Re: [PATCH 00/14] ARM: davinci: step towards removing at24_platform_data

[Bartosz Golaszewski]

2018-06-26 10:38 GMT+02:00 Andrew Lunn <andrew@xxxxxxx>:
> On Tue, Jun 26, 2018 at 09:44:19AM +0200, Bartosz Golaszewski wrote:
>> 2018-06-25 20:02 GMT+02:00 Andrew Lunn <andrew@xxxxxxx>:
>> >> With my patch 1/14 you'll get -EPROBE_DEFER from nvmem_cell_get() if
>> >> the nvmem provider is not yet registered. Will that help in your case?
>> >
>> > I don't think so. My driver instantiates the AT24 device. So if i get
>> > -EPROBE_DEFER, i need to cleanup the probe, and return -EPROBDE_DEFER
>> > to the code. Which means i need to remove the AT24 device...
>> >
>> >        Andrew
>>
>> Are you sure this is the correct approach? I understand that you want
>> to use something like board files for your machine? Wouldn't it be
>> better to register a platform device for at24, register a cell lookup
>> and then depend on that cell from your driver?
>
> Hi Bartosz
>
> The machine is based around a Kontron Com Express module, with an
> intel Ivy Bridge CPU. This is then placed into a custom carrier board,
> which has a number of i2c devices.
>
> I have a platform driver which matches on the DMI system ID for the
> Kontron module.
>
> The Com Express module has a PLD which implements i2c, gpio,
> etc. There is an MFD for this, which instantiates the i2c-kempld i2c
> bus driver.
>
> My platform driver finds this i2c-kempld bus driver. If it does not
> exist yet, it return -EPROBE_DEFER. If it is found it instantiates an
> at24 device on it. I need to look at the content of the EEPROM to
> determine the hardware revision, plus do a checksum.  From that, i
> need to instantiates 1 or 2 additional AT24, up to 4 GPIO expanders,
> and i2c to spi converter, add some gpio-leds on the gpio expanders,
> create a bit-banging MDIO bus, instantiate an Ethernet switch on the
> MDIO bus, maybe add an Ethernet switch to the SPI bus, etc.
>
> As you can see, i have a chain of events. I cannot move onto the next
> part of the chain until i know the probe for the previous part has
> finished. e.g. i cannot add gpio-leds until i know the gpio expander
> has probed. But the gpio expander provides a call back, similar to the
> at24 setup(). The MDIO bus and the SPI bus has a mechanism to register
> an info structure, just like you have done for NVMEM cells. So that
> works out.
>
> The weak link in this chain is that first at24 probe, and knowing when
> i can access the nvmem cells for the revision and checksum
> information. -EPROBE_DEFER does not help me here. I need either some
> sort of blocking wait for the cells to become available, or a callback
> in a context which allows me to instantiate more devices.
>
> I also have some steps which cannot be undone. You don't provide a
> mechanism to unregister the nvmem info structure. The I2C and MDIO
> equivalent also does not provide an unregister for bus info. So once i
> register the first info structure, i'm past the point of no return. I
> cannot return -EPROBE_DEFER because i cannot unregister the info
> structures, so that i can register them again the next time the
> platform driver gets probed.
>
>          Andrew
>

I see. I see it this way: the setup callback comes from the time when
we didn't have nvmem and should go away. I will protest loud whenever
someone will try to use it again and will work towards removing it as
soon as possible.

I will give your problem a thought and will try to get back with some
proposals - maybe we should, as you suggested, extend nvmem even
further to allow to remove nvmem info entries etc.

Best regards,
Bartosz Golaszewski