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

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Date:   Thu, 30 Jun 2022 01:50:15 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        ilpo.jarvinen@...ux.intel.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, vz@...ia.com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-serial@...r.kernel.org, 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 28.06.22 12:03, Andy Shevchenko wrote:
> 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.
>

OK, I see. I will rewrite the commit message then to hopefully make the rationale behind
the time reduction more clear.

Thanks,
Lino