Message-ID: <CABDT8WyUH=z67_bOy-DF9NQn6R0eKsGJPDWSwijrhLRyNLvaSQ@mail.gmail.com>
Date:	Thu, 14 Nov 2013 16:41:56 +0100
From:	Florian Meier <florian.meier@...lo.de>
To:	Tomasz Figa <tomasz.figa@...il.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	devicetree <devicetree@...r.kernel.org>,
	alsa-devel@...a-project.org, Liam Girdwood <lgirdwood@...il.com>,
	linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
	dmaengine <dmaengine@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv4] dmaengine: Add support for BCM2835

2013/11/14 Tomasz Figa <tomasz.figa@...il.com>:
> On Thursday 14 of November 2013 15:44:05 Florian Meier wrote:
>> 2013/11/14 Tomasz Figa <tomasz.figa@...il.com>:
>> > 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@...07000 {
>> >> >>>> +        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.
>>
>
> OK. So the firmware defines what set and clear bits in the mask mean.
> It's fine then.

Yes, this mask is actually an "I am not using this channel"-mask.



>> >> >>> [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'm just presenting you the possible options. You are still free to have
> non-DT support in the driver, but if you don't need it (because you can't
> have any new non-DT platforms in mainline) then you can simplify some
> code.
>
> However the driver shouldn't be left with illusionary support for non-DT
> platforms until you decide to implement that. Instead, if you don't want
> to add non-DT support now, just make the driver DT-only, while keeping
> its design in a way allowing you to add non-DT support in future.

This statement hits the nail on the head. Thank you!

> In other words, a driver should not be able to probe using board files
> if support for such probing method is not available in it yet.

That is meaningful. So would the following be ok?

+ .....
+       if (pdev->dev.of_node) {
+ .....
+       } else {
+               dev_err(&pdev->dev,
+                       "Failed to initialize channels, because device
tree not available: %d\n", rc);
+               bcm2835_dma_free(od);
+               return rc;
+      }
+
+       dev_dbg(&pdev->dev, "Initialized %i DMA channels\n", i);
+
+ .....

>> I am becoming desperate anyway that this migration will ever fully
>> take place....
>
> Why not? It's just a matter of people like you working on this (and
> addressing some review comments ;)).

The most common comment about this is that people will not put effort in
the upstream kernel as long as there is no comfortable way for debugging
in the upstream kernel (i.e. at least USB support).......

Greetings,
Florian
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