Re: Advice on writing driver

[James Courtier-Dutton <James@superbug.co.uk>]

Adrian McMenamin wrote:
> I have asked some the questions below in piecemeal form before and only
> occasionally got an answer, so i'll go through it all now and see if you
> can help.
>
> I am writing a driver for the Sega Dreamcast. Years ago I wrote an OSS
> driver but the sound dma had not been reversed then so it was slower and
> tended to lock the system.
>
> On the Dreamcast (DC) sound the main CPU is an SH4 but there is a
> separate low powered ARM7 to handle sound (PCM). The ARM7 has its own
> memory space which is directly accessible to the SH4 only through a FIFO
> like gateway (so slow) unless DMA is used.
>
> The ARM7 also has to have its own program to be told how to play the
> sound (this looks like firmware to the SH4).
>
> The driver currently allocates a buffer in the main SH4 memory (using
> the DMA buffer allocation APIs). This is then transferred to the ARM7
> space using DMA.
>
> The ARM7 code raises an interrupt to the SH4 whenever it plays 4096
> bytes of the sound - ie every period.
>
> What should my pointer callback return? the number of bytes/frames still
> to be transferred from the DMA buffer to the ARM memory or the number of
> bytes/frames still to play, or what?
>
> Adrian
>
>   
The pointer callback should return the current playback position in the 
sound  ring buffer.
So, NOT the following:

a) the number of bytes/frames still to be transferred from the DMA buffer to the ARM memory
b) the number of bytes/frames still to play

but instead it should return the position in the ring buffer of the 
frame currently being played by the ADC.


James



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