Subject: Re: [PATCH 1/2] i2c: qcom-geni: Provide an option to select FIFO processing

From: Wolfram Sang <[email-masked]>
Date: Thu, 5 Sep 2019 11:16:17 +0200

Hi Lee,

> > It looks like a workaround to me. It would be interesting to hear which
> > I2C client breaks with DMA and if it's driver can't be fixed somehow
> > instead. But even if we agree on a workaround short term, adding a

So, are there investigations running why this reboot happens?

> > Is there no other way to disable DMA which is local to this driver so we
> > can easily revert the workaround later?
> 
> This is the most local low-impact solution (nomenclature aside).

I disagree. You could use of_machine_is_compatible() and disable DMA for
that machine. Less impact because we save the workaround binding.

> The beautiful thing about this approach is that, *if* the Geni SE DMA

I'd say 'advantage' instead of 'beautiful' ;)

> ever starts working, we can remove the C code and any old properties
> left in older DTs just become NOOP.  Older kernels with newer DTs
> (less of a priority) *still* won't work, but they don't work now
> anyway.

Which is a clear disadvantage of that solution. It won't fix older
kernels. My suggestion above should fix them, too.

> The offending line can be found at [0].  There is no obvious bug to
> fix and this code obviously works well on some of the hardware
> platforms using it.  But on our platform (Lenovo Yoga C630 - QCom
> SMD850) that final command, which initiates the DMA transaction, ends
> up rebooting the machine.

Unless we know why the reboot happens on your platform, I'd be careful
with saying "work obviously well" on other platforms.

> With regards to the nomenclature, my original suggestion was
> 'qcom,geni-se-no-dma'.  Would that better suit your request?

My suggestion:

For 5.3, use of_machine_is_compatible() and we backport that. For later,
try to find out the root cause and fix it. If that can't be done, try to
set up a generic "disable-dma" property and use it.

What do you think about that?

Kind regards,

   Wolfram

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