linux-kernel - Re: [PATCH v10 43/55] dt-bindings: input: atmel: support to set max bytes transferred

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Date:   Thu, 9 Apr 2020 18:10:24 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Wang, Jiada" <jiada_wang@...tor.com>, nick@...anahar.org,
        dmitry.torokhov@...il.com, jikos@...nel.org,
        benjamin.tissoires@...hat.com, bsz@...ihalf.com
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        erosca@...adit-jv.com, Andrew_Gabbasov@...tor.com,
        Balasubramani_Vivekanandan@...tor.com
Subject: Re: [PATCH v10 43/55] dt-bindings: input: atmel: support to set max
 bytes transferred

09.04.2020 09:25, Wang, Jiada пишет:
> Hi Dmitry
> 
> On 2020/04/07 23:47, Dmitry Osipenko wrote:
>> 07.04.2020 12:27, Wang, Jiada пишет:
>> ..
>>>> Is this a software (firmware) limitation which varies from version to
>>>> version?
>>>>
>>>
>>> the timeout issue trying to be addressed in this patch is from software,
>>> one of our board a Serializer/Deserializer bridge exists between the SoC
>>> (imx6) and the Atmel touch controller.
>>> imx6 i2c controller driver has a timeout value(100ms) for each i2c
>>> transaction,
>>> Large i2c read transaction failed to complete within this timeout value
>>> and therefore imx6 i2c controller driver aborts the transaction
>>> and returns failure.
>>>
>>> Therefore this patch was created to split the large i2c transaction into
>>> smaller chunks which can complete
>>> within the timeout defined by i2c controller driver.
>>
>> Isn't it possible to use the max_read/write_len of the generic struct
>> i2c_adapter_quirks for limiting the transfer size?
>>
>> BTW, it looks like the i.MX I2C driver doesn't specify the
>> i2c_adapter_quirks, which probably needs to be fixed.
>>
> yes, i.MX I2C driver can specify i2c_adapter_quirks to limit the size be
> transferred in one transaction.
> 
> But even in this case, mxt_process_messages_t44() fails when it tries to
> transfer data count larger than max_read/write_len set in i.MX I2C
> driver, which we would like to avoid.

IIUC, the transfer's limitation is a part of I2C controller hardware and
not the touch controller, so it should be wrong to describe that
limitation in the maxtouch's DT node.

I meant that we probably could set the data->mtu based on
i2c_client->adapter->quirks->max_read and then the DT property shouldn't
be needed, couldn't this be done?

The I2C core only rejects transfers that don't fit into the
max_read/write_len and nothing more.