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Date:   Fri, 11 Dec 2020 09:18:24 +0000
From:   József Horváth <info@...istro.hu>
To:     'Greg Kroah-Hartman' <gregkh@...uxfoundation.org>
Cc:     'Rob Herring' <robh+dt@...nel.org>,
        'Jiri Slaby' <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Serial: silabs si4455 serial driver

On Fri, Dec 11, 2020 at 09:43:31AM +0100, 'Greg Kroah-Hartman' wrote:
> On Fri, Dec 11, 2020 at 08:16:34AM +0000, József Horváth wrote:
> > On Fri, Dec 11, 2020 at 08:33:17AM +0100, 'Greg Kroah-Hartman' wrote:
> > > On Fri, Dec 11, 2020 at 06:37:52AM +0000, József Horváth wrote:
> > > > On Fri, Dec 11, 2020 at 07:20:41AM +0100, 'Greg Kroah-Hartman' wrote:
> > > > > On Fri, Dec 11, 2020 at 06:09:43AM +0000, József Horváth wrote:
> > > > > > On Fri, Dec 11, 2020 at 06:50:58AM +0100, 'Greg Kroah-Hartman' wrote:
> > > > > > > On Thu, Dec 10, 2020 at 07:46:25PM +0000, József Horváth wrote:
> > > > > > > > On Thu, Dec 10, 2020 at 08:03:22PM +0100, 'Greg Kroah-Hartman' wrote:
> > > > > > > > > On Thu, Dec 10, 2020 at 05:04:46PM +0000, József Horváth wrote:
> > > > > > > > > > This is a serial port driver for
> > > > > > > > > > Silicon Labs Si4455 Sub-GHz transciver.
> > > > > > > > > > +
> > > > > > > > > > +#define BASE_TTYIOC_PRIVATE		0xA0
> > > > > > > > > > +/* Set EZConfig.
> > > > > > > > > > + * After this ioctl call, the driver restarts the si4455,
> > > > > > > > > > + * then apply the new configuration and patch.
> > > > > > > > > > + */
> > > > > > > > > > +#define SI4455_IOC_SEZC		_IOW('T', \
> > > > > > > > > > +				     BASE_TTYIOC_PRIVATE + 0x01, \
> > > > > > > > > > +				     struct si4455_iocbuff)
> > > > > > > > > 
> > > > > > > > > Why does a serial driver have private ioctls?  Please no, don't do that.
> > > > > > > > 
> > > > > > > > I checked the ioctl.h and serial_core.h, but I not found any similar definition, like BASE_VIDIOC_PRIVATE in videodev2.h.
> > > > > > > > In this case the name of macro BASE_TTYIOC_PRIVATE means the base value of special ioctl commands owned by this driver.
> > > > > > > 
> > > > > > > My point is, a serial driver should NOT have any custom ioctls.
> > > > > > > 
> > > > > > > > I can change it to BASE_TTYIOC or SI4455_IOC_BASE
> > > > > > > > 
> > > > > > > > > Implement the basic serial driver first, and then we can talk about
> > > > > > > > > "custom" configurations and the like, using the correct apis.
> > > > > > > > 
> > > > > > > > Without the SI4455_IOC_SEZC call, the driver can't configure the Si4455 and not working at all.
> > > > > > > > The cofiguration for interface is provided by user for application.
> > > > > > > 
> > > > > > > That is what a device tree is for, to configure the device to have the
> > > > > > > correct system configuration, why can't that be the same here?
> > > > > > > 
> > > > > > > > It contains the base frequency, channel spacing, modulation, and a lot
> > > > > > > > of more stuff, and generated by Silicon Labs Wireless Development
> > > > > > > > Suite.
> > > > > > > > The generated configuration is in a non public(compressed,
> > > > > > > > encrypted...who knows) format, so without this the driver can't
> > > > > > > > provide configuration parameters to Si4455.
> > > > > > > 
> > > > > > > So we have to take a "custom" userspace blob and send it to the device
> > > > > > > to configure it properly?  Like Jiri said, sounds like firmware, so just
> > > > > > > use that interface instead.
> > > > > > 
> > > > > > I checked Jiri's suggestion, and it is a good solution to replace SI4455_IOC_SEZC(configuration) and SI4455_IOC_SEZP(firmware patch).
> > > > > > I can move SI4455_IOC_SSIZ(package size) to device tree property.
> > > > > > 
> > > > > > Maybe you have good suggestion for the following:
> > > > > > SI4455_IOC_STXC -> Radio transmit channel index. It is a real use case to control this parameter by user at runtime.
> > > > > > SI4455_IOC_SRXC -> Radio receive channel index. It is a real use case to control this parameter by user at runtime.
> > > > > 
> > > > > These are not serial port things, why would a serial port care about
> > > > > these?
> > > > 
> > > > You are right, these are not regular serial port things, but this device is not a regular uart, it is a sub-GHz transciever, digital radio.
> > > > This driver tries to represent it as a serial port to user.
> > > 
> > > Is that the correct representation to be using here?  Why not act like a
> > > proper radio device instead?  That way you get to use the normal kernel
> > > apis for radio devices.
> > 
> > In my mind it is absolute a serial device by the application.
> 
> What is the application?  Traditionally serial ports don't need radio signals :)

The application is connecting newly developed sensors(with only rf interface) and legacy sensors(with regular serial communication over rs-485 with modbus) keeping the legacy user software.

User sw [Java]
	<-> /dev/ttyXXX
		<-> si4455[driver]
			<-> si4455[hardware]
				<---air---> new device[si4455+ARM Cortex-M0] 1
					+-> new device[si4455+ARM Cortex-M0] 2
					+-> new device[si4455+ARM Cortex-M0] n
					+-> gateway[si4455+ARM Cortex-M0]<---RS485--> Legacy device 1
										  +-> Legacy device 2
										  +-> Legacy device n

I think this driver could be a good solution in a lot off applications where the user wants to change from wired to wireless with this(Si4455) device, without changing the user sw.
Using sub GHz transport is better because the band below GHz is less loaded than Wifi or bluetooth.

Thanks to suggestions, I can remove unnecessary ioctl calls as well. This way, it remains fully compatible with user software and does not need to be hacked.

> 
> > > > > > SI4455_IOC_GRSSI -> Last measured RSSI, when packet received. This is a useful information.
> > > > > > (Currently I'm the only one user, and I need this :) )
> > > > > 
> > > > > What is "RSSI"?
> > > > > 
> > > > > And why not debugfs if it's only debugging stuff?
> > > > 
> > > > Received signal strength indication, and not only debugging. It is an information for the end user.
> > > 
> > > How do other radio devices (like wifi controllers) export this
> > > information to userspace?  Don't create custom apis for only a single
> > > device when the goal of a kernel is to make hardware interfaces all work
> > > the same as far as userspace is concerned.
> > 
> > I move the package size, tx/rx channel properties to dt as device
> > parameter, and the user could control these properties in sysfs and
> > get rssi too. Finally I can remove all custom ioctl commands.
> > What do you think?
> 
> I do not know, sorry, please try it and see.
> 
> thanks,
> 
> greg k-h

Üdvözlettel / Best regards:
József Horváth

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