Re: [PATCH v16 5/7] spi: pxa2xx: disable DMA for Apple MacBook8,1

On Sun, Jul 26, 2026 at 06:14:35PM +0200, Lukas Wunner wrote:

I'm not sure why Apple chose to keep the DMA controller in reset,
but have you explored enabling it on boot?

Thinking about this some more and after taking a look at
drivers/input/keyboards/applespi.c, I realize messages exchanged via SPI
are always 256 bytes in size.  That's not a whole lot, and of course
messages are only exchanged when a key is pressed or the trackpad is
used.  So perhaps Apple determined that sending this in PIO mode is
ultimately more efficient than keeping a DMA engine powered on all the
time.  The CPU load saved through offloading the SPI transfer is offset
by having to program the DMA engine for each transfer.

So perhaps usage of PIO mode was indeed an intentional choice by Apple
and need not be second-guessed.  I'm not sure.

Thanks,

Lukas