Re: [PATCH v5] serial: spi: add spi-uart driver for Maxim 3110

[Feng Tang <feng.tang@intel.com>]

> 
> > Here comes another idea, can we add a capability flag in struct
> > spi_master indicating the master supporting poll or dma or both.
> > Also we add similar bits in struct spi_transfer indicating the this
> > transfer wants to be handled in poll or dma mode.
> 
> Let's not do either of those.  There's no need to introduce such
> complexity, or to enable the associated new failure modes and bugs.

This idea has nothing to do with the dma-safe problem you pointed out, I will
make the buffer dma safe anyway.

What I proposed just wants to provide some flexibility for protocol device
drivers, it will use dma-safe buffer always, but it prefer not to use DMA
for its one-word transfers, and hope to have a choice to do that. For current
existing controller and device drivers, they can simply ignore the new 
working mode setting in struct spi_master and spi_transfer and leave them
as 0.

Thanks,
Feng

> 
> Much simpler to just use DMA-safe I/O buffers in the first place ...
> which is what pretty much every driver stack in Linux already expects
> or requires.
> 
> - Dave
>