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

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: gregkh@linuxfoundation.org, jirislaby@kernel.org,
	ilpo.jarvinen@linux.intel.com, robh+dt@kernel.org,
	krzysztof.kozlowski+dt@linaro.org, vz@mleia.com,
	linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	lukas@wunner.de, p.rosenberger@kunbus.com,
	Lino Sanfilippo <l.sanfilippo@kunbus.com>
Subject: Re: [PATCH 5/8] dt_bindings: rs485: Correct delay values
Date: Tue, 28 Jun 2022 13:03:14 +0300	[thread overview]
Message-ID: <YrrR4hItCx56bpxI@smile.fi.intel.com> (raw)
In-Reply-To: <6c50fdca-aac4-aaf5-ad34-18a60fcc0aa0@gmx.de>

On Thu, Jun 23, 2022 at 10:17:06PM +0200, 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:
> >>
> >> 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.

You need to elaborate all this in the commit message to justify the change.

-- 
With Best Regards,
Andy Shevchenko