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

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Date:   Thu, 15 Jun 2017 21:17:43 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     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@...nel.org
Subject: Re: [PATCH 4/4] i2c: rcar: check for DMA-capable buffers

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?

> > -       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?

Thanks for your input,

   Wolfram


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