linux-kernel - Re: [PATCH v2 5/6] i2c: i2c-stm32f7: Add DMA support

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Date:   Wed, 4 Apr 2018 10:20:44 +0200
From:   Pierre Yves MORDRET <pierre-yves.mordret@...com>
To:     Wolfram Sang <wsa@...-dreams.de>
CC:     Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        <linux-i2c@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/6] i2c: i2c-stm32f7: Add DMA support



On 04/03/2018 05:39 PM, Wolfram Sang wrote:
> 
>> +#define STM32F7_I2C_DMA_LEN_MIN			0x1
> ...
> 
>> +	if (i2c_dev->dma && f7_msg->count >= STM32F7_I2C_DMA_LEN_MIN) {
> 
> Are you using DMA for every message with a length >= 1? The setup of
> that might be more expensive than the DMA gain, if so.
> 
Well yes. I am in charge of DMA IPs as well. I2C is the only devices that I had
to test DMA outside standard DMA Engine test. Quite convenient.
I believe to stress both I2C and DMA, this value was relevant.
Now I agree this value can be tuned a little bit. I might raise this threshold
in V3.