linux-kernel - Re: [PATCH 4/4] i2c: rcar: check for DMA-capable buffers

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Date:   Thu, 15 Jun 2017 13:48:17 -0600
From:   Shuah Khan <shuah@...nel.org>
To:     Wolfram Sang <wsa@...-dreams.de>, Shuah Khan <shuahkhan@...il.com>
Cc:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        linux-i2c@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 4/4] i2c: rcar: check for DMA-capable buffers

On 06/15/2017 01:17 PM, Wolfram Sang wrote:
> Hi,
> 
>>> -       /* Do not use DMA if it's not available or for messages < 8 bytes */
>>> -       if (IS_ERR(chan) || msg->len < 8)
>>> +       if (IS_ERR(chan) || msg->len < RCAR_DMA_THRESHOLD || priv->flags & ID_P_NODMA)
>>
>> Might be more efficient to check for ID_P_NODMA first instead of msg->len.
> 
> I think most of the I2C transfers are smaller (like reading/writing one
> register) than the threshold, so this "should" be more efficient. Plus,
> honestly, I also think this is a micro-optimization which is largely
> depending on the use-case. Can we agree on that?

Makes sense.

> 
>>> -       read = msg->flags & I2C_M_RD;
>>> +       /* we need to check here because we need the 'current' context */
>>> +       if (i2c_check_msg_for_dma(msg, RCAR_DMA_THRESHOLD, NULL) == -EFAULT) {
>>> +               dev_dbg(dev, "skipping DMA for this whole transfer\n");
>>
>> Adding reason for skipping will be helpful.
> 
> The I2C core helper will immediately print before that the buffer is not
> DMA capable. Do you think this together will do?

That is sufficient.

thanks,
-- Shuah

> 
> Thanks for your input,
> 
>    Wolfram
>