[PATCH v2 2/6] ASoC: xilinx: xlnx_formatter_pcm: Handle sysclk setting

[Robert Hancock]

On Mon, 2022-01-10 at 20:31 +0000, Mark Brown wrote:
> On Mon, Jan 10, 2022 at 06:28:36PM +0000, Robert Hancock wrote:
> 
> > -On initialization for simple-card, if a clock frequency is specified in
> > device
> > tree, set_sysclk is called on the DAI by asoc_simple_init_dai (called by
> > asoc_simple_dai_init). However, it doesn't appear that it is called on the
> > platform (xlnx_formatter_pcm in this case) at this point.
> > -startup gets called on the DAI from pcm_open, so xlnx_i2s should be able
> > to
> > add its rate constraints properly at that point. However,
> > xlnx_formatter_pcm
> > has no sysclk set at this point, so it couldn't currently enforce any
> > constraint based on that.
> 
> I thought the later patches in your series were intended to fix things
> so that the set_sysclk() does get called?

It does get called during hw_params now, but not during initialization as it is
with the DAI components. I'm not sure that really matters if we don't try to
add constraints in that driver though (see below) and it just needs to know
what sysclk and sample rate are being used.

> 
> > -when the top-level hw_params call is made with simple-card, set_sysclk
> > gets
> > called on everything by asoc_simple_hw_params prior to hw_params calls on
> > all
> > of the components. The sysclk there is based on the device tree mclk-fs and
> > the
> > sample rate which might wipe out the original clock frequency if the
> > constraints don't prevent setting an invalid rate.
> 
> If the device is using mclk-fs then either there's a fixed sample rate
> (in which case simple-card probably ought to force it without the driver
> worrying) or the sysclk will vary in which case simple-card should be
> setting the sysclk to 0 when the card goes idle to clear any
> constraints (which as you say later it does).

It sounds like to fix that, simple-card needs to keep track of whether the DAI
has a fixed clock rate or not. If it does, then it shouldn't be zeroing out the
sysclk when closing the device as that's wiping out information that needs to
be persistent. I guess if the frequency was read from a system-clock-frequency
property then we know it is fixed. There are other cases where it could be a
fixed rate, such as if the clock is connected to a fixed-clock. Maybe we need
an explicit DT flag "system-clock-fixed" or something for those cases?

Then at least in the case where mclk-fs is set and one or more of the DAIs have
a fixed rate, simple-card can add a constraint to restrict the sample rate to
the fixed clock divided by mclk-fs?

> 
> > -In the case of xlnx_formatter_pcm and simple-card, since sysclk is
> > determined
> > by sample rate times mclk-fs, there's no way for userspace to violate the
> > constraint that the sample rate divides evenly into sysclk.
> 
> Only on a system where the sysclk can vary - this is a generic card so
> someone could set a fixed sysclk, and of course the driver could be used
> with other cards.
> 
> > -in order to add any constraints on sample rate based on sysclk in the
> > formatter driver, it would need to get set_sysclk called during
> > initialization
> > which doesn't currently happen. But with simple-card, there's no way those
> > constraints could be violated other than a kernel bug.
> 
> You shouldn't be making assumptions about the machine driver in the DMA
> driver, especially for something like this used in a FPGA product which
> has even more flexibility than most things.

That's true, but the constraint of "sample rate must divide into sysclk" in the
formatter seems like one that should always be the case, and shouldn't really
be the responsibility of the low-level driver level to verify but should be
handled by simple-card or whatever other top-level driver.

> 
> > -xlnx_i2s needs some way to avoid its stored sysclk being wiped out during
> > PCM
> > close so that the constraints are handled properly during subsequent opens.
> 
> Depending on how flexible the system is clearing the sysclk stored in
> the I2S driver may be desirable - if the sysclk rate can be changed then
> you usually don't want to force constraints based on what it was during
> the last stream, you want to relax such constraints so that a new sysclk
> can be chosen where appropriate.
> 
> If that's not possible in this system then it sounds like it should be
> setting system-clock-frequency and simple-card should then not be
> clearing any setting in the components when the stream closes down, it
> should be setting the clock up once at init.  Broadly speaking the
> machine driver is responsible for ensuring that the overall system
> configuration is sensible and coherent (that's what it's there for).
-- 
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com