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. There's the fun with vmalloc() mappings too needing to be handled differently too though that's less likely to bite I2C. > 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. 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.
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