[PATCHv8 2/2] dma: Add Freescale eDMA engine driver support

[Jingchang Lu]

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd at arndb.de]
> Sent: Thursday, January 09, 2014 8:19 PM
> To: Lu Jingchang-B35083
> Cc: vinod.koul at intel.com; dan.j.williams at intel.com; shawn.guo at linaro.org;
> pawel.moll at arm.com; mark.rutland at arm.com; swarren at wwwdotorg.org; linux-
> kernel at vger.kernel.org; linux-arm-kernel at lists.infradead.org;
> devicetree at vger.kernel.org
> Subject: Re: [PATCHv8 2/2] dma: Add Freescale eDMA engine driver support
> 
> On Thursday 09 January 2014 11:44:46 Jingchang Lu wrote:
> > >
> > > On Thursday 09 January 2014, Jingchang Lu wrote:
> > > > > > > +sai2: sai at 40031000 {
> > > > > > > +	compatible = "fsl,vf610-sai";
> > > > > > > +	reg = <0x40031000 0x1000>;
> > > > > > > +	interrupts = <0 86 0x04>;
> > > > > > > +	clocks = <&clks VF610_CLK_SAI2>;
> > > > > > > +	clock-names = "sai";
> > > > > > > +	dma-names = "tx", "rx";
> > > > > > > +	dmas = <&edma0 VF610_EDMA_DMAMUX0
> VF610_EDMA0_MUX0_SAI2_TX>,
> > > > > > > +		<&edma0 VF610_EDMA_DMAMUX0
> VF610_EDMA0_MUX0_SAI2_RX>;
> > > > > > > +	status = "disabled";
> > > > > > > +};
> > > > >
> > > > > It seems wrong to have macros defined like
> VF610_EDMA0_MUX0_SAI2_TX,
> > > > > in particular in the example. These should just be literal
> numbers.
> > > > [Lu Jingchang-b35083]
> > > > This eDMA engineer requires two specifiers, one is a mux group id,
> the
> > > other is a request source id.
> > > > The VF610_EDMA0_MUX0_SAI2_TX is sai's tx dma request source id, it
> is
> > > defined as a literal number.
> > > > There are totally more than 100 request source id, I have them
> macros
> > > defined to make it referenced
> > > > easily and clearly, just like some clock binding done.
> > > > The macros are defined in include/dt-bindings/dma/vf610-edma.h.
> > >
> > > The clock bindings are special because the macros there tend to be
> made
> > > up for controllers that just have a bunch of clocks at random
> register
> > > locations.
> > >
> > > This is not the case for DMA bindings (or some of the more regular
> clock
> > > controllers), so there is absolutely no reason to define those macros
> > > in a header file, it just adds artificial dependencies between the
> > > driver, SoC support and the binding.
> > >
> > > If the numbers are the same as the ones provided in the data sheet,
> > > just use the numbers and remove the macros.
> >
> > Yes, the request source id is defined in the data sheet.
> > each eDMA controller has two muxes, some SoCs have two eDMA controllers
> > while others have only one. The 1st eDMA's muxes are named mux0 and
> mux1,
> > while the 2nd eDMA controller's muxes are named MUX2 and MUX3 in the
> datasheet,
> > and I index them in each eDMA controller from 0 in the driver to handle
> them commonly.
> 
> Right. I don't object to the use of the macros for the muxes, that seems
> fine and appropriate given your description.
> 
> > I defines the macro tending to give the info from the macro name, such
> as
> > for the VF610_EDMA0_MUX0_SAI2_TX request source, implying the eDMA
> engine id 0,
> > and the mux id should be 0. While VF610_EDMA1_MUX1_FTM0_CH0 implying
> eDMA engine id 1,
> > mux 1. The request sources are shared between the two eDMA controller
> but need the user
> > to select one manually when reference.
> 
> Note that the dma binding allows you to actually specify both engines
> in this case. A slave device can have
> 
> 	dma-names = "rx", "rx", "tx", "tx";
> 	dmas = <&edma0 0 12> <&edma1 0 12> <&edma0 0 13> <&edma1 0 13>;
> 
> and the requesting a channel from the slave driver will find
> the first available one. In theory we could have some load-balancing
> as well, but that has not been implemented.
[Lu Jingchang-b35083] 
Yes, the slave can specify mixed dma channel and the eDMA driver can allocate
channels to it properly. But just as you said, the driver doesn't implement
load-balanceing between eDMA controller, this need the user to balance them
in dts and slave driver manually. And on our LS-1 SoC, there is only one eDMA
controller.
For the slave request id macros definitions, I think their reservation could
direct ease the combination of the controller, the mux, and the request in dts.
Thanks.

> 
> > > You are right, your code is actually correct on all combinations
> > > of big-endian and little-endian ARM CPUs. However, I would argue that
> > > it's unusual style, and not portable to other architectures (e.g.
> arm64)
> > > because the definition of readl() is highly architecture dependent.
> > > It would also be problematic if the arm definition has to change
> > > in some form and this driver is overlooked.
> > [Lu Jingchang-b35083]
> > Yes, these definitions are highly depending on the arm32 arch. This
> device is only
> > integrated in our arm32 SoCs currently, so I have only considered arm32.
> > In arm32, it seems that the ioread32 and readl are the same in arm32,
> > I will try your recommendation below to simplify the caller.
> 
> Ok, thanks. I generally insist on writing drivers in a portable way
> if possible because they can serve as examples to other people, and
> hardware often gets reused on other parts. I would expect that it's
> possible that freescale eventually creates powerpc or arm64 devices
> with the same edma controller, even if you are not currently aware
> of any such products.
> 
> > > BTW, I noticed that fsl_edma_set_tcd_params() is calling edma_writew()
> > > and edma_writel() without an endian-swap, so I assume it is still
> > > broken on big-endian CPUs, and likely also on big-endian eDMA engines.
> > The values written in fsl_edma_set_tcd_params() are pre-swapped in
> fill_tcd_params().
> > The eDMA support hardware SGs, but the eDMA engine's sg format is
> required the same as
> > the eDMA Controller's endian, so the SGs in memory to be loaded
> automatically by the eDMA
> > engine also required swapped properly. So they should be swapped
> specifically here.
> 
> Ah, I see now. I had noticed that before but then forgotten about it.
> 
> > > > > > > +static bool fsl_edma_filter_fn(struct dma_chan *chan, void
> *mux)
> > > > > > > +{
> > > > > > > +	 return (chan->chan_id / DMAMUX_NR) == (u32)mux;
> > > > > > > +}
> > > > > > > +
> > > > > > > +static struct dma_chan *fsl_edma_xlate(struct
> of_phandle_args
> > > > > *dma_spec,
> > > > > > > +		struct of_dma *ofdma)
> > > > > > > +{
> > > > > > > +	struct dma_chan *chan;
> > > > > > > +	dma_cap_mask_t mask;
> > > > > > > +
> > > > > > > +	if (dma_spec->args_count != 2)
> > > > > > > +		return NULL;
> > > > > > > +
> > > > > > > +	dma_cap_zero(mask);
> > > > > > > +	dma_cap_set(DMA_SLAVE, mask);
> > > > > > > +	dma_cap_set(DMA_CYCLIC, mask);
> > > > > > > +	chan = dma_request_channel(mask, fsl_edma_filter_fn,
> (void
> > > > > > > *)dma_spec->args[0]);
> > > > > > > +	if (chan)
> > > > > > > +		fsl_edma_chan_mux(to_fsl_edma_chan(chan),
> dma_spec-
> > > >args[1],
> > > > > > > true);
> > > > > > > +	return chan;
> > > > > > > +}
> > > > >
> > > > > Please remove the filter function now and use dma_get_slave_chan
> > > > > with the correct channel as an argument. No need to walk all
> > > > > available channels in the system and introduce bugs by not
> > > > > ignoring other dma engines.
> > > >
> > > > The dma slave request can only be allocated to channel of
> particular
> > > channels group indicated by
> > > > the mux group id specifier. and the second specifier is the request
> id,
> > > not the channel number,
> > > > so to use the dma_get_slave_chan, I would find the channel for this
> > > request by walking all the
> > > > available channels manually.
> > >
> > > Ah, I missed that you only check the mux number, not the channel
> number.
> > >
> > > The current version however is buggy because you don't check that you
> > > are actually looking at the right eDMA instance, or an eDMA at all,
> > > rather
> > > than some other random dma engine that may be connected to an
> external
> > > bus.
> > >
> > > It is possible to fix that, but I suspect that would involve more
> > > complex code than finding an appropriate channel first and then
> > > calling dma_get_slave_chan() on that.
> > > I would suggest keeping a list of channels per dmamux and walking
> that
> > > list until you find one that succeeds in dma_get_slave_chan().
> > The binding in DTS is to the eDMA engine, and the binding indicates the
> eDMA node
> > (by <&edma0 ...>), the dma engine framework would find the proper eDMA
> engine device
> > referenced in dts. This driver only supports dts reference, so I think
> the dma engine
> > framework could guarantee the eDMA instance properly in
> of_dma_find_controller().
> 
> The problem is that dma_request_channel() does not get a reference to the
> eDMA node here. It is a generic API function that returns the first
> channel out of the global list of dma_chan structure that is unused
> and gets a positive return code from the filter function. If you have
> two edma instances in the system, you have a 50% chance to get a channel
> that belongs to the other instance.
> 
> The dma_get_slave_chan() interface was introduced to avoid this problem.
Yes, the public dma_request_channel() will iterate all dma engine devices
when being called. but even I select dma_get_slave_chan() here, the driver
still couldn't prevent others from calling it to request a channel explicitly.
Thanks!


Best Regards,
Jingchang