Re: [PATCH 3/3] dmaengine: tegra-adma: Add support for Tegra210 ADMA

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Friday 25 September 2015 16:38:55 Jon Hunter wrote:
> On 25/09/15 16:17, Jon Hunter wrote:
> > 
> > On 25/09/15 16:03, Arnd Bergmann wrote:
> >> On Friday 25 September 2015 15:56:40 Jon Hunter wrote:
> >>> +       case DMA_MEM_TO_DEV:
> >>> +               burst_size = fls(tdc->config.dst_maxburst);
> >>> +               ch_regs->config = ADMA_CH_CONFIG_SRC_BUF(num_bufs - 1);
> >>> +               ch_regs->ctrl = ADMA_CH_CTRL_XFER_DIR(ADMA_MEM_TO_AHUB) |
> >>> +                               ADMA_CH_CTRL_TX_REQ(tdc->config.slave_id);
> >>> +               ch_regs->src_addr = buf_addr;
> >>> +               break;
> >>> +
> >>> +       case DMA_DEV_TO_MEM:
> >>> +               burst_size = fls(tdc->config.src_maxburst);
> >>> +               ch_regs->config = ADMA_CH_CONFIG_TRG_BUF(num_bufs - 1);
> >>> +               ch_regs->ctrl = ADMA_CH_CTRL_XFER_DIR(ADMA_AHUB_TO_MEM) |
> >>> +                               ADMA_CH_CTRL_RX_REQ(tdc->config.slave_id);
> >>> +               ch_regs->trg_addr = buf_addr;
> >>> +               break;
> >>
> >> Do not use the 'slave_id' field here to identify the slave device, that
> >> concept is broken. Instead, put the slave identification into the
> >> dma specifier in DT and read it out in your xlate function.
> > 
> > Why is it broken?
> > 
> > What happens if I don't know the slave-id? In other words, the slave-id
> > can be dynamically allocated and associated with a given slave.
> 
> I guess thinking about it some more, the driver could assign an id
> itself to a given channel and I could avoid using slave_id here. There
> are 22 channels and 10 tx and 10 rx requests.

This sounds roughly right. So you could pick the ch_regs->ctrl value
when you allocate the tegra_adma_chan structure, and then map it to
the slave in the xlate() function.

> However, I could also
> statically assign a mapping in DT for the clients if that is preferred.

That's not what I was looking for, the DT should contain the information
that the DMA master uses to identify a device, not a setting that it
can freely choose.

	Arnd