[alsa-devel] [PATCH 0/3] ASoC: Enable a new IC master mode: bcm2835<=>IC<=>cs42xx8

Charles Keepax ckeepax at opensource.wolfsonmicro.com
Wed Mar 22 02:43:01 PDT 2017

On Wed, Mar 22, 2017 at 10:29:33AM +1100, Matt Flax wrote:
> On 22/03/17 09:11, Matthias Reichl wrote:
> >On Tue, Mar 21, 2017 at 10:21:04PM +0100, Emmanuel Fusté wrote:
> >>Le 16/03/2017 à 23:14, Matt Flax a écrit :
> >>>On 17/03/17 08:27, Lars-Peter Clausen wrote:
> >>>>On 03/16/2017 09:51 PM, Matt Flax wrote:
> >>>>>On 16/03/17 06:01, Mark Brown wrote:
> >>>>>>On Tue, Feb 28, 2017 at 09:59:29AM +0000, Charles Keepax wrote:
> >>>>>>>On Mon, Feb 27, 2017 at 12:51:08PM +0100, Matthias Reichl wrote:
> >>Re-reading this document, the bcm2835 PCM IP block SHOULD support real DSP
> >>mode, with one BCLK pulsed LRCLK, zero BCLK delay etc...
> >>It just need to be properly setup.
> >I've re-read the document, too, last week and noticed the framesync
> >registers - sorry, I had completely forgotten about these. I guess it
> >should be possible to configure the bcm2835 to DSP mode but it'd still be
> >limited to 2 channel setups - the hardware only has 2 channel position
> >registers for each direction.
> >
> >>According to the same document, you could program the bmc up to 16 32bits
> >>channels when in master mode, so I suspect that you could go up to this
> >>limit in slave mode.
> >>But as it is designed, it could only use up to two of any channels among the
> >>16.
> >I'm not quite sure if I can follow you on this - how would you
> >configure 16 channels when there are only 2 channel position registers?
> >
> >With bclk ratio eg set to 16*32=512 BCM2835 will only transmit 2*32
> >bits of data (at configurable bit positions), the remaining 448
> >bits will be zero.
> >
> The document seems to stipulate that the PCM audio device is an AMBA device
> with 2 APB data channels. The first sync edge marks the beginning of the two
> data words. Their frame lengths can be up to 1024+32 bits in length !
> I think the point is that they intended their PCM audio interface to be
> configurable, they say in their document "It supports many classic PCM
> formats".
> The important point here is that in ALSA we can only have I2S or DSP modes -
> right ?
> Unless we want to create a new ALSA mode (which clearly worries people) then
> we need to support the versatility of the bcm2835 PCM hardware using either
> DSP or I2S modes. Now, we have already implemented the I2S mode, so
> logically the only available mode left is the DSP mode. Using this mode, we
> can implement more features of this device.
> People seem to want to reserve DSP and I2S modes for strictly I2S and DSP
> protocols. At the same time people don't want to allow a looser "APB" mode
> into ALSA. For that reason, we have a lack of functionality for perfectly
> versatile hardware - the bcm2835 hardware.

Apologies but I am still a little unclear as to what actually happens
on the bus here.

Are we saying that what gets transmitted on the bus is neither valid
I2S or DSP mode data? But as you have your custom hardware block in
the middle it interprets this data correctly and converts it to a
regular bus format on the other side that goes to the CODEC?


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