Re: [PATCH v4 01/16] spi: dw: Add Tx/Rx finish wait methods to the MID DMA

[Andy Shevchenko]

On Fri, May 22, 2020 at 05:00:25PM +0300, Serge Semin wrote:
> On Fri, May 22, 2020 at 04:27:43PM +0300, Serge Semin wrote:
> > On Fri, May 22, 2020 at 02:10:13PM +0100, Mark Brown wrote:
> > > On Fri, May 22, 2020 at 03:44:06PM +0300, Serge Semin wrote:
> > > > On Fri, May 22, 2020 at 03:34:27PM +0300, Andy Shevchenko wrote:

...

> > > > > > Realistically it seems unlikely that the clock will be even as slow as
> > > > > > double digit kHz though, and if we do I'd not be surprised to see other
> > > > > > problems kicking in.  It's definitely good to handle such things if we
> > > > > > can but so long as everything is OK for realistic use cases I'm not sure
> > > > > > it should be a blocker.
> > > 
> > > > As I see it the only way to fix the problem for any use-case is to move the
> > > > busy-wait loop out from the tasklet's callback, add a completion variable to the
> > > > DW SPI data and wait for all the DMA transfers completion in the
> > > > dw_spi_dma_transfer() method. Then execute both busy-wait loops (there we can
> > > > use spi_delay_exec() since it's a work-thread) and call
> > > > spi_finalize_current_transfer() after it. What do you think?
> > > 
> > > I'm concerned that this will add latency for the common case to handle a
> > > potential issue for unrealistically slow buses but yeah, if it's an
> > > issue kicking up to task context is how you'd handle it.
> > 
> > I am not that worried about the latency (most likely it'll be the same as
> > before), but I am mostly concerned regarding a most likely need to re-implement
> > a local version spi_transfer_wait(). We can't afford wait for the completion
> > indefinitely here, so the wait_for_completion_timeout() should be used, for which
> > I would have to calculate a decent timeout based on the transfer capabilities,
> > etc. So basically it would mean to partly copy the spi_transfer_wait() to this
> > module.(
> 
> I'd also wait for Andy's suggestion regarding this, since he's been worried
> about the delay length in the first place. So he may come up with a better
> solution in this regard.

The completion approach sounds quite heavy to me.

Since we haven't got any report for such an issue, I prefer as simplest as
possible approach.

If we add might_sleep() wouldn't it be basically reimplementation of the
spi_delay_exec() again?

And second question, do you experience this warning on your system?

My point is: let's warn and see if anybody comes with a bug report. We will
solve an issue when it appears.

-- 
With Best Regards,
Andy Shevchenko