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From: Emmanuel Duros <eduros@chouette.inria.fr>
To: freebsd-hackers@freebsd.org
Subject: Pb using DMA - Physical addressing
Reply-to: Emmanuel.Duros@sophia.inria.fr
Sender: owner-hackers@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk


Hi,

I am currently writing a device driver (isa bus) for a communication
card (satellite reception/emission cards) that uses DMA transfers.

I have got a peace of code that works fine under DOS but as I insert it
into the freebsd device driver code nothing happens.

I think the problem comes from the mapping of virtual addresses into
physical addresses (?). Unfortunately I have not found much
documentation on this...

Here is the simple C code for dos:

------- DOS
/*
 * transfers a 188 byte buffer from
 * memory to the device using drq 6
*/
    u_char buf[188];    
    u_long addr;
    u_char lsb, msb, page, lcnt, hcnt;

    dma_ch = 6;

    addr = ((unsigned long)FP_SEG ( buf ) <<  4)  /* get physical address */
           + FP_OFF ( buf );

    lsb  = (u_char)(addr >>  1);
    msb  = (u_char)(addr >>  9);
    page = (u_char)(addr >>  16);
 
    sz_dma = 188/2 - 1;
    lcnt   = sz_bma; hcnt = sz_dma >> 8;

    _disable();
    outp (SNGL_MASK_reg( dma_ch )    , DISABLE( dma_ch ));
			/* DMA_MODE = BLOCK|ADDR_INC|AUTO_DISABLE|READ|(dma_ch-4) */
    outp (MODE_reg( dma_ch )         , DMA_MODE);  
    outp (CLR_BYTE_PTR_reg( dma_ch ) , 0);
    outp (BASE_ADDR_reg( dma_ch )    , lsb);
    outp (BASE_ADDR_reg( dma_ch )    , msb);
    outp (PAGE_ADDR_reg( dma_ch )    , page);
    outp (CLR_BYTE_PTR_reg( dma_ch ) , 0);
    outp (COUNT_reg( dma_ch )        , lcnt);
    outp (COUNT_reg(dma_ch )         , hcnt);
    _enable();
 
    outp (SNGL_MASK_reg( dma_ch ), ENABLE( dma_ch ));
    outp (SNGL_MASK_reg( dma_ch ), DISABLE( dma_ch ));
--------

Now is the very similare peace of code I wrote for the FreeBSD driver:

-------- FreeBSD
/* 
 * transfers a 188 byte buffer from
 * memory to the device using drq 6
*/
    
   buf = malloc( 188, M_DEVBUF, M_NOWAIT); /* this is from the attach function */
					   /* am I sure that buf < 16M ??? */
   /* filling buffer here */

   addr = pmap_extract(pmap_kernel(), (vm_offset_t) buf);
   /* I found this function, but not quite sure of its behavior)
   /* or addr = (u_int) buf; ??? */

   lsb  = (u_char)(addr >>  1);
   msb  = (u_char)(addr >>  9); 
   page = (u_char)(addr >>  16 &  0xFE);

   sz_dma = 188/2 - 1;
   lcnt   = sz_dma; hcnt = sz_dma >>  8;
 
   outb (SNGL_MASK_reg( dma_ch )   , DISABLE( dma_ch ));
   outb (MODE_reg( dma_ch )        , DMA_MODE);
   outb (CLR_BYTE_PTR_reg( dma_ch ), 0);
   outb (BASE_ADDR_reg( dma_ch )   , lsb);
   outb (BASE_ADDR_reg( dma_ch )   , msb);
   outb (PAGE_ADDR_reg( dma_ch )   , page);
   outb (CLR_BYTE_PTR_reg( dma_ch ), 0);
   outb (COUNT_reg( dma_ch )       , lcnt);
   outb (COUNT_reg( dma_ch )       , hcnt);

   outb (SNGL_MASK_reg( dma_ch ), ENABLE( dma_ch ));
   outb (SNGL_MASK_reg( dma_ch ), DISABLE( dma_ch ));
--------


I think I do not give the DMA the correct address of buf, how can this
be done ?

If someone had any ideas on the problem, I would be very glad to hear
them !

Could anybody explain the relation between physical addressing and
virtual addressing ?

One last question, is it possible to develop in the user space
applications that would handle dma transfers ?  In fact, I am a bit fed
up with rebooting my PC every time I recompile the kernel !

Thanks for your help...

Emmanuel

+--------------------------------------------------------------------+
|Emmanuel Duros                      | Emmanuel.Duros@sophia.inria.fr|
|INRIA Sophia Antipolis              | Projet RODEO                  |
|2004, Route des Lucioles BP 93      | Phone : +33 04 93 65 79 42    |
|06902 Sophia Antipolis CEDEX France | Fax   : +33 04 93 65 77 65    |
+--------------------------------------------------------------------+