Re: [PATCH 5/9] mfd: Add UART support for the ST-Ericsson CG2900.

[Pavan Savoy]

On Tue, Dec 7, 2010 at 4:37 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Monday 06 December 2010 22:24:34 Vitaly Wool wrote:
>> On Mon, Dec 6, 2010 at 5:54 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>
>> >> But I was trying to make a different point here. On a basic level,
>> >> there's this cg2000 chip from STE that does BT, FM and GPS. There's
>> >> the chip from TI that does BT, FM and GPS, and there's the Broadcom
>> >> chip that does BT+FM. They all use HCI to access the other functions
>> >> of the combo chip and they do it in a really simiar way, with the
>> >> differences mostly in power management techniques. So I think it's
>> >> quite sensible to have some kind of framework that is suitable for
>> >> such devices.
>> >
>> > Yes, I agree 100% in principle. I could not find the code that
>> > Broadcom/TI FM and GPS stuff so far, can you point us to that?
>>
>> Sure, the TI "shared transport" code is mostly at drivers/misc/ti-st.
>>
>> Some Broadcom BCM43xx chips work in a similar way AFAIK but I'm not
>> sure about the mainline support for those.
>
> Ah, it had not occured to me to look in drivers/misc. Looking at the
> code over there, it becomes much clearer what your point is. Evidently
> the ti-st driver implements a line discipline similar to, but conflicting
> with hci_ldisc, and it has its own copy of the hci_ll code.
>
> I guess this is exactly what we're trying to avoid having with the
> cg2900 code ;-).
>
>> > The cg2900 solution for this was to use MFD (plus another layer
>> > in the posted version, but that will go away I assume). Using
>> > MFD is not the only possibility here, but I could not see anything
>> > wrong with it either. Do you think we can move them all over to
>> > use MFD infrastructure?
>>
>> I guess so but it's probably more of a detail than what I'm up to now :)
>
> Agreed.
>
>> >> But generally speaking, isn't a line discipline/driver attached to a
>> >> tty? We can use dumb tty for e. g. SPI and still be able to use
>> >> hci_ll, right?
>> >
>> > I suggested that as well, but the point was made that this would
>> > add an unnecessary indirection for the SPI case, which is not
>> > really much like a serial port. It's certainly possible to do it
>> > like you say, but if we add a way to register the high-level
>> > protocols with an HCI-like multi-function device, we could
>> > also do it in a way that does not rely on tty-ldisc but keeps it
>> > as one of the options.
>>
>> I actually don't think it's such a big indirection, I prefer to think
>> of it more as of the abstraction layer. If not use this, are we going
>> to have direct SPI device drivers? I'm afraid we might end up with a
>> huge duplication of code in that case.
>
> The question is how it should be modeled in a better way than today.
>
> I believe we already agree it should be something like
>
>  bt  ti-radio  Âst-e-radio  Âti-gps  st-e-gps Âbroadcom-radio ...
> Â | Â Â Â Â | Â Â Â Â Â| Â Â Â Â Â Â| Â Â Â Â Â| Â Â Â Â Â| Â Â Â Â |
> Â +---------+----------+---------+--+----------+----------+---------+
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â|
> Â Â Â Â Â Â Â Â Â Â Â Â Âcommon-hci-module
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â|
> Â Â Â Â Â Â Â Â Â Â Â+-----------+-----------+
> Â Â Â Â Â Â Â Â Â Â Â| Â Â Â Â| Â Â Â | Â Â Â|
>          Âserial  Âspi   i2c  Â...

Yes, this sort of architecture would certainly help...
However, I suppose the most common question as one of you stated above
was how can the channel -ID and other factors such as offset-header
packet format be generalized?

As in over here, will the common-hci-module work fine, If say ti-radio
says he is expecting packets starting from id-8 which is of length 10?
and also work for st-e-radio which would say expecting data from id-6
with 15 max bytes?
Answering this I suppose would solve a lot of our problems....

Marcel did previously suggest a bus-driver sort of a structure, where
the transport are sort of adapter drivers, the data interpretation
logic like splitting HCI-events, FM CH-8 packets all fall into the
algos drivers and the actual Bluetooth driver or GPS or FM-V4L2
drivers falling into the client/protocol drivers....



> Any objections to this?
>
> If you agree, I think we should discuss the alternatives for the common
> module. I think you are saying we should make the common module a single
> ldisc doing the multiplexing and have all transports register as ttys into
> it, right?
>
> What we discussed before was to split it into multiple modules, one for
> the tty ldisc and one for the common code, and then have the spi/i2c/...
> drivers feed into the common multiplexer without going through tty.
>
> I don't currently see a significant advantage or disadvantage with either
> approach.
>
> Â Â Â ÂArnd
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