SPE to SPE DMA transfer

From Cellbe

This article shows a simple DMA transfer between two SPEs. The synchronization is done via mailbox with the aid of the PPE. The process "PGET" holds the target memory area; it writes 200 in the target area, then waits for the transfer, and prints the value stored in target area. The process "PPUT" writes a value in the target memory area. The order of operations is as follows:

1. The SPE processes are started
2. PGET communicates to the PPE the target address via mailbox, then synchronizes with the PPE with a mailbox read
3. The PPE calculates the effective address of the target, and sends it to PPUT via mailbox, and synchronizes with PPUT with a mailbox read
4. PPUT reads the target's effective address from mailbox, starts the dma trasfer, waits for it to finish and wakes up the PPE with a mailbox write
5. The PPE wakes up, and writes a mailbox to PGET to wake it up.
6. PGET wakes up, and prints the content of the target memory area.

Code snippet: PPE side

/* =============================================================================
 *       Filename:  dmaspe.c							
 *    Description:  Starts the spu threads and cares of synchronizing them with 
 *		    Mailboxes							
 *         Author:  Alessandro Piras (laynor@gmail.com)				
 *        Company:								
 * ===========================================================================*/


#include <libspe2.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include "common.h"

#define MYALIGN(vardecl, n) vardecl\\
	__attribute__ ((__aligned__ (n)))

extern spe_program_handle_t dmaspespu_get;
extern spe_program_handle_t dmaspespu_put;

MYALIGN(unsigned int ls_addr, 128);

/************************************
 * Routine used to start the threads*
 ***********************************/

void *pthrtn(void *arg)
{
	unsigned int entry = SPE_DEFAULT_ENTRY;
	spe_context_ptr_t ctx;

	ctx = *((spe_context_ptr_t *) arg);
	spe_context_run(ctx, &entry, 0, &ls_addr, NULL, NULL);
	pthread_exit(NULL);
}

int main()
{
	spe_context_ptr_t ctx1, ctx2;
	unsigned int shared_lsaddr;
	pthread_t pth1, pth2;
	union eat ls_base_addr;
	union eat shared_ea;
	unsigned int bell;
	
	/* Creating the contexts, loading the programs and starting threads   */
	ctx1 = spe_context_create(0,NULL);
	ctx2 = spe_context_create(0,NULL);
	
	spe_program_load(ctx1, &dmaspespu_get);
	spe_program_load(ctx2, &dmaspespu_put);
	pthread_create(&pth1, NULL, pthrtn, &ctx1);
	pthread_create(&pth2, NULL, pthrtn, &ctx2);

	/* 1. Getting the ls address of the memory area where the put will be */
	/*    executed (via mailboxes)                                        */
	

	while(!spe_out_mbox_status(ctx1));
	spe_out_mbox_read(ctx1, &shared_lsaddr, 1);
	/* 2. constructing the effective address adding the base address of   */
	/*    LS area of the "get" process to the LS address just received    */
	ls_base_addr.e = 0;
	ls_base_addr.e =(unsigned long long) spe_ls_area_get(ctx1);
	shared_ea.e = shared_lsaddr + ls_base_addr.e;
	
	/* 3. Sending the calculated address to the "put" process.            */
	spe_in_mbox_write(ctx2, &shared_ea.h, 1, 1);
	spe_in_mbox_write(ctx2, &shared_ea.l, 1, 1);

	/* synchronizing with the put process for the termination of the dma  */
	/* transfer.                                                          */
	while(!spe_out_mbox_status(ctx2));
	spe_out_mbox_read(ctx1, &bell, 1);

	/* Waking up the "get" process                                        */
	spe_in_mbox_write(ctx1, &bell, 1, 1);

	/* Joining the threads                                                */
	pthread_join(pth1, NULL);
	pthread_join(pth2, NULL);
	spe_context_destroy(ctx1);
	spe_context_destroy(ctx2);
	return 0;

}

Code snippet: process PGET, SPE side

/* =============================================================================
 *										
 *       Filename:  dmaspespu_get.c						
 *										
 *    Description:  waits for transfer and prints the value.			
 *										
 *         Author:  Alessandro Piras (laynor@gmail.com)				
 *        Company:								
 *										
 * ===========================================================================*/

#include <stdio.h>
#include <spu_intrinsics.h>
#include <spu_mfcio.h>
#include <commonlib.h>

#define MYALIGN(vardecl, n) vardecl\\
	__attribute__ ((__aligned__ (n)))



MYALIGN(volatile unsigned int cane, 128);
int main(unsigned long long spe_id, unsigned long long argp, unsigned long long envp)
{
	unsigned int cane_addr;
	cane = 200;
	/* spu_dsync();                                                   */
	/* I'm not sure if it's indeed useful/required, it worked without */
	cane_addr = &cane;
	/* Sending the address of the memory area where the other spe     */
	/* process will transfer its data                                 */
	spu_write_out_mbox((unsigned int)cane_addr);
	/* Synchronizing with the ppe                                     */
	spu_read_in_mbox();
	printf("(spu1) value: %u\
", cane);
	return 0;
}

Code snippet: process PPUT, SPE side

/* =============================================================================
 *										
 *       Filename:  dmaspespu_put.c						
 *										
 *    Description:  issues a dma on the LS of spe1.				
 *										
 *         Author:  Alessandro Piras (laynor@gmail.com)				
 *        Company:								
 *										
 * ===========================================================================*/

#include <stdio.h>
#include <spu_intrinsics.h>
#include <spu_mfcio.h>
#include <commonlib.h>
#include "../common.h"	

MYALIGN(volatile unsigned int x, 128);
int main(unsigned long long spe_id, unsigned long long argp, 
	unsigned long long envp)
{
	union eat shared_ea;
	/* Getting the target LS area effective address from ppe              */
	shared_ea.h = spu_read_in_mbox();
	shared_ea.l = spu_read_in_mbox();
	
	x = 210;
	/* Starting the dma trasnfer                                          */
	mfc_put((volatile void *)&x, shared_ea.e, 
		sizeof(unsigned int), 31, 0, 0);
	/* Waiting for the dma transfer to finish                             */
	mfc_write_tag_mask(1<<31);
	mfc_read_tag_status_all();
	printf("value = %d\
", x);
	/* Synchronizing with the ppe                                         */
	spu_write_out_mbox(10);
	return 0;
}