Re: [PATCH] i2c: mediatek: modify threshold passed to i2c_get_dma_safe_msg_buf()

From: Wolfram Sang <wsa@the-dreams.de>
To: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Ryder Lee <ryder.lee@mediatek.com>,
	Jun Gao <jun.gao@mediatek.com>,
	linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
	linux-i2c@vger.kernel.org,
	Matthias Brugger <matthias.bgg@gmail.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] i2c: mediatek: modify threshold passed to i2c_get_dma_safe_msg_buf()
Date: Fri, 15 Feb 2019 17:36:57 +0100	[thread overview]
Message-ID: <20190215163657.fs6t7p3vqez53su3@ninjato> (raw)
In-Reply-To: <20190215092500.6q3hqnoufyowiqaz@ninjato>

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>> > Ok, I can add a check in another patch. Should we return NULL pointer
>> > if msg->len is 0 or print out some warnings? Thanks.
>> 
>> No warning, msg->len == 0 is a valid setting. But interesting question:
>> I was about to say NULL, but your driver would assume ENOMEM there and
>> discard the message which is also not correct since msg->len == 0 is a
>> valid setting. So, should we just return msg->buf then? Will this work
>> with your driver? Can it handle zero-length transfers?
> 
> dma_map_single(i2c->dev, msg->buf , msgs->len, DMA_TO_DEVICE); breaks
> kernel if msgs->len is 0, so I think it doesn't handle zero-length transfer.

Please don't drop the lists.

Then, the correct solution is to forbid those transfer with this
controller. Check I2C_AQ_NO_ZERO_LEN. Also, update the functionality
like this .. (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK).

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