Re: [PATCHv4] dmaengine: Add support for BCM2835

[Tomasz Figa]

On Thursday 14 of November 2013 08:12:46 Florian Meier wrote:
> On 13.11.2013 21:39, Tomasz Figa wrote:
> > On Wednesday 13 of November 2013 20:35:22 Florian Meier wrote:
> >> On 13.11.2013 19:43, Tomasz Figa wrote:
> >>>> ....
> >>>> diff --git a/Documentation/devicetree/bindings/dma/bcm2835-dma.txt b/Documentation/devicetree/bindings/dma/bcm2835-dma.txt
> >>>> new file mode 100644
> >>>> index 0000000..bca5e84
> >>>> --- /dev/null
> >>>> +++ b/Documentation/devicetree/bindings/dma/bcm2835-dma.txt
> >>>> @@ -0,0 +1,59 @@
> >>>> +* BCM2835 DMA controller
> >>>> +
> >>>> +Required properties:
> >>>> +- compatible: Should be "brcm,bcm2835-dma".
> >>>> +- reg: Should contain DMA registers location and length.
> >>>> +- interrupts: Should contain the DMA interrupts associated
> >>>> +		to the DMA channels in ascending order.
> >>>> +		First cell is the IRQ bank.
> >>>> +		Second cell is the IRQ number.
> >>>> +- #dma-cells: Must be <1>, used to represent the number of integer cells in
> >>>> +		the dmas property of client devices.
> >>>> +- dma-channels: Maximum number of DMA channels available.
> >>>> +- dma-requests: Number of DMA Requests.
> >>>
> >>> The two properties above do not seem to be used anywhere in the driver.
> >>
> >> Aren't they necessary for the DMA engine core?
> > 
> > Honestly, looking at the DMA bindings documentation, these two are quite
> > a mystery for me. They don't seem to be used in DMA engine core, but
> > instead several drivers use them for private purposes.
> > 
> > So, since they are optional and you don't seem to need them, I wouldn't
> > list them.
> 
> Ok
> 
> >>>> +- brcm,dma-channel-mask: Bit mask representing the channels available.
> >>>
> >>> What does the value of this property depend on? Could you describe the
> >>> structure of this DMA controller?
> >>>
> >>>> +
> >>>> +Example:
> >>>> +
> >>>> +dma: dma@7e007000 {
> >>>> +	compatible = "brcm,bcm2835-dma";
> >>>> +	reg = <0x7e007000 0xf00>;
> >>>> +	interrupts = <1 16
> >>>> +		      1 17
> >>>> +		      1 18
> >>>> +		      1 19
> >>>> +		      1 20
> >>>> +		      1 21
> >>>> +		      1 22
> >>>> +		      1 23
> >>>> +		      1 24
> >>>> +		      1 25
> >>>> +		      1 26
> >>>> +		      1 27
> >>>> +		      1 28>;
> >>>
> >>> There are 13 interrupts specified here, but...
> >>>
> >>>> +
> >>>> +	#dma-cells = <1>;
> >>>> +	dma-channels = <16>;
> >>>
> >>> ...16 channels here...
> >>>
> >>>> +	dma-requests = <32>;
> >>>> +	brcm,dma-channel-mask = <0x7f35>;
> >>>
> >>> ...and 11 set bits here. May I ask you to explain this to me, please?
> >>
> >> How I understand this DMA controller:
> >> There are 16 DMA channels in the DMA controller, but only 13 interrupts
> >> are available at the IRQ controller. Therefore, the upper DMA channels
> >> can just not be used. Maybe because there are to many other IRQs and
> >> they didn't want to implement another IRQ bank.
> >> Furthermore, some of the DMA channels are already used by the
> >> VideoCore/GPU/firmware. This is what dma-channel-mask indicates. This
> >> should be automatically set by the firmware in the future.
> >> Finally, there are some channels with special functionality that should
> >> not be used by DMA engine, too. Therefore, these lines:
> >> 		/* do not use the FIQ and BULK channels */
> >> 		chans_available &= ~0xD;
> > 
> > OK, this makes it much more clear.
> > 
> > So, my only comment remaining here is that you shouldn't include the
> > channels without interrupt signal in the mask. This would allow you
> > to define it as a mask of channels that are operable and then just
> > iterate over all set bits in the driver, instead of using tricks with
> > interrupt resources. What do you think?
> 
> Since the mask will come directly from the firmware, this would require
> patching the firmware. I think that is not worth the effort.

Now I'm slightly confused. Do you already have code in your firmware that
adds this property to your device tree?

Otherwise in what circumstances such patching would take place? On given
hardware (unless it's an FPGA) the configuration of available DMA channels
that have interrupt signals should not change.

> 
> >>> [snip]
> >>>> diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> >>>> new file mode 100644
> >>>> index 0000000..baf072e
> >>>> --- /dev/null
> >>>> +++ b/drivers/dma/bcm2835-dma.c
> >>> [snip]
> >>>> +static int bcm2835_dma_probe(struct platform_device *pdev)
> >>>> +{
> >>>> +	struct bcm2835_dmadev *od;
> >>>> +	struct resource *dma_res = NULL;
> >>>> +	void __iomem *dma_base = NULL;
> >>>> +	int rc = 0;
> >>>> +	int i = 0;
> >>>> +	int irq;
> >>>> +	uint32_t chans_available;
> >>> [snip]
> >>>> +	if (pdev->dev.of_node) {
> >>>
> >>> Is this driver supposed to support non-DT based instantation (aka board
> >>> files)? If not, maybe it would be cleaner to simply check for
> >>> !pdev->dev.of_node at the beginning of probe and return an error?
> >>
> >> I would like to maintain the possibility for board file based
> >> instatiation, because the Raspberry Pi downstream kernel still doesn't
> >> support device tree. If this is a no-go, I will accept that.
> > 
> > Sure, you are free to do so.
> > 
> > What I meant is that your probe won't call bcm2835_dma_chan_init() at all
> > if there is no pdev->dev.of_node, because the loop iterating over channels
> > is under the if clause.
> 
> Yes you are right, but I think it will make the patching easier, later.
> Currently, nothing bad happens without device tree - it just allocates
> no channels.

But isn't it really an error condition, if no channels are allocated?

Anyway, back to my point about leaving non-DT support in a driver, the
point is still valid only for drivers, not for platforms/boards. So if
there are no boards supported using board files in mainline that could
benefit from this driver, then this driver can be safely made DT-only,
because no new non-DT platforms/boards can be added.

Best regards,
Tomasz

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