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Subject: Re: [PATCH 5/8] dt_bindings: rs485: Correct delay values
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>
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>
References: <20220622154659.8710-1-LinoSanfilippo@gmx.de>
 <20220622154659.8710-6-LinoSanfilippo@gmx.de>
 <YrSU4eL9hgISg3Y1@smile.fi.intel.com>
From:   Lino Sanfilippo <LinoSanfilippo@gmx.de>
Message-ID: <6c50fdca-aac4-aaf5-ad34-18a60fcc0aa0@gmx.de>
Date:   Thu, 23 Jun 2022 22:17:06 +0200
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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@kunbus.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 al=
lowed 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 delay=
s read from
DT/ACPI either, the only exception I found is max310x which clamps it to 1=
5 ms).

We already have a maximum of 100 ms defined for RTS delays set via TIOCSRS=
485. To be
consistent with TIOCSRS485 the same limit is used for DT/ACPI values in th=
is 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 cl=
amped 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 value=
s are clamped
that are higher than that.


Regards,
Lino