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

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Message-Id: <EA228ADD-F03A-49C0-9A0B-E768D7A7A466@goldelico.com>
Date:   Mon, 22 Aug 2016 23:23:26 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Sebastian Reichel <sre@...nel.org>
Cc:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Peter Hurley <peter@...leysoftware.com>,
        NeilBrown <neil@...wn.name>, Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:BLUETOOTH DRIVERS" <linux-bluetooth@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] UART slave device bus

Hi Sebastian,

> Am 22.08.2016 um 22:39 schrieb Sebastian Reichel <sre@...nel.org>:
> 
> Hi,
> 
> On Sun, Aug 21, 2016 at 09:50:57AM +0200, H. Nikolaus Schaller wrote:
>>> Am 20.08.2016 um 15:34 schrieb One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>:
>>>> What it is not about are UART/RS232 converters connected through USB or virtual
>>>> serial ports created for WWAN modems (e.g. /dev/ttyACM, /dev/ttyHSO). Or BT devices
>>>> connected through USB (even if they also run HCI protocol).
>>> 
>>> It actually has to be about both because you will find the exact same
>>> device wired via USB SSIC/HSIC to a USB UART or via a classic UART. Not is
>>> it just about embedded boards.
>> 
>> Not necessarily.
>> 
>> We often have two interface options for exactly the sam sensor chips. They can be connected
>> either through SPI or I2C. Which means that there is a core driver for the chip and two different
>> transport glue components (see e.g. iio/accel/bmc150).
>> 
>> This does not require I2C to be able to handle SPI or vice versa or provide a common API.
> 
> I don't understand this comparison. I2C and SPI are different
> protocols,

Yes, they are different on protocol level, but on both you transfer blocks of data from/to a slave device
which usually can be addressed. And for some chips they are just two slightly alternative serial interfaces.

> while native UART and USB-connected UART are both UART.

I see what you mean, but kernel divides between directly connected UART and USB-connected UART.

drivers/usb/serial/ vs. drivers/tty/serial/

to implement two different groups of UARTs. Although on user space level they are harmonized again.
This is why I compare with i2c and spi. But each such comparison is not perfect.

Anyways, to me it looks as if everybody wants to make the solution work for usb-uarts as well
(although I still would like to see a real world use-case).

> 
>> And most Bluetooth devices I know have either UART or a direct
>> USB interface. So in the USB case there is no need to connect
>> it through some USB-UART bridge and treat it as an UART at all.
> 
> I think having support for USB-UART dongles is useful for
> driver development and testing on non-embedded HW.

Hm. I assume you mean the Bluetooth situation where both, embedded UART
connected chips and USB dongles are available. 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.

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.

So to me it looks as if you need to develop different low-level drivers anyways.

BR,
Nikolaus

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