linux-kernel - Re: [PATCH 5/8] dt_bindings: rs485: Correct delay values

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Date:   Sat, 25 Jun 2022 12:54:39 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, vz@...ia.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, lukas@...ner.de,
        p.rosenberger@...bus.com, Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH 5/8] dt_bindings: rs485: Correct delay values

On Thu, 23 Jun 2022, Lino Sanfilippo wrote:

> On 23.06.22 at 18:29, Andy Shevchenko wrote:
> > On Wed, Jun 22, 2022 at 05:46:56PM +0200, Lino Sanfilippo wrote:
> >> From: Lino Sanfilippo <l.sanfilippo@...bus.com>
> >>
> >> The maximum allowed delay for RTS before and RTS after send is 100 ms.
> >> Adjust the documentation accordingly.
> >
> >
> > Is it only documentation issue? If the code allows this to be set higher
> > than 100, we may not change the documentation since this an ABI (from
> > firmware <--> kernel perspective) we need to support old variants.
> >
> 
> Well currently the documentation claims that a maximum of 1000 msecs is allowed but
> nothing actually checks the values read from device tree/ACPI and so it is possible
> to set much higher values (note that the UART drivers dont check the delays read from
> DT/ACPI either, the only exception I found is max310x which clamps it to 15 ms).
> 
> We already have a maximum of 100 ms defined for RTS delays set via TIOCSRS485. To be
> consistent with TIOCSRS485 the same limit is used for DT/ACPI values in this patch.
> 
> I am aware that this changes the firmware/kernel ABI. But we had a similar situation when
> the sanity checks for TIOCSRS485 were introduced
> (see https://lore.kernel.org/all/20220410104642.32195-2-LinoSanfilippo@gmx.de/)
> since before we did not have those limits for all drivers (some drivers clamped the
> values itself but many did not care).
> Furthermore 100 ms is already a very high value for RTS delays (which are usually rather
> in usecs range). So IMHO the risk is very low to break anything when values are clamped
> that are higher than that.

Did you see this development direction (from Lukas):

https://lore.kernel.org/linux-serial/20220309125908.GA9283@wunner.de/

?

Effectively, he wants to making a compat threshold at 1msec and beyond 
that the input value would be interpreted as nsecs.

-- 
 i.