Re: DMA

Rene Herman <rene.herman@xxxxxxxxxxxx> · Fri, 25 Apr 2008 15:05:23 +0200

On 24-04-08 08:28, jeyram jadenthradevan wrote:

i am a beginner level in driver programming. please help to solve this 

problem.

this is a part of the internal modem driver coding. this is for intel 

chip set internal modem.

-------------------------------------------------------------------------------------------------------------------------------------------------------------

// ZT_MAX_CHUNKSIZE = 8

wc->writechunk = (int *)pci_alloc_consistent(pdev, ZT_MAX_CHUNKSIZE * 2 

* 2 * 2 * 4, &wc->writedma);

    if (!wc->writechunk) {
        printk("wcfxo: Unable to allocate DMA-able memory\n");
        if (wc->freeregion)
            release_region(wc->ioaddr, 0xff);
        return -ENOMEM;
    }

    wc->readchunk = wc->writechunk + ZT_MAX_CHUNKSIZE * 4;    /* in 

doublewords */

    wc->readdma = wc->writedma + ZT_MAX_CHUNKSIZE * 16;        /* in 

bytes */

---------------------------------------------------------------------------------------------------------------------------------------------------------------

here they are creating 2 chunks for writing and reading.

but i couldn't understand this line.
wc->readdma = wc->writedma + ZT_MAX_CHUNKSIZE * 16;

why are they using ZT_MAX_CHUNKSIZE * 16 for allocating read chunk?

I gather "a chunk" is some entity consisting of 4 32-bit datums.

wc->writechunk is an int * I see from the cast and (I suppose) wc->writedma 

is a char *. C pointer arrithmetic says	p + i = (char *)p + i * sizeof(*p) 

meaning both would end up at the same numeric value. I guess that was the 

question?

Rene.

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