Re: [PATCHv4] dmaengine: Add support for BCM2835

[Florian Meier]

2013/11/14 Tomasz Figa <tomasz.figa@xxxxxxxxx>:
> 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:
>> >>>> +- 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.

It is very confusing. I agree.
There is already a DMA driver with a proprietary API in the downstream
kernel. The firmware already creates this mask and passes it to this
proprietary driver.
There was already a discussion about this in the first version thread
that (as long as I understand it) resulted in "we should pass this
mask on to the driver via device tree". So I did that. I have no idea
about how this firmware->devicetree interface will take place, but
since I didn't want to run in circles I hardcoded it in the device
tree.


>> >>> [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?

A fridge is still a working fridge, even if no beer is inside ;-)
Ok, bad example, but you will get an error message anyway when you try
to get a channel.

> 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.

I don't have a telling argument against this, but just thought writing
it this way will
make the migration of the downstream kernel to upstream easier, but if you say I
should change it, I will of course do that.
I am becoming desperate anyway that this migration will ever fully
take place....

Greetings,
Florian
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