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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