Hi Rob, Hi Leo,
Am 2020-02-28 00:03, schrieb Rob Herring:
On Thu, Feb 27, 2020 at 4:49 PM Li Yang <leoyang.li@xxxxxxx> wrote:
On Thu, Feb 27, 2020 at 4:35 PM Rob Herring <robh+dt@xxxxxxxxxx>
wrote:
>
> On Fri, Feb 21, 2020 at 11:48 AM Michael Walle <michael@xxxxxxxx> wrote:
> >
> > The DMA channel might not be available at the first probe time. This is
> > esp. the case if the DMA controller has an IOMMU mapping.
> >
> > Use the new dma_request_chan() API and handle EPROBE_DEFER errors. Also
> > reorder the code a bit, so that we don't prepare the whole UART just to
> > determine that the DMA channel is not ready yet and we have to undo all
> > the stuff. Try to map the DMA channels earlier.
>
> Changing this means you never probe successfully if you boot a kernel
> with the DMA driver disabled (or it's IOMMU disabled). Some other
> drivers request DMA in open() and can work either way.
Oh, I see.
We got this exact issue previously with another driver. When the
What driver is it? I've been working on the i2c-mxs.c driver which has
the same problem. Ie. its not working with DMA when the IOMMU is
enabled.
Now that I've learned that dma_request_chan() will return EPROBE_DEFER
if the actual DMA driver is not available, I don't think there is any
trick like this there. There is no function which would be called late
except you'd do something like on the first master_xfer() try to request
the DMA channels. But I don't think that would be the way to go.
-michael
required DMA driver is disabled, the DMA framework cannot figure out
this situation and keeps returning EPROBE_DEFER. I'm wondering if we
should update the DMA framework to use your deferred probe timeout
mechanism. Is it still only used for debug purpose?
It's undergoing some rework ATM to not just be for debug. However,
it's not really going to help you if you care about the console
because waiting for the timeout will be too late to register the
console.
