Re: [RFC PATCH 0/3] UART slave device bus

H. Nikolaus Schaller
2016-08-23T07:29:48Z

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Hi,

> Am 23.08.2016 um 00:42 schrieb Sebastian Reichel <sre@kernel.org>:
>
>> I am not a specialist for such things, but I think you have three
>> options to connect bluetooth:
>>
>> a) SoC-UART <-> BT-Chip-UART-port
>> b) USB-UART (FT232, PL2303 etc.) <-> BT-Chip-UART-port
>> c) USB <-> BT-Chip-USB-port (not UART involved at all)
>>
>> Case c) IMHO means you anyways need a special USB driver for the BT-Chip connected
>> through USB and plugging it into a non-embedded USB port does not automatically
>> show it as a tty interface. So you can't use it for testing the UART drivers.
>>
>> BTW: the Wi2Wi W2CBW003 chip comes in two firmware variants: one for UART and
>> one for USB. So they are also not exchangeable.
>
> Yes, let's ignore option c).

> I'm talking about UART only. If the
> chip has native USB support, then that's a different driver.

Exactly.

>
>> Variant b) is IMHO of no practical relevance (but I may be wrong)
>> because it would mean to add some costly FT232 or PL2302 chip
>> where a different firmware variant works with direct USB
>> connection.
>
> Well for some chips there is not native USB support. But my scenario
> was about development. Let's say I have a serial-chip and I want to
> develop a driver for it. It would be nice if I can develop the
> driver with a USB-UART

Yes it would be nice, but is this a thing with significant practical relevance?

Usually you have to write drivers for a complete device where the slave
chip is already wired up to a SoC-UART.

Sometimes you can get a bare chip where you can connect to an
USB-UART. But someone has to design that piece of special hardware
for you. If you are really lucky there is an evaluation board.

And in that case I would use a RasPi or BeagleBone and tie up directly
to some SoC-UART instead of using an intermediate USB-UART adapter.
Because it is more close to timing relations to the final SoC based design.

> and then use it on my embedded system.
>
> There are usb-serial devices, which could benefit from support
> btw. I would find it really useful, if the Dangerous Prototype's
> Bus Pirate would expose native /dev/i2c and /dev/spi and it's
> based on FT232.

Oh, that is an interesting device I didn't know yet.

>
>> So to me it looks as if you need to develop different low-level
>> drivers anyways.
>
> No. You say, that option b) is irrelevant and assume, that every
> serial chip also has native USB support.

I just assume that b) is rarely used because there are alternatives.
Although it would be a nice option.

Anyways, while following the discussion this is not the most important
facet of the overall topic.

BR,
Nikolaus

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