Re: [RFC PATCH v4 3/6] i2c: add docs to clarify DMA handling

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

Em Sat, 9 Sep 2017 17:27:06 +0200
Wolfram Sang <wsa@the-dreams.de> escreveu:

> > Yes, but the statistics that 10% of I2C bus master drivers
> > are DMA-safe is not true, if you take those into account ;-)
> > 
> > Perhaps you could write it as (or something similar):
> > 
> > 	At this time of writing, only +10% of I2C bus master
> > 	drivers for non-remote boards have DMA support implemented.    
> 
> No, this is confusing IMO. Being remote has nothing to with DMA. What
> has to do with DMA is that these are using USB as communication channel.
> And USB clearly needs DMA-safe buffers. The encapsulation needs DMA-safe
> buffers. So, I think the new sentence "other subsystems might impose"
> mentions that.
> 
> Let me recap what this series is for:
> 
> a) It makes clear that I2C subsystem does not require DMA-safety because
> for the reasons given in the textfile.
> 
> If I2C is encapsulated over USB, then DMA-safety is of course required,
> but this comes from USB. So, those USB-I2C master drivers need to do
> something to make sure DMA-safety is guaranteed because i2c_msg buffers
> don't need to be DMA safe because of a). They could use the newly
> introduced features, but they don't have to.
> 
> b) a flag for DMA safe i2c_msgs is added
> 
> So, for messages we know to be DMA safe, we can flag that. Drivers
> could check that and use a bounce buffer if the flag is not set.
> However, this is all optional. Your drivers can still choose to not
> check the flag and everything will stay as before. Check patch 5 for a
> use case.
> 
> c) helper functions for bounce buffers are added
> 
> These are *helper* functions for drivers wishing to do DMA. A super
> simple bounce buffer support. Check patch 4 for a use case. Again, this
> is optional. Drivers can implement their own bounce buffer support. Or,
> as in your case, if you know that your buffers are good, then don't use
> any of this and things will keep going.
> 
> 
> This all is to allow bus master drivers to opt-in for DMA safety if they
> want to do DMA directly. Because currently, with a lot of i2c_msgs on
> stack, this is more or less working accidently.
> 
> And, yes, I know I have to add this new flag to a few central places in
> client drivers. Otherwise, master drivers checking for DMA safety will
> initially have a performance regression. This is scheduled for V5 and
> mentioned in this series.
> 
> > In the past, on lots of drivers, the i2c_xfer logic just used to assume
> > that the I2C client driver allocated a DMA-safe buffer, as it just used to
> > pass whatever buffer it receives directly to USB core. We did an effort to
> > change it, as it can be messy, but I'm not sure if this is solved everywhere.  
> 
> Good, I can imagine this being some effort. But again, this is because
> USB needs the DMA-safety, not I2C. AFAICS, most i2c_msgs are register
> accesses and thus, small messages.
> 
> > The usage of I2C at the media subsystem predates the I2C subsystem.
> > at V4L2 drivers, a great effort was done to port it to fully use the
> > I2C subsystem when it was added upstream, but there were some problems
> > a the initial implementation of the i2c subsystem that prevented its
> > usage for the DVB drivers. By the time such restrictions got removed,
> > it was too late, as there were already a large amount of drivers relying
> > on i2c "low level" functions.  
> 
> Didn't know that, but this is good to know! Thanks.
> 
> > Even on the drivers that use i2c_add_adapter(), the usage of DMA can't
> > be get by the above grep, as the DMA usage is actually at drivers/usb.  
> 
> Ok. But as said before, what works now will continue to work. 

OK, good!

> This series is about clarifying that i2c_msg buffers need not to be DMA safe
> and offering some helpers for those bus master drivers wanting to opt-in
> to be sure.
> 
> Clearer now?

Yeah, it is clearer for me. Thanks!

Anyway, it doesn't hurt if you could improve the documentation patch to
mention the above somewhow, as I want to avoid patches trying to make
existing media USB drivers to use the new flag without changing the
already existing DMA-safe logic there (causing double-buffering), or, 
even worse, making the I2C bus DMA-unsafe because someone misread
this document.


Regards,
Mauro