amba-pl011 complains about DMA for bcm2835 where it is not supported

amba-pl011 complains about DMA for bcm2835 where it is not supported

[Piotr Król]

Hi all,
according to BCM2835 ARM Peripherals spec DMA is optional functionality
for ARM PL011 UART and it is not supported on this SoC. It looks like
amba-pl011 search for device tree property (dma-names) unconditionally,
what leads to error message:

of_dma_request_slave_channel: dma-names property of node '/soc/uart@7e201000' missing or empty
uart-pl011 20201000.uart: no DMA platform data

I assume that if it is possible that amba-pl011 can be without DMA we
should check if DMA is supported and then try to read properties. Is
there any known method for checking dma support ? If yes that I would be
glad for any pointers.

Let me know if I should ignore/accept this error message or there is
some code fix needed.

Thanks,
Piotr Król
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Re: amba-pl011 complains about DMA for bcm2835 where it is not supported

[Russell King - ARM Linux]

On Tue, Oct 28, 2014 at 12:44:31AM +0100, Piotr Król wrote:
> Hi all,
> according to BCM2835 ARM Peripherals spec DMA is optional functionality
> for ARM PL011 UART and it is not supported on this SoC. It looks like
> amba-pl011 search for device tree property (dma-names) unconditionally,
> what leads to error message:
> 
> of_dma_request_slave_channel: dma-names property of node '/soc/uart@7e201000' missing or empty
> uart-pl011 20201000.uart: no DMA platform data
> 
> I assume that if it is possible that amba-pl011 can be without DMA we
> should check if DMA is supported and then try to read properties. Is
> there any known method for checking dma support ? If yes that I would be
> glad for any pointers.
> 
> Let me know if I should ignore/accept this error message or there is
> some code fix needed.

A great many implementations of PL011 do not have DMA support (some have
DMA support but its well broken.)  The driver has always supported PIO
mode, and continues to do so.

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Re: amba-pl011 complains about DMA for bcm2835 where it is not supported

[Piotr Król]

On Mon, Oct 27, 2014 at 11:59:42PM +0000, Russell King - ARM Linux wrote:
> On Tue, Oct 28, 2014 at 12:44:31AM +0100, Piotr Król wrote:
> > Hi all,
> > according to BCM2835 ARM Peripherals spec DMA is optional functionality
> > for ARM PL011 UART and it is not supported on this SoC. It looks like
> > amba-pl011 search for device tree property (dma-names) unconditionally,
> > what leads to error message:
> > 
> > of_dma_request_slave_channel: dma-names property of node '/soc/uart@7e201000' missing or empty
> > uart-pl011 20201000.uart: no DMA platform data
> > 
> > I assume that if it is possible that amba-pl011 can be without DMA we
> > should check if DMA is supported and then try to read properties. Is
> > there any known method for checking dma support ? If yes that I would be
> > glad for any pointers.
> > 
> > Let me know if I should ignore/accept this error message or there is
> > some code fix needed.
> 
> A great many implementations of PL011 do not have DMA support (some have
> DMA support but its well broken.)  The driver has always supported PIO
> mode, and continues to do so.
> 

Driver works fine but IMHO this error message confuse user. Should I assume
that error message is expected and we should not avoid it by trying to
check if DMA is supported ?

BTW above error message do not indicate that DMA is not supported but
indicates that something is wrong with DT, what in this case is not true.

Thanks,
Piotr Król


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Re: amba-pl011 complains about DMA for bcm2835 where it is not supported

[Russell King - ARM Linux]

On Tue, Oct 28, 2014 at 01:09:23AM +0100, Piotr Król wrote:
> On Mon, Oct 27, 2014 at 11:59:42PM +0000, Russell King - ARM Linux wrote:
> > On Tue, Oct 28, 2014 at 12:44:31AM +0100, Piotr Król wrote:
> > > Hi all,
> > > according to BCM2835 ARM Peripherals spec DMA is optional functionality
> > > for ARM PL011 UART and it is not supported on this SoC. It looks like
> > > amba-pl011 search for device tree property (dma-names) unconditionally,
> > > what leads to error message:
> > > 
> > > of_dma_request_slave_channel: dma-names property of node '/soc/uart@7e201000' missing or empty
> > > uart-pl011 20201000.uart: no DMA platform data
> > > 
> > > I assume that if it is possible that amba-pl011 can be without DMA we
> > > should check if DMA is supported and then try to read properties. Is
> > > there any known method for checking dma support ? If yes that I would be
> > > glad for any pointers.
> > > 
> > > Let me know if I should ignore/accept this error message or there is
> > > some code fix needed.
> > 
> > A great many implementations of PL011 do not have DMA support (some have
> > DMA support but its well broken.)  The driver has always supported PIO
> > mode, and continues to do so.
> > 
> 
> Driver works fine but IMHO this error message confuse user. Should I assume
> that error message is expected and we should not avoid it by trying to
> check if DMA is supported ?
> 
> BTW above error message do not indicate that DMA is not supported but
> indicates that something is wrong with DT, what in this case is not true.

Unfortunately, the DT lobby believe that their way is the only way, and
that drivers should always complain if DT stuff is missing.  They seem
to loose sight of what behaviour an "optional" property should have.
I've seen this happen several times now, and I've commented on patches I
saw today which had made driver initialisation /fail/ if various optional
properties weren't specified!

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according to speedtest.net.
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