Re: [PATCH v1 1/2] spi: dw: Request DMA channels at runtime instead of probe

[Changhuang Liang]

Hi, Mark

Thanks for the review.

> On Wed, Sep 23, 2026 at 02:57:04AM -0700, Changhuang Liang wrote:
> > The DW SPI controller requests its DMA channels in
> > dw_spi_add_controller(), which ties them up for the entire lifetime of
> > the controller. Even a controller that is only ever used for standard
> > SPI transfers and never for enhanced SPI transfers holds the channels
> > exclusively from probe onwards, preventing them from being shared with
> > other users.
> 
> > +static int dw_spi_prepare_hardware(struct spi_controller *ctlr) {
> > +	struct dw_spi *dws = spi_controller_get_devdata(ctlr);
> > +	int ret;
> > +
> > +	if (!ctlr->can_dma)
> > +		return 0;
> > +
> > +	ret = dws->dma_ops->dma_init(ctlr->dev.parent, dws);
> > +	if (ret) {
> > +		/*
> > +		 * DMA is optional: fall back to the PIO/IRQ path instead of
> > +		 * failing the message. Use dev_dbg() since this may happen
> > +		 * on every prepare.
> > +		 */
> > +		dev_dbg(&ctlr->dev, "DMA init failed (%d), using PIO\n", ret);
> > +
> > +		return 0;
> > +	}
> > +
> > +	return 0;
> > +}
> 
> This seems like a lot of overhead for every existing controller with DMA, we'll
> have to go through the request/release cycle whenever DMA gets used which
> feels like a bunch of overhead for a hot path - especially in the fast path in
> spi_sync().  If some platforms need it they should be able to opt into it rather
> than forcing it on every single platform.
> 

I will reorganize this series, trying to avoid affecting the current flow.

> > +static int dw_spi_unprepare_hardware(struct spi_controller *ctlr) {
> 
> Adding this also causes overhead since we do the unprepare in the thread.

Best Regards,
Changhuang