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From: Wolfram Sang <wsa@the-dreams.de>
To: Mark Brown <broonie@kernel.org>
Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>,
	linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-renesas-soc@vger.kernel.org, linux-iio@vger.kernel.org,
	linux-input@vger.kernel.org, linux-media@vger.kernel.org
Subject: Re: [PATCH v6 0/9] i2c: document DMA handling and add helpers for it
Date: Mon, 27 Nov 2017 19:51:16 +0100	[thread overview]
Message-ID: <20171127185116.j2vmkhbik33vk4f7@ninjato> (raw)
In-Reply-To: <20171108225037.i4dx5iu5zxc542oq@sirena.co.uk>

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On Wed, Nov 08, 2017 at 10:50:37PM +0000, Mark Brown wrote:
> On Sat, Nov 04, 2017 at 09:20:00PM +0100, Wolfram Sang wrote:
> 
> > While previous versions until v3 tried to magically apply bounce buffers when
> > needed, it became clear that detecting DMA safe buffers is too fragile. This
> > approach is now opt-in, a DMA_SAFE flag needs to be set on an i2c_msg. The
> > outcome so far is very convincing IMO. The core additions are simple and easy
> > to understand. The driver changes for the Renesas IP cores became easy to
> > understand, too.
> 
> It would really help a lot of things if there were a way to detect if a
> given memory block is DMA safe, having to pass the information around
> causes so much pain.

I so agree.

> > I am still not sure how we can teach regmap this new flag. regmap is a heavy
> > user of I2C, so broonie's opinion here would be great to have. The rest should
> > mostly be updating individual drivers which can be done when needed.
> 
> We pretty much assume everything is DMA safe already, the majority of
> transfers go to/from kmalloc()ed scratch buffers so actually are DMA
> safe but for bulk transfers we use the caller buffer and there might be
> some problem users.

So, pretty much the situation I2C was in before this patch set: we
pretty much assume DMA safety but there might be problem users.

> I can't really think of a particularly good way of
> handling it off the top of my head, obviously not setting the flag is
> easy but doesn't get the benefit while always using a bounce buffer
> would involve lots of unneeded memcpy().  Doing _dmasafe() isn't
> particularly appealing either but might be what we end up with.

Okay. That sounds you are fine with the approach taken here, in general?

Thanks,

   Wolfram


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  reply	other threads:[~2017-11-27 18:51 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-04 20:20 [PATCH v6 0/9] i2c: document DMA handling and add helpers for it Wolfram Sang
2017-11-04 20:20 ` Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 1/9] i2c: add a message flag for DMA safe buffers Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 2/9] i2c: add helpers to ease DMA handling Wolfram Sang
2017-11-04 20:20   ` Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 3/9] i2c: dev: mark RDWR buffers as DMA_SAFE Wolfram Sang
2017-11-04 20:20   ` Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 4/9] i2c: refactor i2c_master_{send_recv} Wolfram Sang
2017-11-11  0:03   ` Jonathan Cameron
2017-11-11  0:03     ` Jonathan Cameron
2017-11-04 20:20 ` [PATCH v6 5/9] i2c: add i2c_master_{send|recv}_dmasafe Wolfram Sang
2017-11-11  0:08   ` Jonathan Cameron
2017-11-11  0:08     ` Jonathan Cameron
2017-11-04 20:20 ` [PATCH v6 6/9] i2c: smbus: use DMA safe buffers for emulated SMBus transactions Wolfram Sang
2017-11-11  0:13   ` Jonathan Cameron
2017-11-11  0:13     ` Jonathan Cameron
2017-11-04 20:20 ` [PATCH v6 7/9] i2c: add docs to clarify DMA handling Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 8/9] i2c: sh_mobile: use core helper to decide when to use DMA Wolfram Sang
2017-11-04 20:20 ` [PATCH v6 9/9] i2c: rcar: skip DMA if buffer is not safe Wolfram Sang
2017-11-08 22:50 ` [PATCH v6 0/9] i2c: document DMA handling and add helpers for it Mark Brown
2017-11-27 18:51   ` Wolfram Sang [this message]
2017-11-28 15:34     ` Mark Brown
2017-12-03 19:43       ` Wolfram Sang
2017-12-04 22:05         ` Mark Brown
2017-12-04 22:05           ` Mark Brown
2017-12-05 11:00           ` Jonathan Cameron
2017-12-05 11:00             ` Jonathan Cameron
2017-12-04 21:25 ` Wolfram Sang

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