)
>>>> +{
>>>> +       return "Broadcom Starfighter 2";
>>>> +}
>>>> +
>>> I hadn't noticed before but with this driver it seems like you could
>>> potentially load on any DSA enabled device could you not?  It seems
>>> like this would be problematic since you could end up registering
>>> before another DSA driver and prevent it from being able to load since
>>> you always return success.  Isn't there any test you could run to
>>> determine if the switch is actually there or not?
>> Unfortunately the current DSA device/driver model is kind of messed up
>> for that, which is something I plan on fixing, although it would take a
>> little bit more time. The way it works currently is:
>>
>> - you register a DSA platform device, feed it with Device Tree or
>> C-struct configuration data
>> - you register a switch driver
>> - the DSA platform code will eventually iterate over all switch devices,
>> call into their probe function and based on a non-NULL return, accept to
>> register this switch device
>> - the probe function only accepts MDIO connected switches, anything else
>> has to find another way to tell that it is there
>>
>> so all of this works okay until you have a switch which is memory-mapped
>> into the CPU address space and which is not on the MDIO bus.
>>
>> A short term solution could be to change the probe argument to be more
>> generic and pass a void *bus pointer or something allowing us to do a
>> tad more things, including verifying a register to see if the switch is
>> there.
>
> I would probably just rewrite the call to accept dsa_chip_data instead
> of passing it the mii_bus and sw_addr.  Then you can just access data
> like the of_node directly.  I'm also thinking it might make more sense
> to make the mii_bus pointer in the dsa_chip_data a bit more type
> agnostic by simply treating it as a parent device.  It seems like most
> of the code is already there in dsa via the dev_find_class check that is
> checking for "mdio_bus".

Yes, I like that.

>
>> The way I would like to fix this model though is to allow switch drivers to:
>>
>> - specify their own configuration data, since for instance, external
>> switches usually have a pretty fixed set of configuration options:
>> number of ports, fixed CPU port, while keeping platform-driven
>> configuration data as well
>>
>> - be backed by their host interface device/driver, e.g: allow a SPI,
>> PHY, PCI(e), USB drivers to register a switch driver, such that there
>> really is a struct device pointer we can refer to for various operations
>> (DMA, PM...)
>
> This is the kind of situation I am looking at.  In my case I have a PCIe
> interface with one of the BARs providing access to switch registers.  As
> such I would want to be able to provide a PCI device and sort out the
> eligibility to run the driver by checking for the PCI vendor and device ID.

I see, bcm_sf2 is kind of similar here, thanks to Device Tree we can do 
a lot of things without being backed by an actual struct device, but 
there are other situations where this is not desirable, like yours. In 
my case a platform_device/driver would be more appropriate anyway.

>
>> I will cook some patches that do that in the next few days.
>> --
>> Florian
>
> I'll keep an eye open for them.  I might start submitting a few patches
> myself as I should be pushing my driver in the next week or two.

Great! I will also keep an eye on it too, I got some patches I would 
like to send that add suspend/resume support, Wake-on-LAN and EEE to 
DSA/bcm_sf2; but we should probably get the device/driver model right first.

Those are pretty trivial patches anyway that just add some layering 
around the DSA and the DSA switch drivers.

NB: I have not yet addressed your suggestion of replacing tag_protocol 
with an enum, feel free to send that first.
--
Florian
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